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The role of palmitoylation in endoplasmic reticulum transport and quality control of the yeast polytopic protein Chs3 Lam, Karen Kar Yan


All secretory proteins must pass the strict quality control (QC) imposed by the endoplasmic reticulum (ER), where they are first synthesized. Various chaperones, degradation machinery, and vesicular transport factors act together to ensure only properly folded proteins can leave this compartment for subsequent sorting. Failure in QC contributes to misfolding, intracellular retention, and frequently degradation, all of which are known to cause disease. QC is particularly crucial for polytopic proteins, which often represent plasma membrane transporters and channels important for cell function. Recent work suggests ER surveillance systems for polytopic proteins are specialized towards substrates and specific misfolding defects. The underlying mechanisms, especially the roles of post-translational modifications, are poorly understood, thus necessitating examination of various model proteins. Here, the yeast chitin synthase Chs3 was used as a paradigm for polytopic protein trafficking. By high-throughput analysis of the yeast deletion collection, a novel Chs3 ER transport factor was identified. This protein, Pfa4, contains a conserved DHHC-domain, signifying its putative function as a protein acyltransferase. These enzymes of protein palmitoylation were only recently discovered, and few substrates are known. The work described here showed that Chs3 was palmitoylated by Pfa4, and this modification was required for ER export. Both palmitoylation and association with the chaperone Chs7 were necessary for preventing Chs3 aggregation at the ER, indicating that palmitoylation is required for Chs3 to attain an export-competent conformation. Retention of misfolded Chs3 appeared independent of known ER-associated degradation machinery. Instead, a high-throughput search identified the Ubp3 deubiquitination enzyme as a retention factor; deletion of UBP3 restored ER export of unpalmitoylated Chs3 through palmitoylation-independent means. Ubp3-mediated deubiquitination may be regulating the levels of proteins involved in both Chs3 folding and Golgi-to-ER retrieval of misfolded Chs3. The role of palmitoylation in folding at the ER is not well known, and many substrate-specific retention pathways for polytopic proteins have not been identified. These findings suggest palmitoylation can contribute to ERQC of polytopic proteins, and point to potentially novel QC factors that are regulated by deubiquitination. A better understanding of these fundamental molecular mechanisms could contribute to discovery of therapeutic targets for ER misfolding diseases.

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