Rpn10, a subunit of the 26S proteasome, has been proposed to act as a receptor for multiubiquitin chains in ubiquitin-dependent proteolysis. However, studies on RPN10-deleted mutants in yeasts have suggested the presence of other multiubiquitin chain-binding factors functioning in ubiquitin-dependent proteolysis. Here, we report that a mutant with a triple deletion of RAD23, DSK2, and RPN10 genes accumulates large amounts of polyubiquitinated proteins, as is the case with a mutant with RAD23 and DSK2 deletions under restrictive conditions. Dsk2, Rad23, and Rpn10 have different capacities to bind multiubiquitin chains. Another ubiquitin-like protein. Ddi1, has similar activity to those of Rad23 and Dsk2. Taken together, the results suggest that ubiquitin-like proteins. Rad23, Dsk2, possibly Ddi1, and Rpn10 play cooperative roles in ubiquitin-dependent proteolysis, serving as multiubiquitin chain-binding proteins. (C) 2002 Elsevier Science (USA). All rights reserved.