Here, there, everywhere. mRNA localization in budding yeast

Abstract

mRNA localization and localized translation is a common mechanism that contributes to cell polarity and cellular asymmetry. In metazoan, mRNA transport participates in embryonic axis determination and neuronal plasticity. Since the mRNA localization process and its molecular machinery are rather complex in higher eukaryotes, the unicellular yeast Saccharomyces cerevisiae has become an attractive model to study mRNA localization. Although the focus has so far been on the mechanism of ASH1 mRNA transport, it has become evident that mRNA localization also assists in protein sorting to organelles, as well as in polarity establishment and maintenance. A diversity of different pathways has been identified that targets mRNA to their destination site, ranging from motor protein-dependent trafficking of translationally silenced mRNAs to co-translational targeting, in which mRNAs hitch-hike to organelles on ribosomes during nascent polypeptide chain elongation. The presence of these diverse pathways in yeast allows a systemic analysis of the contribution of mRNA localization to the physiology of a cell.

Keywords: ASH1; Myo4p; Saccharomyces cerevisiae; She2p; endoplasmic reticulum; mitochondria; polarity.

Figure 1.

(A) Bud-directed mRNA transport occurs simultaneously with ER inheritance. During polarized growth, POL mRNAs targeting and local translation contribute to POL protein deposition within the emerging bud. This mRNP targeting correlates with cER inheritance and depends on SHE1–5, the secretory pathway and a polarized actin cytoskeleton. (B) Shmoo-directed mRNPs use Scp160p and Myo4p for asymmetric targeting. During mating, targeted mRNA transport is mediated by Scp160p, a 14 KH-domain protein that associates with ER and Myo4p. The pheromone-induced RNA-binding of Scp160p is relevant for cell polarization, chemotropism and mating efficiency. Polarized localization of some POL factors, like Cdc42p, likely occurs via secretion. (C) Pre- and co-translational targeting of mRNAs encoding mitochondrial proteins (mMPs). Targeting of mMPs to the outer mitochondrial membrane and the TOM complex likely combines at least two mechanisms: recognition of a signal within the proximal region of the 3′-UTR recognized by the RNA-binding protein Puf3p and active translation of the peptide sequence including the mitochondrial targeting sequence (MTS). (D) Co-translational targeting of ABP140 RNA to the distal pole of the mother cell via actin retrograde flow. Asymmetric ABP140 mRNA localization requires active translation of Abp140p and tethering the nascent chain of Abp140p to actin cables via its N-terminal actin-binding domain (ABD) and a subsequent amino acid stretch.