Budding yeast Dsk2 is a family of UbL-UBA proteins that can interact with both polyubiquitin and the proteasome, and is thereby thought to function as a shuttle protein in the ubicluitin-proteasome pathway. Here we show that Dsk2 can homodimerize via its C-terminal UBA domain in the absence of ubiquitin. Dsk2 mutants defective in the UBA domain do not dimerize and do not bind polyubiquitin. The expression of Dsk2 UBA mutants fails to restore the growth defect caused by DSK2 disruption although that of wild-type Dsk2 can restore the defect. These results suggest that Dsk2 homodimerization via the UBA domain plays a role in regulating polyubiquitin binding in the ubiquitin-proteasome pathway. (c) 2005 Elsevier Inc. All rights reserved.