Hrs-binding protein (Hbp) is a Src homology 3 (SH3) domain-containing protein that tightly associates with Hrs. Hbp together with Hrs is thought to play a regulatory role in endocytic trafficking of growth factor-receptor complexes through early endosomes. Association of Hbp with a binding partner(s) via the SH3 domain seems to be essential for Hbp to exert its function. In this study, we searched for Hbp-binding proteins by a far Western screening and isolated a mouse cDNA clone encoding a deubiquitinating enzyme mUBPY as an Hbp SH3-binding protein. mUBPY has two Hbp-SH3 domain binding sites. Mutagenic analysis identified a consensus sequence PX(V/I)(D/N)RXXKP as the Hbp-SH3 domain binding motif. It is a novel SH3-binding motif and does not contain the canonical proline-rich consensus binding motif, PXXP. Ubiquitination of growth factor receptors is thought to regulate their intracellular degradation. Thus, UBPY may play a regulatory role in the degradation by interaction with the SH3 domain of Hbp via the novel SH3-binding motif.