Ubiquitin is a small (8.5 kDa) protein that is attached singly, or in chains, to lysine residues of other proteins via a complex, three step mechanism. The pattern of ubiquitin addition, referred to as ubiquitination, marks the protein for degradation, alters its function and interactions with other proteins, or modulates its transit via endocytosis. Recent research suggests that abnormalities in the ubiquitin pathway play a role in diseases that involve aberrant accumulation of proteins as is seen in Alzheimer’s, Parkinson’s, and Huntington’s diseases. To better understand the role that ubiquitination may play in such diseases requires a full understanding of the dynamics of the ubiquitin protein itself, leading Vanderbilt Basic Sciences investigators Jason MacGurn and Walter Chazin along with their laboratories to explore mechanisms of ubiquitin turnover. The studies began with the discovery of two phosphatase enzymes that affect the turnover of ion channels in yeast. Phosphoproteomics analysis of yeast mutants lacking these enzymes revealed an increased level of phosphorylation of ubiquitin at Ser-57. Further studies in yeast expressing a Ser57Ala mutant ubiquitin, which cannot be phosphorylated, or a Ser57Asp mutant ubiquitin, which mimics permanent phosphorylation, showed that phosphorylation at Ser-57 increases the turnover rate of the protein. Phosphorylation at Ser-57 also increased the rate of turnover of membrane-associated ion channels through the endocytic pathway. To explain the link between endocytic turnover of membrane proteins and ubiquitin, the investigators showed that phosphorylation of Ser-57 prevents the enzyme Doa4 from removing ubiquitin from proteins in endosomes. Prior research had shown that addition of a single ubiquitin molecule to a membrane protein tags it for uptake and degradation via the endocytic pathway. Removal of the ubiquitin by Doa4 just before protein degradation preserves the ubiquitin for reuse; however, phosphorylation at Ser-57 prevents this removal, leading to destruction of the ubiquitin along with the tagged protein. Follow up studies demonstrated that the endocytic pathway is the primary mechanism of ubiquitin turnover in yeast. These findings provide important new information on ubiquitin dynamics and their relationship to the turnover of other cellular proteins. Further research is needed to determine how this pathway may relate to abnormalities in the ubiquitin pathway in various disease states. The work is published in the journal eLife [S. Lee, et al., (2017) eLife, 6, e29176].
Figure reproduced under the Creative Commons Attribution (CC-BY) from S. Lee et al. eLife, 2017, 6, e29176.