Sam68 from an immortalised B-cell line associates with a subset of SH3 domains

The binding of proteins from an immortalised B-cell line to a panel of SH3 domains was investigated in vitro, One of the most prominent SH3 domain binding proteins was a 68 kD polypeptide which strongly associated with the SH3 domains of c-src, p85 alpha and p47(phox) and weakly with the SH3 domain of PLC gamma and n-src with undetectable binding to the other SH3 domains tested, Immunoblotting identified this protein as human Sam68, The ability of proline-rich peptides homologous to the Sam68 primary sequence to inhibit the binding of Sam68 to SH3 domains was investigated, Only one peptide inhibited binding of Sam68 to the p85 alpha: SH3 domain, whereas several peptides inhibited binding of Sam68 to c-src SH3 domain, suggesting that Sam68 uses different proline-rich motifs to bind to different SH3 domains, A peptide derived from residues 32-44 of Sam68,which fits the class II SH3 domain binding consensus sequence inhibited binding of Sam68 to both p85 alpha SH3 domain and c-src SH3 domain, but with differential potency, suggesting a differential affinity of these SH3 domains for this proline-rich motif.