The covalent modification of proteins plays a central role in regulating most cellular processes. A newly discovered protein modification, involving attachment of the small ubiquitin-related protein SUMO, is found in all eukaryotes and has been implicated in the yeast cell cycle and in pathologic states including inflammation and tumorigenesis. The set of known sumoylated proteins is growing rapidly, suggesting that sumoylation may be a widespread mechanism for controlling protein activity. In the background work to this proposal, the first three Saccharomyces cerevisiae SUMO substrates have been identified as members of the septin family of cytoskeletal proteins. SUMO conjugation is tightly regulated; septins are sumoylated exclusively during mitosis, and the overall pattern of yeast SUMO conjugates changes dramatically during the cell cycle. However, it is not known how individual proteins are targeted for sumoylation, how this process is regulated, or what factors are required. The hypothesis guiding this work is that there are specific targeting factors that select individual proteins for sumoylation and that sumoylation is regulated by modulating the interaction between the targeting factor and its substrate. No such targeting factors have yet been identified for any SUMO substrate. The yeast septins are an outstanding model for studying targeting and regulation of sumoylation, as septin sumoylation is intricately regulated. Septins are also the only known yeast SUMO substrates and are by far the most abundant SUMO conjugates in G/M-arrested yeast, making them uniquely amenable to both molecular genetic and biochemical analyses.
The aims of this project are to identify the targeting factors that select septins for sumoylation, to determine how septin sumoylation is regulated, to dissect the domains in the targeting factor and in the septins that confer specificity on their interaction, and to analyze the physiological role of septin sumoylation. An attractive candidate to be the targeting factor has already been identified, which will facilitate many of these studies. The SUMO conjugation pathway is a highly conserved system that is only at the early stages of exploration. The basic components and mechanisms discovered by investigating septin sumoylation are likely to be common features in the sumoylation of all eukaryotic SUMO substrates. Consequently, the fundamental understanding of SUMO conjugation provided by these studies should illuminate the role of SUMO not only in yeast septin function but also in inflammation and cancer.
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