Drosophila Ulp1, a nuclear pore-associated SUMO protease, prevents accumulation of cytoplasmic SUMO conjugates.
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.
Department of Chemistry and Biochemistry, University of California, Los Angeles, 90095-1569, USA.
Matthew
M
Smith
Vinay
V
Bhaskar
Joseph
J
Fernandez
Albert J
AJ
Courey
D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D002467:Cell Nucleus / Q000502:physiology; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D003593:Cytoplasm / Q000502:physiology; D004330:Drosophila / Q000502:physiology; D029721:Drosophila Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D022022:Nuclear Pore / Q000502:physiology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D025841:Small Ubiquitin-Related Modifier Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism
D029721:Drosophila Proteins; D011994:Recombinant Proteins; D025841:Small Ubiquitin-Related Modifier Proteins; D025801:Ubiquitin; D003546:Cysteine Endopeptidases; C118083:Ulp1 protease