hHR23B is one of two human homologs of the Saccharomyces cerevisiae
nucleotide excision repair (NER) gene product RAD23 and a component of a
protein complex that specifically complements the NER defect of xeroderma
pigmentosum group C (XP-C) cell extracts in vitro. Although a small
proportion of hHR23B is tightly complexed with the XP-C responsible gene
product, XPC protein, a vast majority exists as an XPC-free form,
indicating that hHR23B has additional functions other than NER in vivo.
Here we demonstrate that the human NER factor hHR23B as well as another
human homolog of RAD23, hHR23A, interact 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 centrifugation of HeLa S100
extracts. Intriguingly, hHR23B showed the inhibitory effect on the
degradation of (125)I-lysozyme in the rabbit reticulocyte lysate. hHR23
proteins thus appear to associate with 26 S proteasome in vivo. From
co-precipitation experiments using several series of deletion mutants, we
defined the domains in hHR23B and S5a that mediate this interaction. From
these results, we propose that part of hHR23 proteins are involved in the
proteolytic pathway in cells