Ubiquitin-like proteins Rad23 and Dsk2 have recently been shown to be capable of binding both polyubiquitin chains and the 26S proteasome. The ubiquitin-like domains (Ubls) of Rad23 and Dsk2 are indispensable for their interaction with the 26S proteasome, but the proteasome subunits capable of binding the Ubl have not been identified. Here, we report that the Ubls of both Rad23 and Dsk2 can bind with the 19S regulatory particle (RP) of the 26S proteasome in vivo and in vitro. A competition assay using the respective Ubls of Rad23 and Dsk2 revealed that they bind to the RP in a competitive manner. The base subcomplex of the RP was found to have the ability to bind the Ubl. By cross-linking experiments, Rpn1 and Rpn2 were identified as Ubl-binding subunits. Taken together, the results suggest that the Rpn1 and Rpn2 in the base subcomplex form the receptor for the ubiquitin-like protein. (C) 2002 Elsevier Science (USA). All rights reserved.