title: SUMO-independent in vivo activity of a SUMO-targeted ubiquitin ligase toward a short-lived transcription factor.
abstract: Many proteins are regulated by ubiquitin-dependent proteolysis. Substrate ubiquitylation can be stimulated by additional post-translational modifications, including small ubiquitin-like modifier (SUMO) conjugation. The recently discovered SUMO-targeted ubiquitin ligases (STUbLs) mediate the latter effect; however, no endogenous substrates of STUbLs that are degraded under normal conditions are known. From a targeted genomic screen, we now identify the yeast STUbL Slx5-Slx8, a heterodimeric RING protein complex, as a key ligase mediating degradation of the MATalpha2 (alpha2) repressor. The ubiquitin-conjugating enzyme Ubc4 was found in the same screen. Surprisingly, mutants with severe defects in SUMO-protein conjugation were not impaired for alpha2 turnover. Unmodified alpha2 also bound to and was ubiquitylated efficiently by Slx5-Slx8. Nevertheless, when we inactivated four SUMO-interacting motifs (SIMs) in Slx5 that together account for its noncovalent SUMO binding, both in vitro Slx5-Slx8-dependent ubiquitylation and in vivo degradation of alpha2 were inhibited. These data identify alpha2 as the first native substrate of the conserved STUbLs, and demonstrate that its STUbL-mediated ubiquitylation does not require SUMO. We suggest that alpha2, and presumably other proteins, have surface features that mimic SUMO, and therefore can directly recruit STUbLs without prior SUMO conjugation.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: Yang
authors.initials: Y
authors.lastname: Xie
authors.forename: Eric M
authors.initials: EM
authors.lastname: Rubenstein
authors.forename: Tanja
authors.initials: T
authors.lastname: Matt
authors.forename: Mark
authors.initials: M
authors.lastname: Hochstrasser
mesh_terms: D000595:Amino Acid Sequence; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D016415:Sequence Alignment; D025841:Small Ubiquitin-Related Modifier Proteins / Q000378:metabolism*; D044763:Ubiquitin-Conjugating Enzymes / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000737:chemistry / Q000378:metabolism*; D054875:Ubiquitination
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D029701:Saccharomyces cerevisiae Proteins; D025841:Small Ubiquitin-Related Modifier Proteins; D044763:Ubiquitin-Conjugating Enzymes; D044767:Ubiquitin-Protein Ligases; C062928:MATA2 protein, S cerevisiae; C501306:Ubc4 protein, S cerevisiae; C515574:Slx8 protein, S cerevisiae; C515573:Slx5 protein, S cerevisiae
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