Protein phosphorylation in regulation of photosynthesis

The chloroplast is the site of multiple physiological processes, including photosynthesis, pigment biogenesis, and starch metabolism. One of the most striking features of the chloroplast is its complex membrane organization, with its intricate network of sheet-like thylakoid membranes. Thylakoids have evolved this specific architecture to scaffold photosynthetic complexes and provide an environment for converting the energy of photons into energy-rich chemical compounds. In this chapter, we describe the membrane organization of the Chlamydomonas reinhardtii chloroplast, including the chloroplast envelope, thylakoids, and pyrenoid tubules. We detail our current knowledge of molecular mechanisms that build and maintain thylakoid membranes, enforce thylakoid stacking and lateral heterogeneity, and regulate light-induced membrane remodeling. Throughout this chapter, we provide a historical perspective on the parallel development of Chlamydomonas research and new structural biology techniques, which have combined to shape our understanding of chloroplast molecular architecture and function.

In the photosynthetic electron transfer chain, PSII and PSI are connected in series through the plastoquinone (PQ) pool, the cytochrome b₆f complex and plastocyanin. Optimal electron flow requires a balanced redox poise of these intermediate carriers. State transitions reflect a regulatory feedback loop that adjusts within minutes the redox balance of the PQ pool in response to changes in external conditions and cellular demands for ATP and NADPH. Thus, state transitions are tightly integrated into the regulation of cellular energy metabolism. State transitions involve the dynamic allocation of components of the light-harvesting complex II (LHCII) antenna to PSI or PSII. They are regulated by a protein kinase (STT7) and two counteracting phosphatases (CrPPH1 and CrPBCP) that control LHCII phosphorylation. Specific phosphorylated subunits of LHCII direct its docking to PSI, forming PSI-LHCI-LHCII supercomplexes. The segregation of PSI and PSII in different domains of the thylakoid membranes imposes the need to laterally redistribute LHCII during state transitions.

A very high rate for cyclic electron flow (CEF) around PSI (~180 s⁻1 or 210 s⁻¹ in minimum medium or in the presence of a carbon source respectively) is measured in the presence of methyl viologen (MV) in intact cells of Chlamydomonas reinhardtii under anaerobic conditions. The observation of an efficient CEF in the presence of methyl viologen is in agreement with the previous results reports of Asada et al. in broken chloroplasts (Plant Cell Physiol. 31(4) (1990) 557–564). From the analysis of the P700 and PC absorbance changes, we propose that a confinement between 2 PC molecules, 1 PSI and 1 cytb₆f corresponding to a functional supercomplex is responsible for these high rates of CEF. Supercomplex formation is also observed in the absence of methyl viologen, but with lower maximal CEF rate (about 100 s⁻¹) suggesting that this compound facilitates the mediation of electron transfer from PSI acceptors to the stromal side of cytb₆f. Further analysis of CEF in mutants of Chlamydomonas defective in state transitions shows the requirement of a kinase-driven transition to state 2 to establish this functional supercomplex configuration. However, a movement of the LHCII antennae is not involved in this process. We discuss the possible involvement of auxiliary proteins, among which is a small cytb₆f-associated polypeptide, the PETO protein, which is one of the targets of the STT7 kinase.

The persistent low temperature that characterize polar habitats combined with the requirement for light for all photoautotrophs creates a conundrum. The absorption of too much light at low temperature can cause an energy imbalance that decreases photosynthetic performance that has a negative impact on growth and can affect long-term survival. The goal of this review is to survey the mechanism(s) by which polar photoautotrophs maintain cellular energy balance, that is, photostasis to overcome the potential for cellular energy imbalance in their low temperature environments. Photopsychrophiles are photosynthetic organisms that are obligately adapted to low temperature (0⁰- 15 °C) but usually die at higher temperatures (≥20 °C). In contrast, photopsychrotolerant species can usually tolerate and survive a broad range of temperatures (5⁰- 40 °C). First, we summarize the basic concepts of excess excitation energy, energy balance, photoprotection and photostasis and their importance to survival in polar habitats. Second, we compare the photoprotective mechanisms that underlie photostasis and survival in aquatic cyanobacteria and green algae as well as terrestrial Antarctic and Arctic plants. We show that polar photopsychrophilic and photopsychrotolerant organisms attain energy balance at low temperature either through a regulated reduction in the efficiency of light absorption or through enhanced capacity to consume photosynthetic electrons by the induction of O₂ as an alternative electron acceptor. Finally, we compare the published genomes of three photopsychrophilic and one photopsychrotolerant alga with five mesophilic green algae including the model green alga, Chlamydomonas reinhardtii. We relate our genomic analyses to photoprotective mechanisms that contribute to the potential attainment of photostasis. Finally, we discuss how the observed genomic redundancy in photopsychrophilic genomes may confer energy balance, photoprotection and resilience to their harsh polar environment. Primary production in aquatic, Antarctic and Arctic environments is dependent on diverse algal and cyanobacterial communities. Although mosses and lichens dominate the Antarctic terrestrial landscape, only two extant angiosperms exist in the Antarctic. The identification of a single ‘molecular key’ to unravel adaptation of photopsychrophily and photopsychrotolerance remains elusive. Since these photoautotrophs represent excellent biomarkers to assess the impact of global warming on polar ecosystems, increased study of these polar photoautotrophs remains essential.

We extended, for the first time, the Michaelis-Menten (M-M) model to describe the kinetics of photosystem I (PSI) complexes using light as a substrate. Our work is novel as it can be useful for studying the phenomenon of “state transitions” because it quantifies the affinity of light to PSI reaction centers depending on the associated light harvesting complex II (LHCII) antennas. We verified our models by measuring the PSI activity as a function of light intensity using an oxygen electrode for chloroplast from plants grown in low light conditions and treated with far red light. We determined the kinetics constant K_M for: PSI-LHCI, PSI-LHCI-LHCII and PSI-PSII megacomplexes and have shown that K_M for PSI located in the megacomplexes was smaller in magnitude than PSI-LHCI, thus demonstrating that LHCII antennas are functionally associated with PSI. The parameter [S]_1/2used in our models is the equivalent of M-M constant. Far red light increases [S]_1/2, which indicates that transition from state 1 to state 2 leads to an energy gain while reaching the PSI reaction centers. We also observed that redistribution of the absorbed excitation energy is realized not only by LHCII migration but also by association of the photosystems in the megacomplexes.

Photosystem II (PSII), a light-driven water: plastoquinone oxidoreductase, resides in the thylakoid membrane of prokaryotic cyanobacteria and eukaryotic chloroplasts performing oxygenic photosynthesis. The synthesis and assembly of a eukaryotic PSII require cooperation between the chloroplast and nuclear genomes. Besides structural protein subunits, PSII contains pigments and cofactors that are mainly ligated to its reaction center proteins. Such an arrangement, together with the strongly oxidizing photochemistry, renders the reaction center proteins vulnerable to photodamage and accelerates their turnover. The primary target of photodamage is the D1 protein, which is constantly replaced in a dynamic process known as the PSII photoinhibition repair cycle.