Abstract
Protein sumoylation plays an important but poorly understood role in controlling genome integrity. In
Saccharomyces cerevisiae
, the Slx5-Slx8 SUMO-targeted Ub ligase appears to be needed to ubiquitinate sumoylated proteins that arise in the absence of the Sgs1 DNA helicase.
WSS1
, a high-copy-number suppressor of a mutant SUMO, was implicated in this pathway because it shares phenotypes with
SLX5-SLX8
mutants, including a
wss1Δ sgs1Δ
synthetic-fitness defect. Here we show that Wss1, a putative metalloprotease, physically binds SUMO and displays
in vitro
isopeptidase activity on poly-SUMO chains. Like that of
SLX5
, overexpression of
WSS1
suppresses
sgs1Δ slx5Δ
lethality and the
ulp1ts
growth defect. Interestingly, although Wss1 is relatively inactive on ubiquitinated substrates and poly-Ub chains, it efficiently deubiquitinates a Ub-SUMO isopeptide conjugate and a Ub-SUMO fusion protein. Wss1 was further implicated in Ub metabolism on the basis of its physical association with proteasomal subunits. The results suggest that Wss1 is a SUMO-dependent isopeptidase that acts on sumoylated substrates as they undergo proteasomal degradation.