Of social molecules: The interactive assembly of ASH1 mRNA-transport complexes in yeast

Abstract

Asymmetric, motor-protein dependent transport of mRNAs and subsequent localized translation is an important mechanism of gene regulation. Due to the high complexity of such motile particles, our mechanistic understanding of mRNA localization is limited. Over the last two decades, ASH1 mRNA localization in budding yeast has served as comparably simple and accessible model system. Recent advances have helped to draw an increasingly clear picture on the molecular mechanisms governing ASH1 mRNA localization from its co-transcriptional birth to its delivery at the site of destination. These new insights help to better understand the requirement of initial nuclear mRNPs, the molecular basis of specific mRNA-cargo recognition via cis-acting RNA elements, the different stages of RNP biogenesis and reorganization, as well as activation of the motile activity upon cargo binding. We discuss these aspects in context of published findings from other model organisms.

Keywords: ASH1 mRNA; Loc1p; Myo4p; RNA-binding protein; She2p; She3p; endoplasmic reticulum,; mRNA localization; mRNP; myosin.

Figure 1.

Schematic drawings of ASH1 mRNA localization in budding yeast. (A) Overview cartoon showing the major components of the transport complex and their sites of action. Left side indicates the mother cell, right side the daughter cell. (B) Directionality of ASH1 mRNP assembly is warranted by a gradient of affinity and specificity from the initial assembly of the co-transcriptional mRNP to the mature cytoplasmic transport complex. (C) Schematic representation of complex reorganization at the nuclear pore and at the bud tip. (D) Cartoon showing the stoichiometries and number of molecules assembling into a motile transport complex. (E) Scheme of the ER association of the ASH1 mRNP and its sub-complexes. For a more detailed assessment of ER inheritance, please see the concomitant review by Singer-Krüger and Jansen.

Figure 2.

Secondary structures of zip-code elements (A) Selection of known zip-code elements bound by the She2p-She3p complex in yeast, as predicted by the mfold web server. Cartoon on top shows that in the ASH1 mRNA, the zip-code elements E1, E2A, and E2B are part of the open reading frame, whereas E3 is located directly 3′ to the stop codon. Also the EAR1 mRNA contains an experimentally confirmed zip-code element. In the secondary structure predictions, boxed base triplets and a single encircled cytosine mark a previously identified consensus recognition motif. Note that the EAR1 zip-code lacks this motif, that the motif can occur in inversed 3′ to 5′orientation, and that the base triplet is also found in permutations of its sequence. Also the location of the bipartite motif with regard to the secondary structure varies, indicating that the basic features of specific zip-code recognition are still not well understood. (B) OES secondary structure from chemical probing data. (C) Secondary structure (left) and three-dimensional NMR structure (right) of fs(1)K10 TLS (PDB-identifier: 2KE6). Asterisk shows widened major grove that is required for dynein-dependent localization.