Induction of secretory pathway components in yeast is associated with increased stability of their mRNA.

The overexpression of certain membrane proteins is accompanied by a striking proliferation of intracellular membranes. One of the best characterized inducers of membrane proliferation is the 180-kD mammalian ribosome receptor (p180), whose expression in yeast results in increases in levels of mRNAs encoding proteins that function in the secretory pathway, and an elevation in the cell's ability to secrete proteins. In this study we demonstrate that neither the unfolded protein response nor increased transcription accounts for membrane proliferation or the observed increase in secretory pathway mRNAs. Rather, p180-induced up-regulation of certain secretory pathway transcripts is due to a p180-mediated increase in the longevity of these mRNA species, as determined by measurements of transcriptional activity and specific mRNA turnover. Moreover, we show that the longevity of mRNA in general is substantially promoted through the process of its targeting to the membrane of the endoplasmic reticulum. With respect to the terminal differentiation of secretory tissues, results from this model system provide insights into how the expression of a single protein, p180, could result in substantial morphological and functional changes.