Vol. 55 No. 4/2008, 649–662 Review

Rsp5 is a conserved ubiquitin ligase involved in regulation of numerous cellular processes. A growing number of publications describing new functions of the ligase have appeared in recent years. Rsp5 was shown to be involved in the control of intracellular trafficking of proteins via endocytosis and multivesicular body sorting. Moreover, nuclear functions of Rsp5 in response to various stresses have been discovered. Rsp5 is also involved in the regulation of unsaturated fatty acid and sterol synthesis and phospholipid composition. Here, an overview of Rsp5 functions with emphasis on its involvement in the regulation of lipid biosynthesis will be presented. UbiqUiTinATion Numerous cellular proteins are modified post-translationally by conjugation of the polypep-tide ubiquitin. Among them are cell cycle regulators , transcription activators, signaling proteins, and enzymes involved in metabolic pathways. Therefore, the ubiquitination system regulates a broad array of cellular processes. Aberrations in the system have been implicated in the pathogenesis of major diseases such as cancer, diabetes, and neurodegenerative disorders (reviewed by Weissman, 2001). Ubiquitination is a process of ubiquitin conjugation to the protein substrate (Hereshko & Ciech-anover, 1998). The process is carried out by a multi-enzyme cascade involving enzymes from different classes. First, ubiquitin is activated by E1 activating enzyme. In yeast cells there is only one E1 enzyme — Uba1, which is essential for growth. The activation is ATP-dependent and occurs with the formation of a thioester bond between a cysteine in the active center of E1 and the C-terminus of ubiquitin. The next step is the transfer of activated ubiquitin to a cysteine residue located in the active center of conjugating enzyme E2. Thirteen E2 enzymes have been identified in yeast. Ubiquitin is then transferred to the acceptor protein either directly from the E2 enzyme or indirectly with an involvement of a ubiquitin ligase, the E3 enzyme. Isopeptide bond is formed between the C-terminal glycine of ubiquitin and a lysine of the substrate protein or from another ubiquitin molecule. The E3 ligases play important roles in recognition and binding specific substrates in a particular moment and compartment of the cell. They are grouped in two classes: protein complexes with a RING-finger catalytic domain, such as APC (anaphase promoting complex) (Jackson et al., 2000), and ligases containing a Hect domain (homologus


UbiqUiTinATion
Numerous cellular proteins are modified post-translationally by conjugation of the polypeptide ubiquitin.Among them are cell cycle regulators, transcription activators, signaling proteins, and enzymes involved in metabolic pathways.Therefore, the ubiquitination system regulates a broad array of cellular processes.Aberrations in the system have been implicated in the pathogenesis of major diseases such as cancer, diabetes, and neurodegenerative disorders (reviewed by Weissman, 2001).
Ubiquitination is a process of ubiquitin conjugation to the protein substrate (Hereshko & Ciechanover, 1998).The process is carried out by a multienzyme cascade involving enzymes from different classes.First, ubiquitin is activated by E1 activating enzyme.In yeast cells there is only one E1 enzyme -Uba1, which is essential for growth.The activa-tion is ATP-dependent and occurs with the formation of a thioester bond between a cysteine in the active center of E1 and the C-terminus of ubiquitin.The next step is the transfer of activated ubiquitin to a cysteine residue located in the active center of conjugating enzyme E2.Thirteen E2 enzymes have been identified in yeast.Ubiquitin is then transferred to the acceptor protein either directly from the E2 enzyme or indirectly with an involvement of a ubiquitin ligase, the E3 enzyme.Isopeptide bond is formed between the C-terminal glycine of ubiquitin and a lysine of the substrate protein or from another ubiquitin molecule.The E3 ligases play important roles in recognition and binding specific substrates in a particular moment and compartment of the cell.They are grouped in two classes: protein complexes with a RING-finger catalytic domain, such as APC (anaphase promoting complex) (Jackson et al., 2000), and ligases containing a Hect domain (homologus 2008 P. Kaliszewski and T. Żołądek to E6-AP carboxy terminus) (Huibregtse et al., 1995).Five ligases from the Hect class are known in yeast: Rsp5, Ufd4, Tom1, Hul4, and Hul5 (Wang et al., 1999).
Ubiquitin is a 76 amino acid-long polypeptide that is highly evolutionarily conserved.It contains seven lysine residues K6, K11, K27, K29, K33, K48, and K63 (Arnason & Elison, 1994), all of which can be used for conjugation with other ubiquitin molecules.There are three types of ubiquitination: monoubiquitination, the attachment of a single ubiquitin, multiubiquitination -attachment of multiple ubiquitin molecules to a protein, and polyubiquitination, in which ubiquitin is attached to a lysine of another ubiquitin and a long polyubiquitin chain is formed on the protein.Mono-or polyubiquitination of proteins by K63-linked ubiquitins are signals for endocytosis, vacuolar degradation and chromatin remodeling (see a review by Hicke, 2001).Polyubiquitination affects also proteins involved in DNA repair, transcription, cell cycle, and endocytosis of plasma membrane proteins (Weissman, 2001;Lindsten et al., 2002).Polyubiquitination through K48 or K29 of ubiquitin is a signal for 26S proteasomal degradation of short-lived or misfolded proteins (Hochstrasser, 1996;Hershko & Ciechanover, 1998).The functions of the polyubiquitin chain linked by other lysines: K6, K11, K27, and K33 have not been discovered yet.

Rsp5 ubiquitin-protein ligase and its domain structure
The best-studied ubiquitin ligase in yeast, the eukaryotic model organism, is Rsp5 (Huibregtse et al., 1995).It belongs to the Nedd4 family of ubiquitin ligases implicated in diverse cellular functions.Nedd4-like proteins are found in eukaryotes from yeasts to mammals and are defined by a similar domain organization (reviewed by Ingham et al., 2004).In the baker's yeast there is only one protein from the Nedd4 family (Rsp5), but this family has expanded further in higher eukaryotes, for example there are nine paralogous proteins in humans (Fig. 1).The Nedd4 (neural cell-expressed developmentally downregulated) (Kumar et al., 1992) ligase (also referred to as Nedd4-1) is the founding member of the Nedd4 family.Several substrates and binding partners of Nedd4 have been identified and its function in signal transduction, protein trafficking and oncogenesis is documented (Shearwin-Whyatt et al., 2006;Wang et al., 2007).Nedd4 is involved in the regulation of endocytosis of the plasma membrane sodium channel ENaC and implicated in pathogenesis of a hereditary hypertension in humans, the Liddle syndrome (Hamilton & Butt, 2000;Rotin et al., 2000).Besides that Nedd4 is also involved in bud-ding of retroviruses (Segura-Morales et al., 2005).All proteins from the Nedd4 family possess a C2 domain, several WW domains and a catalytic Hect domain.The C2 domain is located at the N-terminus of the protein, multiple WW domains are in the middle, and the Hect domain is at the C-terminus (Harvey & Kumar, 1999).
The C2 domain, approximately 130 amino acids long, is a conserved lipid-and protein-interaction module that is often regulated by calcium (Nalefski & Falke, 1996;Hurley & Misra, 2000).Many C2 domains bind to membranes through electrostatic interactions between basic amino acids and negatively charged lipids (Cho, 2001).It has been shown that mutation of five lysine residues to glutamine within the C2 domain of Rsp5 abolishes its binding to the membranes.The C2 domain of Rsp5 interacts with phosphorylated phosphatidylinositols and is important for localization of Rsp5 to endosomal membranes (Dunn et al., 2004).The C2 domain of Rsp5 is not necessary for the essential function of Rsp5 in standard conditions, but is implicated in Rsp5dependent sorting of biosynthetic cargo proteins in multivesicular bodies (MVB, late endosomes) (Dunn et al., 2004).Moreover, deletion of the Rsp5 C2 domain impairs internalization of Gap1, a general amino acid permease, without detectably affecting its ubiquitination, suggesting that Rsp5 participates via its C2 domain in endocytosis of ubiquitinated permeases (Springael et al., 1999a).
The WW domains were first described by Sudol (1996) as small modules composed of about 40 amino acids.The WW domains mediate protein-protein interactions and recognize proline-rich sequences called PY motifs.These domains are folded into three-stranded anti-parallel β-sheets forming a hydrophobic pocket (Macias et al., 1996).WW domains are divided into four groups according to their binding specificities (Bedford et al., 2000).Group I bind the PXY, LPXY and PPXY motifs.Group II bind to PPLP, group III recognize the PPR motif and group IV bind short sequences containing phosphoserine or phosphothreonine followed by proline (Lu et al., 1999).Rsp5 contains three WW domains which belong to group I, but not all Rsp5 substrates known contain a PXY, PPXY or LPXY motif (Gupta et al., 2007).These proteins may bind Rsp5 via other domains than WW or use adaptor proteins which contain Rsp5-binding motifs.One of these adaptor proteins is Bsd2, which is crucial for Rsp5-dependent ubiqutination of Cps1, a vacuolar carboxypeptidase, and another Tre1, a protein important for ubiqutination and vacuolar degradation of the metal transporter Smf1 (Sullivan et al., 2007).
The catalytic Hect domain of about 350 residues is situated at the C-terminus of Rsp5/Nedd4 proteins.This domain is essential for the ubiquitina- The role of Rsp5 in yeast tion activity of Rsp5.The conserved cysteine in the active center of the Hect domain forms a thioester bond with ubiquitin upon its transfer from E2 enzymes (Huibregtse et al., 1995).Mutation of this cysteine results in a complete loss of ubiquitination activity of Rsp5 that leads to a dominant negative effect on cell growth.

Roles of Rsp5
Rsp5 is a key regulatory protein in the cell, which ubiquitinates numerous proteins and is involved in regulation of a broad array of cellular processes.It is capable of modifying proteins in different cellular compartments, for example on the plasma membrane and in the nucleus.

Intracellular trafficking of proteins
The involvement of Rsp5 in intracellular trafficking of proteins, particularly in endocytosis and MVB (multi vesicular body) sorting, is well studied.Endocytosis is a process by which cells internalize portions of the plasma membrane with proteins and lipids and surrounded molecules from outside the cell.Endocytosis allows cells to remove no longer needed plasma membrane proteins (ion channels, receptors, etc.) but also to supply them with nutrients from the environment.Moreover, it is important for modulation of the cellular responses to external stimuli.Endocytosis starts with invagination of the plasma membrane which buds off and forms an internal vesicle which is later fused with a cellular compartment -early endosome.The early endosome is then converted into MVB.The MVB forms when portions of the late endosome membrane invaginate and pinch off into the lumen, thus forming intralumenal vesicles (Katzmann et al., 2002;Raiborg et al., 2003).The cargo from MVB can be transported either to a vacuole or to the Golgi apparatus.Proteins from endosomes may be also recycled back to the plasma membrane.It has been shown that Rsp5 is important for ubiquitin-mediated endocytosis of several proteins, including the general amino acid permease Gap1 (Springael et al., 1999b), uracil permease Fur4 (Galan et al., 1996, Hein & Andre, 1997), maltose permease Mal61 (Medintz et al., 1998), hexose transporter Hxt6/7, tryptophan permease Tat2 (Beck et al., 1999), zinc transporter Zrt1 (Gitan & Eide, 2000), and the pheromone receptor Ste2 (Dunn & Hicke, 2001).In addition, mutations in the RSP5 gene cause defects in fluid phase endocytosis as monitored by uptake of the fluorescent dye Lucifer Yellow (Żołądek et al., 1997).The WW domains, but not C2, are important for internalization of Fur4 and Ste2 and for fluid phase endocytosis (Gajewska et al., 2001;Dunn & Hicke, 2001).Plasma membrane transporters and receptors are polydiubiquitinated and the ubiquitin chain is formed via lysine K63 (see a rewiev by Hicke, 2001).
The biosynthetic route is the main route for the delivery of resident vacuolar proteins and lipids from their site of synthesis in the ER, via Golgi and MVB, to their site of action in the vacuole.Sorting of biosynthetic and endocytic transmembrane proteins into MVB vesicles is controlled by the addition of a single ubiquitin moiety to a cytoplasmic domain of these proteins (Katzmann et al., 2001;Reggiori & Pelham, 2001;Urbanowski & Piper 2001;Morvan et al., 2004).Recent data indicate that Rsp5 is essential for the MVB sorting of the biosynthetic cargo.A mutant lacking the Rsp5 C2 or with mutations in the WW1, 2 or 3 domain was unable to ubiquitinate or properly sort Cps1 into MVB vesicles (Dunn et al., 2004).Other authors showed that Sna3 protein is also diverted from its route in case of Rsp5 deficiency (reviewed by Piper & Katzmann, 2007;Stawiecka-Mirota et al., 2007).Sna3 carries a PPXY motif which mediates its P. Kaliszewski and T. Żołądek interaction with Rsp5 WW domains.Mutation of either the Sna3 PPXY motif or the Rsp5 WW3 domain or reduction in the amount of Rsp5 results in mistargeting of Sna3 to multiple mobile vesicles and prevents its sorting to the endosomal pathway.Sna3 is polyubiquitinated on one target lysine, and a mutant Sna3 lacking this lysine displays defective MVB sorting.Sna3 undergoes Rsp5-dependent polyubiquitination with K63-linked ubiquitin chains (Stawiecka-Mirota et al., 2007).Rsp5-dependent ubiquitination is also involved in sorting of the amino acid permease Gap1 at the Golgi apparatus (Helliwell et al., 2001).
Rsp5 is also implicated in activation of the plasma membrane H + -ATPase Pma1 by glucose (de la Fuente et al., 1997), but the mechanism of this regulation is not known.Pma1 is ubiquitinated, which does not affect its stability, but it does affect the stability of a mutant protein Pma1-7 (Pizzirusso & Chang, 2004).Moreover, the G(653)V substitution in the ATP-binding domain of Pma1 suppresses the temperature sensitivity phenotype of rsp5 mutations (Kamińska et al., 2000).

nuclear functions
The cell nucleus is delimited by a double membrane also called the nuclear envelope.This double membrane contains nuclear pores which are gates allowing active and selective transport of macromolecules such as proteins and RNAs.Numerous highly regulated processes take place in the nuclear compartment, such as transcription, DNA replication, chromosome segregation, etc.In normal growth condition Rsp5 is localized to multiple cytoplasmic complexes (Gajewska et al., 2001;Katzmann et al., 2004).However, many nuclear functions of Rsp5 have been discovered, which implies that Rsp5 may be a shuttling protein.
Rsp5 affects transcription by regulation of the large subunit of RNA polymerase II (Rpb1 of RNAPII), which is ubiquitinated and targeted for degradation in 26S proteasome in stress conditions.This regulation is mediated by interaction of Rsp5 domains WW2 and WW3 with the C-terminus of Rpb1, CTD (C-terminal domain).CTD is composed of the sequence YSPTSPS repeated 26 times and a core including the PXY motif, which is essential for the interaction with the WW domains.Mutation in the Rsp5 WW2 domain abolishes its interaction with Rpb1 in vitro (Wang et al., 1999;Beaudenon et al., 1999).It has been shown that phosphorylation of serine, threonine and tyrosine residues within CTD inhibits its interaction with Rsp5.Dephosphorylation of this domain could be a primary signal targeting Rpb1 to proteasomal degradation (Chang et al., 2000).Def1, an RNAPII degradation factor, is required for the recruiting of Rsp5 to effect RNAPII ubiquitination and subsequent degradation (Reid & Svejstrup, 2004).There are two ubiquitination sites in the yeast Rbp1 and they both play an important role in the elongation step of transcription and the DNA-damage response (Somesh et al., 2007).
Nuclear accumulation of poly(A) + RNA was observed in a temperature sensitive rsp5-1 mutant strain (mutation in the Hect domain) at the non-permissive temperature (37°C) (Rodriguez et al., 2003).Then, Rsp5-dependent regulation of the nuclear export factor Hpr1 was discovered (Gwizdek et al., 2005).Hpr1 is a member of the THO/TREX (transcription/export) complex which has been implicated in transcription elongation, transcription-dependent recombination, and mRNA export (Zenklusen et al., 2002;Strasser et al., 2002).The THO complex component Hpr1 is ubiquitinated and degraded both in vitro and in vivo by Rsp5 in conjunction with the E1 and Ubc4p as an E2 (Gwizdek et al., 2005).
Recent data indicate that Rsp5 can affect tRNA localization.Neuman and coworkers (2003) noticed nuclear accumulation of immature tRNA in the rsp5-3 mutant which contains three mutations of which one lies in the catalytic Hect domain of Rsp5.The rsp5-3 mutant not only shows strong nuclear accumulation of tRNAs at the restrictive temperature, but also is severely impaired in the nuclear export of mRNAs and 60S pre-ribosomal subunits.Strikingly, the nuclear RNA export defects seen in the rsp5-3 strain are accompanied by a dramatic inhibition of both rRNA and tRNA processing.Thus, the ubiquitin ligase Rsp5 plays a role in controlling the major nuclear RNA biogenesis/export pathways in yeast.Other authors showed that the rsp5-19 mutation (P418L substitution in WW3 domain) alters cell sensitivity to antibiotics that affect translation and that rsp5-19 also increases the fidelity of translation (Kwapisz et al., 2005).Nuclear accumulation of tRNA in this mutant was also observed.Moreover, an additional copy of TEF2 gene encoding elongation factor eEF1A which delivers tRNAs to the ribosome, suppressed the rsp5-19 growth defects, translational phenotypes and nuclear accumulation of tRNA.This suggests that nuclear tRNA accumulation may be the primary reason for the altered translational decoding accuracy of rsp5-19 mutant cells (Kwapisz et al., 2005).
The Rsp5 ligase together with APC (ang.anaphase promoting complex), a ligase from the RING family, are both required for chromatin condensation (Altheim & Schultz, 1999;Harkness et al., 2002).Moreover, it has been shown recently that Rsp5 and Apc5, a subunit of the APC, interact genetically and that Rsp5 acts upstream of Apc5 (Arnason et al., 2005).Those authors also identified an E2 enzyme, Ubc7, implicated in chromatin assembly.Further- The role of Rsp5 in yeast more, they demonstrated that Ubc7 physically and genetically interacts with Rsp5, suggesting that Ubc7 acts as an E2 for Rsp5 at least in this process.

Rsp5 functions in response to various stresses
Cells in nature are exposed to various environmental stresses, for example changes in temperature, osmolarity, concentration of nutrients or toxic substances, etc. Stress induces protein denaturation, generates damaged proteins, and leads to growth inhibition or cell death.Two major transcription factors, Hsf1 and Msn2/4, appear to be responsible for stress-induced gene expression (Hashikawa & Sakurai, 2004;Ferguson et al., 2005).Hsf1 binds to heat shock elements (HSEs) and Msn2/4 binds to stress response elements (STREs) found in the promoters of many heat-inducible genes encoding stress proteins.The transcription of genes encoding stress proteins: HSP42 (containing HSE), DDR2 (containing STRE) and HSP12 (containing both HSE and STRE) in the rsp5(A401E) mutant was significantly lower than that in the wild-type strain when exposed to a temperature up-shift or 9% ethanol (Haitani et al., 2006).Moreover, the amounts of transcription factors Hsf1 and Msn4 were remarkably decreased in the rsp5(A401E) mutant in these stress conditions (Haitani et al., 2006) whereas the respective mRNA levels were only slightly lower than those in wildtype cells (Haitani & Takagi, 2008).The mRNAs of HSF1 and MSN2/4 were accumulated in the nucleus of rsp5(A401E) cells after exposure to temperature up-shift or ethanol, suggesting that Rsp5 is required for the nuclear export of these mRNAs.Those results indicated that, in response to environmental stresses, Rsp5 primarily regulates the expression of HSF1 and MSN2/4 at a post-transcriptional level (Haitani & Takagi, 2008).

Regulation of unsaturated fatty acid synthesis
The regulation of enzymes involved in lipid metabolism is an essential process that affects membrane lipid composition and has an impact upon many cell processes, such as cell growth, organelle function and response to stress (Schneiter & Kohlwein, 1997;Carman & Henry, 1999).Therefore, eukaryotes have developed complex mechanisms to regulate lipid biosynthetic pathways.Deregulation of lipid metabolism has been reported in many human diseases, including obesity and atherosclerosis, one of the diseases with the highest morbidity in developed countries (Ntambi, 1999).The ratio of saturated to monounsaturated fatty acids that are incorporated into cell membranes contributes to fluidity of the membrane.In the yeast Saccharomyces cerevisiae, this ratio also affects mitochondrial inheritance (Stewart & Yaffe, 1991) and stress responses (Carratu et al., 1996).The enzyme involved in fatty acid desaturation is the D-9 fatty acid desaturase encoded by the essential OLE1 gene (Stukey et al., 1989).Ole1 protein converts saturated fatty acyl-CoA (palmityland stearyl-) to monounsaturated fatty acid species (palmitoleoyl-and oleoyl-) in an oxygen-dependent manner (Stukey et al., 1989).The regulation of OLE1 expression is physiologically very important since unsaturated fatty acids contribute 70-80% of the fatty acyl groups in membrane lipids.The expression of OLE1 is regulated by nutrient fatty acids and molecular oxygen (Vasconcelles et al., 2001) and other physiological conditions, both at the transcriptional and mRNA stability levels (Gonzalez & Martin, 1996;Choi et al., 1996).Unsaturated fatty acid-dependent repression is mediated by FAR (fatty acid-regulated) elements (Choi et al., 1996) and hypoxic activation is mediated by LORE (low-oxygen response elements) (Vasconcelles et al., 2001).
One essential function of Rsp5 is the regulation of unsaturated fatty acid biosynthesis; the rsp5∆ strain is inviable unless the medium is supplemented with oleic acid (Hoppe et al., 2000).Rsp5 controls the activation of two homologous ER-localized transcriptional activators, Spt23 and Mga2 (Hoppe et al., 2000;Shcherbik et al., 2003;2004), which play a role in the up-regulation of expression of OLE1 gene (Zhang et al., 1999).They are both functionally redundant since neither of the two genes, SPT23 and MGA2, is essential for viability, whereas the double spt23∆ mga2∆ mutation is lethal (Zhang et al., 1999).This lethality is suppressed by the presence of oleic acid in the growth medium.Spt23 and Mga2 are produced as p120 precursors which are anchored as homodimers in the membrane of the ER via their C-terminal transmembrane domains (Hoppe et al., 2000;Shcherbik et al., 2003).When unsaturated fatty acids become limiting, the Spt23 and Mga2 precursors are ubiquitinated, and one of the dimer subunits is processed into a mature p90 form.Subsequently, with the assistance of the chaperone complex Cdc49/ Ufd1/Npl4 and the ubiquitin-proteasome pathway, p90 is released from the membrane-bound p120 subunit and transported into the nucleus where it functions as a transcriptional activator of OLE1 (Shcherbik et al., 2003).Mga2 is also essential for the hypoxic induction of OLE1 expression and is a component of the LORE-bound complex (Nakagawa et al., 2002).The WW2 or WW3 domain of Rsp5 binds Spt23 and Mga2 via the LPKY motif and ubiquitination takes place enabling release of these processed proteins from the ER (Shcherbik et al., 2003;Bhattacharya et al., 2008).In the rsp5-19 temperature sensitive mutant saturated fatty acid accumulation contributes to cell lethality at elevated temperatures (Kaliszewski et al., 2006).

P. Kaliszewski and T. Żołądek
A recent study has revealed several classes of genes, including those for ribosomal proteins, mating-type and lipid metabolism genes, with which activated Spt23 and Mga2 associate (Auld et al., 2006).Most of the lipid metabolism genes also bound proteasome components suggesting that the ubiquitinproteasome pathway might have a role in regulation of these genes downstream of Spt23 and Mga2 activation.The list of lipid metabolism genes bound by Spt23 and Mga2 includes the OLE1 gene, other fatty acid biosynthesis genes (SUR4, FAS1, ELO1, FAA4, ACC1, FAA3, OAR1) and genes encoding enzymes of the mevalonate pathway (see below).This suggests that Rsp5 is not only involved in the regulation of unsaturated fatty acid content, but due to Spt23 and Mga2 regulation it is important for the maintenance of lipid homeostasis.
Rsp5 regulates the synthesis of unsaturated fatty acids (see previous paragraph) which are built into phospholipids and therefore it also affects the phospholipids' composition.It has been shown that rsp5-3 cells grown at the restrictive temperature exhibit significantly reduced levels of di-unsaturated PE species (Neumann et al., 2003).Moreover, accu- The role of Rsp5 in yeast mulation of saturated fatty acids in particular phospholipids can be one of the reasons of the decreased viability of the rsp5-19 mutant cells at the restrictive temperature.Interestingly, this growth defect can be suppressed by overexpression of the PIS1 gene encoding an enzyme involved in PI synthesis (Fig. 2) (Kaliszewski et al., 2006).The PIS1 gene appeared to be a nonspecific suppressor since it suppressed also growth defects of other rsp5 mutants at the restrictive temperature, suggesting that the suppression mechanism was not connected with a particular rsp5 mutation.It was demonstrated that enhanced phosphatidylinositol synthesis was important for the suppression because expression of PIS1 was higher in the rsp5-19 mutant than in the wild-type, whereas the introduction of PIS1 on a multicopy plasmid resulted in a further increase of the Pis1 level in both backgrounds and the catalytic activity of Pis1 was essential for the suppression (Kaliszewski et al., 2006).Moreover, the synthesis and utilization of inositol (a substrate of Pis1) was increased, since the expression of INO1 (inositol synthase, see Fig. 2) was elevated in the rsp5-19 mutant, and inositol added to the medium improved growth of rsp5 mutants at the restrictive temperature.Finally, it was shown that overexpression of PIS1 did not correct the cellular unsaturated fatty acid content in rsp5-19; however, the rsp5-19 mutation induced saturated fatty acid accumulation in PE, a phenomenon that could be fully suppressed by overexpression of PIS1 due to rerouting of saturated acyl chains towards PI (Kaliszewski et al., 2006).This suggests that the primary reason of rsp5 mutant lethality at the restrictive temperature can be accumulation of saturated fatty acids in PE, the phospholipid which normally is the most unsaturated one (Ferreira et al., 2004).
Triacylglycerols (TAG) serve as a storage of energy and of fatty acids required for the synthesis of membrane lipids in cells.TAG cannot integrate into a phospholipid bilayer membrane, so they are deposited in lipid particles.TAG synthesis in yeast is mainly catalyzed by two enzymes: Dga1, DAGacyltransferase which catalyzes acyl-CoA-dependent acylation of DAG, and Lro1, which is a phospholipid: DAG acyltransferase (Oelkers et al., 2002).Lro1p converts DAG to TAG in an acyl-CoA-independent reaction and uses glycerophospholipids, preferentially PC and PE, as the acyl source (Dahlqvis et al., 2000;Oelkers et al., 2000).These two enzymes play different roles in the cell.Lro1 is mainly responsible for TAG synthesis during logarithmic phase of growth, whereas Dga1 is more active in the stationary phase of growth.Another difference between them is their subcellular distribution.Dga1 is located in the ER and lipid particles, the storage compartment for neutral lipids, whereas Lro1 seems to be located only in the ER (Sorger & Daum, 2002;Sorg-er & Daum, 2003).Lro1 and Dga1 are not the only TAG-synthesizing enzymes in yeast cells because when both LRO1 and DGA1 genes are disrupted the cells retain approximately 5% of the DAG esterification activity as compared to wild type (Sorger & Daum, 2002;Oelkers et al., 2002).For that activity Are1 and Are2 sterol acyl transferases are responsible (see Fig. 3) which mainly use activated fatty acids to synthesize steryl esters (SE), another form of lipids stored in lipid particles (Sandager et al., 2002;Sorger et al., 2004).TAG in yeast cells mainly contain unsaturated fatty acids and they cannot accommodate too much saturated ones (Ferreira et al., 2004).In agreement with this finding it has been shown that TAG amount is decreased in the rsp5-19 mutant which shows elevated levels of saturated fatty acids (Kaliszewski et al., 2006;2008).Overproduction of Spt23 or Mga2 devoid of transmembrane domain and constitutively active (see previous paragraph) enhanced TAG synthesis in the wild type and the rsp5-19 mutant and led to an accumulation of unsaturated fatty acids stored within TAG.Those results indicate that Rsp5 via Spt23 and Mga2 affects not only the unsaturation ratio but also the TAG level.The overproduction of Spt23 or Mga2 was also accompanied by the appearance of large lipid particles in the wild type and rsp5-19 strains, probably as a result of enhanced TAG synthesis (Kaliszewski et al., 2008).

Regulation of mevalonate pathway
The mevalonate (MVA) pathway supplies the cell with sterols and isoprenoid precursors which are used to produce dolichols, prenylated proteins, ubiquinone and heme.The first step in the pathway is the synthesis of acetoacetyl-CoA by the Erg10 enzyme -acetoacetyl-CoA thiolase (see a review by Kornblatt & Rudney, 1971;Daum et al., 1998).Then acetoacetyl-CoA is converted to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) in a multistep reaction catalyzed by six enzymes: Erg13 -hydroxymethylglutaryl-CoA (HMGCoA) synthase, Hmg1/Hmg2 HMGCoA reductases, Erg12 MVA kinase, Erg8 phosphomevalonate kinase, Erg19 MVA pyrophosphate decarboxylase, and Idi1 isopentenyl diphosphate isomerase (see a review by Daum et al., 1998) (Fig. 3).The branch point enzyme of the isoprenoid pathway is farnesyl diphosphate synthase (Erg20) which catalyses the sequential condensation of DMAPP with IPP to form geranyl diphosphate (GPP) and further farnesyl diphosphate (FPP) (Song & Poulter, 1994).DMAPP used by Erg20 is also a substrate for Mod5, a tRNA isopentenyltransferase (Dihanich et al., 1987), an enzyme which is dually localized in the cytoplasm and mitochondria.It has been shown that rsp5 mutant cells exhibit a P. Kaliszewski and T. Żołądek decreased mitochondrial pool of Mod5 as compared to wild type (Żołądek et al., 1995).
Sterol depletion in mammalian cells causes activation of the transcription factors known as sterol regulatory element (SRE)-binding proteins (SREBPs) The role of Rsp5 in yeast (reviewed in Edwards et al., 2000).When sterols are abundant the SREBPs are inactive and tethered to the endoplasmic reticulum membrane by their transmembrane domains.When sterol level drops, regulated proteolysis releases the transcrpitional activation domain of SREBPs allowing its nuclear transport.SREBPs activate transcription of genes involved in sterol and fatty acid synthesis.The human gene encoding FPP synthase contains a SRE sequence (Sato et al., 1996).However, less is known about this regulatory mechanism in yeast.Many genes of the mevalonate pathway are transcriptionaly regulated in response to erg mutations, inhibitors of MVA pathway, and anaerobiosis, as determined by genome-wide expression profile analyses (Dimster-Denk et al., 1999;Bammert & Fostel, 2000;Kwast et al., 2002;Agarwal et al., 2003).On the other hand, ERG20 is constitutively expressed after inhibition of the downstream part of the ergosterol synthesis pathway by azoles (Bammert & Fostel, 2000;Agarwal et al., 2003) and in anaerobiosis (Kwast et al., 2002), and only 2-3-fold upregulated by lovastatin, an inhibitor of HMG-CoA reductase in the upstream part of the pathway (Dimster-Denk et al., 1999).An about two-fold increase of ERG20 expression was also observed in an RNA polymerase III regulatory mutant that shows enhanced tRNA synthesis (Kamińska et al., 2002).Many ERG genes (ERG1, ERG2, ERG3, ERG7, ERG25, ERG26, and ERG27) are activated by the Upc2 and Ecm22 transcription factors which bind yeast SRE (Vik & Rine, 2001), and are repressed by Mot3 and Rox1 (Kwast et al., 2002;Henry et al., 2002).One of the enzymes of the isoprenoid pathway, HMG-CoA reductase (Hmg2), is physiologically regulated by ubiquitination and degradation in proteasomes (Gardner et al., 2001).It was observed that in the rsp5-19 mutant the level of sterols was lower compared to wild type and the steady state level of ERG20 transcript was diminished, but this latter effect appeared to be Spt23-independent (Kamińska et al., 2005).It was demonstrated that the rsp5-19 strain had a decreased level of ergosterol and its intermediates downstream from lanosterol in the pathway (Kaliszewski et al., 2008), which implies that Rsp5 may affect the level of FPP.
It has been shown that activated Spt23 and Mga2 bind to genes involved in the ergosterol biosynthetic pathway: ERG1, ERG3, ERG5, and ERG26 (Auld et al., 2006).Moreover, Mga2 binds to the ERG19 gene encoding an enzyme which acts upstream of Erg20 in the ergosterol pathway (see Fig. 3).Recently it was observed that overproduction of Spt23 and Mga2 transcriptional activators increased the level of sterols in the wild type and, to a lower extent, in the rsp5-19 mutant strain (Kaliszewski et al., 2008), which led to the conclusion that Rsp5 regulates sterol synthesis via activation of Spt23 and Mga2 and via other post-activation step(s) (Kaliszewski et al., 2008).
Ubiquinone, another product of the MVA pathway, is present in all cells and membranes and in addition to being a component of the mitochondrial respiratory chain it has other functions as well: it participates in extra-mitochondrial electron transport, is the only lipid-soluble antioxidant, a regulator of the physicochemical properties of membranes, etc. (see a review by Turunen et al., 2004).Ubiquinone is composed of a benzoquinone moiety and an isoprenoid side chain.The number (n) of isoprene units in the polyprenyl tail (Qn) is species specific, in humans it is 10 and in S. cerevisiae 6.In the yeast the isoprenoid chain is formed by Coq1, a transprenyltransferase (Ashby & Edwards, 1990) which catalyzes the condensation of FPP with three IPPs, all in the trans configuration.The isoprenoid chain is then transfered to the benzoquinone precursor 4hydroxybenzoic acid by Coq2 (Ashby et al., 1992).The final steps of ubiquinone synthesis are subsequent ring modifications by the Coq3, Coq5, Coq6, and Coq7/Clk-1 enzymes (see a review by Turunen et al., 2004).
Dolichol is a long-chain polyprenol with a saturated α-isoprene unit, and its phosphorylated form (dolichyl phosphate, Dol-P) participate in the synthesis of N-or O-glycosidically linked oligosaccharide chains of glycoproteins and in the formation of glycosylphosphatidylinositol (GPI) membrane anchors (Herscovics & Orlean, 1993; reviewed by Grabinska & Palamarczyk, 2002).Dolichol synthesis is catalyzed by a cis-prenyltransferase enzyme which catalyses successive condensations of IPP with FPP in the cis configuration to form long-chain polyprenyl diphosphate which is further converted to dolichol by dephosphorylation and saturation of the αisoprene unit (Chojnacki & Dallner, 1988;Sagami et al., 1993;1996).There are two cis-prenyltransferases, Rer2 and Srt1, in yeast (Sato et al., 2001).The polyprenol product of Srt1 is longer (19-24 isoprene units) than that of Rer2 (14-18 isoprene units) (Sato et al., 2001).The expression of these two cis-prenyltransferases is differently regulated during the yeast life cycle.The cellular level of Srt1 is maximal in the late-logarithmic and stationary phases, while the level of Rer2 is the highest in the early logarithmic phase (Sato et al., 2001).
It was shown that in the rsp5-19 mutant strain, in addition to a decreased sterol level, the level of dolichols and ubiquinone was also decreased (Kaliszewski et al., 2008).This suggests that the synthesis of FPP (the common substrate for these products, see Fig. 3) could be diminished by the rsp5-19 mutation.Moreover, the Spt23 and Mga2 transcriptional activators appeared to play a role in the regulation of dolichol synthesis.The level of dolichols P. Kaliszewski and T. Żołądek was decreased in rsp5-19 and wild type strain overproducing Spt23 or Mga2, which could be an effect of enhanced utilization of FPP in sterol synthesis (Kaliszewski et al., 2008).Moreover, the Spt23 and Mga2 factors affected the synthesis of long chain dolichols, products of the Srt1 cis-prenyl transferase (Kaliszewski et al., 2008).Spt23 or Mga2 overproduction resulted in lowering of the pool of long-chain dolichols from 30% to 3-5% of total polyprenols.Similarly, the rsp5-19 strain transformed with plasmids encoding Spt23 or Mga2 exhibited no Srt1 activity (Kaliszewski et al., 2008).These results indicate that Rsp5 together with Spt23 and Mga2 have broad physiological effects on lipid homeostasis.

ConCLUSionS
Ubiqutination is an extensively studied process in yeast and mammalian cells.It has been shown that deregulation of this pathway is implicated in pathogenesis of many diseases, including neurodegenerative diseases and cancer.Therefore, discovering new functions of highly homologous ligases, such as Rsp5 in yeast, provides useful information which can be easily utilized in the deciphering of similar processes in higher eukaryotes.Moreover, the knowledge of lipid synthesis regulation can be used to construct yeast strains with high lipid content which can be useful in biotechnology.

Figure 2 .
Figure 2. biosynthesis of phospholipids and triacylglycerol.Description in the text.

Figure 3 .
Figure 3.The mevalonate pathway.Underlined enzymes are encoded by genes which are bound by Spt23 and Mga2.