P62, a phosphotyrosine indpendent ligand of p56Lck SH2 domain interacts with ubiquitin

p62 is a novel protein which binds to the SH2 domain of p56lck in a phosphotyrosine-independent manner. For further understanding of the function of p62, proteins binding to p62 other than the Ick SH2 domain have been investigated. Yeast two hybrid system using p62 as a bait shows that p62 specifically interacts with ubiquitin in vivo. This result was confirmed by in vitro interaction of these two proteins; p62 binds to ubiquitin conjugated sepharose beads but not to sepharose bead itself and the binding can be completely inhibited by soluble ubiquitin. Association and dissociation of p62 and ubiquitin do not require ATP and dithiothreitol respectively, suggesting that these two proteins interact noncovalently. Further analysis using a series of deletion mutants of p62 shows that ubiquitin binding site is located in the C-terminal 150 amino acids, while the Ick SH2 domain binds to a region in the N-terminal 50 amino acids. These results indicate a role for p62 in the protein ubiquitination process during signal transduction.