Proteins containing the UBA domain are able to bind to multi-ubiquitin chains

The UBA domain is a motif found in a variety of proteins, some of which are associated with the ubiquitin-proteasome system(1,2). We describe the isol ation of a fission-yeast gene, mud1(+), which encodes a UBA domain containi ng protein that is able to bind multi-ubiquitin chains. We show that the UB A domain is responsible for this activity. Two other proteins containing th is motif, the fission-yeast homologues of Rad23 and Dsk2, are also shown to bind multi-ubiquitin chains via their UBA domains. These two proteins are implicated, along with the fission-yeast Pus1(S5a/Rpn10) subunit of the 26 S proteasome, in the recognition and turnover of substrates by this proteol ytic complex.