Interaction of hHR23 with S5a - The ubiquitin-like domain of hHR23 mediates interaction with S5a subunit of 26 S proteasome

hHR23B is one of two human homologs of the Saccharomyces cerevisiae nucleot ide excision repair (NER) gene product RAD23 and a component of a protein c omplex that specifically complements the NER defect of xeroderma pigmentosu m group C (XP-C) cell extracts in vitro. Although a small proportion of hHR 23B is tightly complexed with the XP-C responsible gene product, XPC protei n, a vast majority exists as an XPC-free form, indicating that hHR23B has a dditional functions other than NER in vivo. Here we demonstrate that the hu man NER factor hHR23B as well as another human homolog of RAD23, hHR23A, in teract specifically with S5a, a subunit of the human 26 S proteasome using the yeast two-hybrid system. Furthermore, hHR23 proteins were detected with S5a at the position where 26 S proteasome sediments in glycerol gradient c entrifugation of HeLa S100 extracts, intriguingly, hHR23B showed the inhibi tory effect on the degradation of I-125-lysozyme in the rabbit reticulocyte lysate. hHR23 proteins thus appear to associate with 26 S proteasome in vi vo. From co-precipitation experiments using several series of deletion muta nts, we defined the domains in hHR23B and S5a that mediate this interaction . From these results, we propose that part of hHR23 proteins are involved i n the proteolytic pathway in cells.