Review
Why cells move messages: The biological functions of mRNA localization

https://doi.org/10.1016/j.semcdb.2007.01.010Get rights and content

Abstract

RNA localization is a widespread mechanism that allows cells to spatially control protein function by determining their sites of synthesis. In embryos, localized mRNAs are involved in morphogen gradient formation or the asymmetric distribution of cell fate determinants. In somatic cell types, mRNA localization contributes to local assembly of protein complexes or facilitates protein targeting to organelles. Long-distance transport of specific mRNAs in plants allows coordination of developmental processes between different plant organs. In this review, we will discuss the biological significance of different patterns of mRNA localization.

Section snippets

mRNA localization and the formation of protein gradients

Gradients of morphogens pattern the embryo body plan [6]. They can act as regulators of gene expression that switch on different sets of genes dependent on their concentration along a defined embryonic axis. The first molecularly defined morphogen, Bicoid in Drosophila embryos is encoded by a localized mRNA (for a recent review see Ref. [7]). Bicoid mRNA is localized during oogenesis to the anterior tip of the oocyte where it is anchored. After fertilization, it is translated and a gradient of

mRNA localization and asymmetric distribution of cell fate determinants

Asymmetric cell divisions are widespread in the bacterial and eukaryotic kingdoms [16], [17], [18]. Generally this achieves unequal distribution of different cellular components including nucleic acids and proteins. Numerous mRNAs are asymmetrically distributed upon cell division, especially during early embryogenesis. For many, however, the reason for their asymmetric partitioning is not understood (see, e.g., Ref. [19]). Even in cases where the encoded protein functions as a cell fate

mRNA localization and local assembly of protein complexes

In polarized somatic cells, localized mRNAs can serve as template for spatially restricted synthesis of proteins to maintain cellular asymmetry. In some cases, mRNA localization might also help to facilitate the compartmentalized assembly of multifactor complexes. One of the best-characterized examples for this aspect of mRNA localization is β-actin mRNA in motile cells (Fig. 1C). In several motile cell types, β-actin mRNA is targeted to the leading edge of lamellipodia [32]. These flattened

mRNA localization to the vicinity of organelles

mRNA localization can also facilitate protein sorting to organelles. Generally, protein sorting is based on peptide signals within the protein to be sorted. However, increasing evidence suggests that localization of mRNAs encoding proteins destined to organelles like ER, mitochondria or nuclei supports protein targeting or allows refined targeting that cannot occur on the protein level (see also Dahm et al., this issue).

Vimentin, α-actin, and γ-actin mRNAs display perinuclear localization [33],

Long distance transport of mRNAs and small RNAs in plants

Less is known about RNA localization in the plant kingdom compared to animal cells but a role for RNAs as long-distance information molecules is emerging. The framework for such a mechanism is provided by the plant vascular system. In contrast to animals, higher plants have evolved a unique vascular system that consists of xylem and phloem. Xylem cells allow the delivery of water and minerals, whereas phloem, which consists of living but enucleated sieve elements, transports photo assimilates

Outlook

mRNA localization serves as a sophisticated cellular tool to optimize protein expression on a post-transcriptional level. Besides well-understood functions like generating morphogen gradients or sorting determinants, mRNA localization seems to fulfil a variety of additional roles ranging from facilitated assembly of protein complexes to long-distance signalling and coordination of developmental processes. In addition, new functions have emerged like modulating synaptic plasticity in neurons or

Acknowledgements

R.P.J. and T.G.D. are supported by grants from the Deutsche Forschungsgemeinschaft (DFG JA696/4-4 and SFB646 TPA5).

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