The full-length cDNA clone which encodes a novel 824-amino acid protein was characterized. The predicted protein contains ten 34-amino acid repeats characteristic of the tetratricopeptide repeat protein family. The sequence homology and organization of the 10 repeats are similar to those of the nuc2 protein of fission yeast and bimA protein of Aspergillus, which suggests that the newly identified protein could be the human homologue of nuc2 (H-NUC). Consistent with this notion, the M(r) 95,000 H-NUC is a nuclear protein with DNA binding activity. This protein binds to hypophosphorylated Rb protein in a region indistinguishable from that to which SV40 large T antigen binds. However, Rb also binds to H-NUC at the tetratricopeptide repeat motif, a region which contains sequences different from the binding motifs of either T-antigen or E2F-1. To mimic the temperature-sensitive mutant of yeast nuc2, an H-NUC mutant was made in which the highly conserved glycine 640 residue was changed to aspartic acid. In contrast to wild-type H-NUC, the mutant was temperature sensitive in binding to Rb protein. These results, taken together, suggest that the interaction between H-NUC and Rb may be significant.