The extensive flow of proteins through the Golgi, the protein sorting hub in the cells presents challenges in the retention of Golgi-resident transmembrane proteins whose correct localizations in the Golgi are critical for their functions such as post-translational modifications. Diverse retention mechanisms adopted by cells have been proposed in the past decades and they have demonstrated the role of transmembrane domains and sorting motifs in the retention. However, current theories are not sufficient to explain the localization of all Golgi transmembrane proteins because of the diversity. Recent works from my lab identified a novel retention mechanism for Golgi-resident glycosyltransferases mediated by the Bre5p/Ubp3p de-ubiquitinase in Saccharomyces cerevisiae and I conducted a syst...[ Read more ]

The extensive flow of proteins through the Golgi, the protein sorting hub in the cells presents challenges in the retention of Golgi-resident transmembrane proteins whose correct localizations in the Golgi are critical for their functions such as post-translational modifications. Diverse retention mechanisms adopted by cells have been proposed in the past decades and they have demonstrated the role of transmembrane domains and sorting motifs in the retention. However, current theories are not sufficient to explain the localization of all Golgi transmembrane proteins because of the diversity. Recent works from my lab identified a novel retention mechanism for Golgi-resident glycosyltransferases mediated by the Bre5p/Ubp3p de-ubiquitinase in Saccharomyces cerevisiae and I conducted a systematic study in which I identified a subset of type II transmembrane proteins that require Bre5p/Ubp3p for their Golgi retention. Furthermore, I found that Bre5p/Ubp3p-dependent clients can interact with COPI coatomer via their N-terminal cytoplasmic domains. To shed light on the molecular details of this retention mechanism, I mainly took biochemical approaches to show the interaction between Bre5p and COPI coatomer and to specify that Bre5p/Ubp3p-dependent clients bind to the same binding sites on α-COP (Ret1p) and β’-COP (Sec27p) as the well-studied COPI binding motif di-lysine motif. My results suggest that ubiquitination on Ret1p and Sec27p may block the binding between Golgi transmembrane proteins and COPI coatomer, thus the de-ubiquitination mediated by Bre5p/Ubp3p is required for the normal retrograde transport. My study reveals a new mechanism for the retention of Golgi transmembrane proteins and shows a regulatory role of ubiquitination on the function of COPI coatomer.