A Novel Mechanism for SUMO System Control: Regulated Ulp1 Nucleolar Sequestration

Abstract

The small ubiquitin-related modifiers (SUMOs) are evolutionarily conserved polypeptides that are covalently conjugated to protein targets to modulate their subcellular localization, half-life, or activity. Steady-state SUMO conjugation levels increase in response to many different types of environmental stresses, but how the SUMO system is regulated in response to these insults is not well understood. Here, we characterize a novel mode of SUMO system control: in response to elevated alcohol levels, the Saccharomyces cerevisiae SUMO protease Ulp1 is disengaged from its usual location at the nuclear pore complex (NPC) and sequestered in the nucleolus. We further show that the Ulp1 region previously demonstrated to interact with the karyopherins Kap95 and Kap60 (amino acids 150 to 340) is necessary and sufficient for nucleolar targeting and that enforced sequestration of Ulp1 in the nucleolus significantly increases steady-state SUMO conjugate levels, even in the absence of alcohol. We have thus characterized a novel mechanism of SUMO system control in which the balance between SUMO-conjugating and -deconjugating activities at the NPC is altered in response to stress via relocalization of a SUMO-deconjugating enzyme.

We are grateful to C. Boone, B. Andrews, R. Wozniak, and M. Tyers for strains and plasmids.

Y.S. was funded by an Amgen/OCI postdoctoral fellowship. T.S. and S.M.J. were funded by Canadian Institutes of Health Research (CIHR) student fellowships, and S.W. was funded by an Ontario Graduate Student fellowship. We thank I. Jurisica for use of the confocal microscope, funded by the Canada Foundation for Innovation and Ontario Ministry of Innovation, grant 12301. A.C.G. holds the Canada Research Chair in Functional Proteomics and the Lea Reichmann Chair in Cancer Proteomics and is supported by CIHR MOP-84314. B.R. holds the Canada Research Chair in Proteomics and Molecular Medicine and was supported by funding from the Canada Foundation for Innovation, the Ontario Ministry of Innovation, the Ontario Ministry of Health and Long Term Care, and CIHR grant MOP-81268.