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J. Biol. Chem., Vol. 279, Issue 42, 43805-43814, October 15, 2004

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Drosophila Ulp1, a Nuclear Pore-associated SUMO Protease, Prevents Accumulation of Cytoplasmic SUMO Conjugates^*

Matthew Smith, Vinay Bhaskar, Joseph Fernandez¶, and Albert J. Courey||

From the Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569 and the ^¶Proteomics Resource Center, The Rockefeller University, New York, New York 10021

SUMO is a small ubiquitin-like protein that becomes covalently conjugated to a variety of target proteins, the large majority of which are found in the nucleus. Ulp1 is a member of a family of proteases that control SUMO function positively, by catalyzing the proteolytic processing of SUMO to its mature form, and negatively, by catalyzing SUMO deconjugation. In Drosophila S2 cells, depletion of Ulp1 by RNA interference results in a dramatic change in the overall spectrum of SUMO conjugates, indicating that SUMO deconjugation is substrate-specific and plays a critical role in determining the steady state targets of SUMO conjugation. Ulp1 normally serves to prevent the accumulation of SUMO-conjugated forms of a number of proteins, including the aminoacyl-tRNA synthetase EPRS. In the presence of Ulp1, most SUMO conjugates reside in the nucleus. However, in its absence, SUMO-conjugated EPRS accumulates in the cytoplasm, contributing to an overall shift of SUMO from the nucleus to the cytoplasm. The ability of Ulp1 to restrict SUMO conjugates to the nucleus is independent of its role as a SUMO-processing enzyme because Ulp1-dependent nuclear localization of SUMO is even observed when SUMO is expressed in a preprocessed form. Studies of a Ulp1-GFP fusion protein suggest that Ulp1 localizes to the nucleoplasmic face of the nuclear pore complex. We hypothesize that, as a component of the nuclear pore complex, Ulp1 may prevent proteins from leaving the nucleus with SUMO still attached.

Received for publication, May 4, 2004 , and in revised form, August 3, 2004.

^* This work was supported by Grant GM63596 (to A. J. C.) from the National Institutes of Health. 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 Supplementary Figs. A, B, and C.

Current address: Protein Design Labs, Inc., 34801 Campus Dr., Fremont, CA 94555.

^|| To whom correspondence should be addressed: Dept. of Chemistry and Biochemistry, University of California, Los Angeles, P. O. Box 951569, 607 Charles E. Young Dr., E., Los Angeles, CA 90095-1569. Tel.: 310-825-2530; Fax: 310-206-4038; E-mail: courey{at}chem.ucla.edu.

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