The mechanisms employed in the retention of Golgi resident membrane proteins are diverse and include features such as the composition and length of the protein’s transmembrane domain and motifs that mediate direct or indirect associations with COPI-coatomer. However, in sum the current compendium of mechanisms cannot account for the localization of all Golgi membrane proteins, and this is particularly the case for proteins such as the glycosyltransferases. Here I describe a novel mechanism that mediates the steady-state retention of a subset of glycosyltransferases in the Golgi of budding yeast cells. This mechanism is mediated by a de-ubiquitination complex comprised of Bre5p and Ubp3p. I show that in the absence of Bre5p/Ubp3p mediated de-ubiquitination activity, certain glyco...[ Read more ]

The mechanisms employed in the retention of Golgi resident membrane proteins are diverse and include features such as the composition and length of the protein’s transmembrane domain and motifs that mediate direct or indirect associations with COPI-coatomer. However, in sum the current compendium of mechanisms cannot account for the localization of all Golgi membrane proteins, and this is particularly the case for proteins such as the glycosyltransferases. Here I describe a novel mechanism that mediates the steady-state retention of a subset of glycosyltransferases in the Golgi of budding yeast cells. This mechanism is mediated by a de-ubiquitination complex comprised of Bre5p and Ubp3p. I show that in the absence of Bre5p/Ubp3p mediated de-ubiquitination activity, certain glycosyltransferases are mislocalized to the vacuole where they are degraded. I also show that Bre5p/Ubp3p clients bind to COPI-coatomer via a series of positively charged amino acids in their cytoplasmically exposed N-termini. Furthermore, I identify Rsp5p as the corresponding E3 ligase that shows a requirement for the retention of Bre5p/Ubp3p clients and is recruited by PXY motif on Sec27p. I also identify potential ubiquitination sites on Sec27p and Ret1p that are required for the retention of Bre5p/Ubp3p clients. Moreover, ubiquitination of these ubiquitination sites together with competition between cytoplasmic tails of Golgi resident glycosyltransferases and proteins bearing a C-terminal di-lysine motif are key for the retention of glycosyltransferases in the Golgi. My study expands the repertoire of mechanisms mediating the retention of Golgi membrane proteins and provides mechanistic insights about how differential sorting could be achieved through reversible post-translational modifications.