Functional analysis of the sumoylation pathway in Saccharomyces cerevisae

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Ubiquitin and ubiquitin-like proteins (e.g. SUMO-1) modulate protein function in the cell via reversible post-translational modification. Unlike ubiquitinylation, which targets proteins for proteosome-dependent degradation, sumoylation is thought to modulate the intracellular location, protein-protein interaction and stability of target proteins. However, only a few substrate proteins for SUMO-1 are known, but it is expected that many SUMO-1 carrying targets exist in yeast and metazoans. Thus, the identification of a broad range of SUMO-1 substrate proteins is expected to give insight into the regulatory aspects of sumoylation in a eukaryotic cell. We have performed a proteome-wide analysis of sumoylated substrate proteins in yeast. Employing an efficient affinity-purification of ProtA-Smt3 (Smt3 is the yeast homologue of SUMO-1) from yeast lysates in combination with mass spectrometry, we have isolated 100 potential Smt3-carrying substrate proteins and show for a representative number of them that they indeed carry the SUMO-1 modification. Notably, the list of sumoylated proteins points to diverse functions in all major cellular pathways, metabolism, anabolism, proteolysis, DNA replication and repair, chromatin organization, transcription activation, PolI, PolII and PolIII transcription, 5'-pre-mRNA capping, 3'-pre-mRNA processing, proteasome function, chromatin remodeling, actin and tubulin folding, aminoacylation, translation, secretion, vacuolar function and cellular organization. In overall, our findings suggest that sumoylation is an important mechanism to regulate and coordinate diverse cellular pathways in eukaryotes. In our future research, we aim to gain insight into the functional significance of this post-translational modification to target proteins, which are connected to gene expression.