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Published online 15 November 2004. doi:10.1083/jcb.200406001
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 167, Number 4, 649-659
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Article

 Ubiquitin charging of human class III ubiquitin-conjugating enzymes triggers their nuclear import

Scott M. Plafker1, Kendra S. Plafker1, Allan M. Weissman2, and Ian G. Macara3

1 Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104
2 Laboratory of Protein Dynamics and Signaling, Center for Cancer Research, National Cancer Institute at Frederick, Frederick, MD 21702
3 Center for Cell Signaling and Department of Microbiology, University of Virginia, Charlottesville, VA 22908

Correspondence to Scott M. Plafker: scott-plafker{at}ouhsc.edu

Ubiquitin is a small polypeptide that is conjugated to proteins and commonly serves as a degradation signal. The attachment of ubiquitin (Ub) to a substrate proceeds through a multi-enzyme cascade involving an activating enzyme (E1), a conjugating enzyme (E2), and a protein ligase (E3). We previously demonstrated that a murine E2, UbcM2, is imported into nuclei by the transport receptor importin-11. We now show that the import mechanism for UbcM2 and two other human class III E2s (UbcH6 and UBE2E2) uniquely requires the covalent attachment of Ub to the active site cysteine of these enzymes. This coupling of E2 activation and transport arises from the selective interaction of importin-11 with the Ub-loaded forms of these enzymes. Together, these findings reveal that Ub charging can function as a nuclear import trigger, and identify a novel link between E2 regulation and karyopherin-mediated transport.

Abbreviations used in this paper: ß-ME, ß-mercaptoethanol; CBB, Coomassie brilliant blue; E1, ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; E3, ubiquitin protein ligase; HEK, human embryonic kidney; RRL, rabbit reticulocyte lysate; Ub, ubiquitin; UBC, catalytic core domain; wt, wild type.


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