Insights into the mechanisms of endoplasmic reticulum proliferation in Saccharomyces cerevisiae

• Main Author: Block-Alper, Laura M
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2001
• Subjects: Cellular biology; Molecular biology
• ISBN: 9780493595641; 0493595643

The endoplasmic reticulum (ER) is a dynamic organelle necessary for protein secretion and the majority of lipid biosynthesis in eukaryotes. The yeast Saccharomyces cerevisiae is an organism of limited secretory capacity and little visible ER. However, expression of the mammalian ribosome receptor (p180) causes yeast cells to adopt unique ER membrane morphologies accompanied by an increased secretory capacity. It is the aim of this dissertation to reveal the underlying mechanisms of how p180-expression leads to ER proliferation in yeast. Little is known about the biogenesis and assembly of ER. To identify genes involved in rough and smooth ER biogenesis, microarray analysis was performed using RNA isolated from yeast strains expressing different regions of p180. A catalog of potential candidate genes was generated with unique and overlapping functions. Genes involved in lipid metabolism were differentially expressed in all p180-expressing strains. Many of these genes are activated by the helix-loop-helix transcription factors Ino2p and Ino4p, which are known to function as a heterodimer. Expression of p180 in ino2Δ cells failed to give rise to the proliferated ER membranes seen in wild-type cells, whereas p180 expression in ino4Δ cells gave rise to membranes indistinguishable from wild-type. Here we define a role for Ino2p in the formation of p180-induced membranes. Conditions that cause ER stress, such as the accumulation of unfolded proteins in the lumen, activate a signal transduction cascade called the unfolded protein response (UPR). Induction of the UPR subsequently activates transcription of genes involved in lipid biosynthesis and protein translocation and processing, presumably to expand the ER membrane and its lumenal components. Because p180 expression also leads to increased levels of these transcripts, we asked whether p180-induced ER proliferation occurs via the UPR. Our results show that a key regulator of the UPR is dispensable for p180-induced membrane biogenesis and elevated mRNA levels encoding components of the secretory pathway. Furthermore we demonstrate that certain secretory mRNAs are stabilized on the ER membrane in a p180-dependent fashion. The increased half-lives of these mRNAs may account for increased production of ER-localized proteins, and ultimately up-regulation of the secretory pathway in yeast.