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J. Biol. Chem., Vol. 283, Issue 29, 19912-19921, July 18, 2008

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Activation of the Slx5–Slx8 Ubiquitin Ligase by Poly-small Ubiquitin-like Modifier Conjugates^*

From the Department of Molecular Biology and Biochemistry, Rutgers University, Piscataway, New Jersey 08854

Protein sumoylation is a regulated process that is important for the health of human and yeast cells. In budding yeast, a subset of sumoylated proteins is targeted for ubiquitination by a conserved heterodimeric ubiquitin (Ub) ligase, Slx5–Slx8, which is needed to suppress the accumulation of high molecular weight small ubiquitin-like modifier (SUMO) conjugates. Structure-function analysis indicates that the Slx5–Slx8 complex contains multiple SUMO-binding domains that are collectively required for in vivo function. To determine the specificity of Slx5–Slx8, we assayed its Ub ligase activity using sumoylated Siz2 as an in vitro substrate. In contrast to unsumoylated or multisumoylated Siz2, substrates containing poly-SUMO conjugates were efficiently ubiquitinated by Slx5–Slx8. Although Siz2 itself was ubiquitinated, the bulk of the Ub was conjugated to SUMO residues. Slx5–Slx8 primarily mono-ubiquitinated the N-terminal SUMO moiety of the chain. These data indicate that the Slx5–Slx8 Ub ligase is stimulated by poly-SUMO conjugates and that it can ubiquitinate a poly-SUMO chain.

Received for publication, April 8, 2008 , and in revised form, May 21, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant GM071268. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2.

¹ To whom correspondence should be addressed: 679 Hoes Lane, Center For Advanced Biotechnology and Medicine, Piscataway, NJ 08854. Tel.: 732-235-4197; Fax: 732-235-4880; E-mail: brill{at}mbcl.rutgers.edu.

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