Sequence requirements for association of protein-tyrosine phosphatase PEP with the Src homology 3 domain of inhibitory tyrosine protein kinase p50csk

Previously, we reported that the inhibitory tyrosine protein kinase p50(csk) is physically associated with the protein-tyrosine phosphatase PEP in hematopoietic cells. This interaction was shown to involve the Src homology 3 (SH3) region of Csk and a proline-rich sequence of PEP termed P1 (SRRTDDEIPPPLPERTPESFIVVEE). In this report, we have attempted to understand the structural basis for the highly specific association of these two molecules in vivo. Our studies revealed that the proline-rich core of the P1 region of PEP (PPPLPERT) was necessary but not sufficient for binding to p50(csk). Additional sequences located carboxyl to this motif were also needed for binding to the Csk SH3 domain in vitro and in vivo. Further analyses revealed that two aliphatic residues (isoleucine 625 and valine 626; PESFIVVEE) mere especially important for this effect. In addition to clarifying the molecular basis for the selective ability of PEP to associate with Csk, these results constitute further evidence that sequences outside proline-rich cores dictate the specificity of SH3 domain-mediated interactions in vivo.