Abstract

In eukaryotic cells, integral membrane protein (IMP) turnover is an important contributor to cell physiology. Whereas the only known mechanism for IMP degradation is the multivesicular body (MVB) pathway, there are examples of IMPs that eschew this pathway and travel directly to the vacuole surface, suggesting that a second mechanism acts at the vacuole. Previously, it was shown that homotypic vacuole fusion yields an intralumenal membrane fragment within the fusion product, providing the cell with a unique opportunity to internalize (and thus, degrade) a portion of vacuolar membrane and its protein cargoes. Based on this result, we hypothesized that vacuole IMPs are sorted for internalization during the sub-reactions that lead to vacuole fusion. Using epifluorescence microscopy, I confirmed that Fth1-GFP and Ybt1-GFP localize exclusively to the vacuole membrane, and are sorted into, or out of, respectively, the membrane domain that is internalized upon homotypic vacuole fusion. Additionally, I demonstrated that Mup1, a surface methionine transporter, colonizes the vacuole limiting membrane upon disruption of the MVB pathway, and also becomes internalized during homotypic vacuole fusion. This study provides the first evidence of a novel selective IMP degradation mechanism, which works independently or complementarily to the MVB pathway. Further studies are required to confirm whether vacuole IMPs are sorted based on their ubiquitylation status (as in the MVB pathway), or whether the vacuole fusion protein machinery has a role within this process.