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Schmid, Maria (2008): Coordination of endoplasmic reticulum and mRNA localization in Saccharomyces cerevisiae. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

mRNA localization is a widespread mechanism in most eukaryotic cells to spatially restrict protein synthesis. During cell propagation of the budding yeast S. cerevisiae, mRNA localization is the basis for an asymmetric, stem-cell like division process. At least 24 transcripts are known to be localized to the yeast bud tip and the common core machinery mediating this mRNA translocation pathway consists of three components: the type V motor protein Myo4p, its adaptor She3p and the mRNA binding protein She2p. Recently, Myo4p and She3p were identified as essential factors of another bud-directed transport process, the inheritance of cortical endoplasmic reticulum (ER). In addition, live cell imaging revealed a co-localization and co-migration of localizing mRNPs with ER tubules that move to the yeast bud, implying for the first time that there might be a common transport of mRNAs and ER membranes. Within the scope of this study it was demonstrated that mutants defective in ER segregation are simultaneously impaired in mRNA localization indicating that there is indeed a connection between the two processes. Additionally, the RNA binding protein She2p associates with ER membranes in different sucellular fractionation assays and it does so independently of polysomes (thus, ongoing translation), mRNA and the Myo4p/She3p complex. During in vitro binding assays, recombinant She2p binds to flotation purified ER microsomes and protease treated membranes suggesting that She2p is not tethered to ER via a protein factor. Finally, She2p was found to have an inherent membrane binding activity since it directly associated with synthetic lipid vesicles in flotation assays. She2p attaches to liposomes also in presence of its RNA ligand excluding the possibility of an unspecific binding via its basic mRNA binding moiety. In summary, these data indicate that mRNA trafficking and ER inheritance are coordinated processes in S. cerevisiae and that She2p is the factor that tethers localizing mRNPs to the ER membranes. Consistent with this observation there is a growing number of examples from higher eukaryotes for a connection between membrane and mRNA transport. This in turn suggests that it is not a yeast specific phenomenon but rather might be a common theme throughout all kinds of eukaryotic species.