SOLUTION STRUCTURE AND PEPTIDE BINDING OF THE SH3 DOMAIN FROM HUMAN FYN

Background: The Src family of tyrosine kinases is involved in the prop agation of intracellular signals from many transmembrane receptors. Ea ch member of the family contains two domains that regulate interaction s with other molecules, one of which is the Src homology 3 (SH3) domai n. Although structures have previously been determined for SH3 domains , and ideas about peptide-binding modes have been proposed, their phys iological role is still unclear. Results: We have determined the solut ion structure of the SH3 domain from the Src family tyrosine kinase Fy n in two forms: unbound acid complexed with a peptide corresponding to a putative ligand sequence from phosphatidylinositol 3' kinase. Fyn S H3 shows the typical SH3 topology of two perpendicular three-stranded beta sheets and a single turn of 3(10) helix. The interaction of SH3 w ith three potential ligand peptides was investigated, demonstrating th at they all bind to the same site on the molecule. A previous model fo r ligand binding to SH3 domains predicts binding in one of two orienta tions (class I or II), each characterized by a consensus sequence. The ligand with the closest match to the class I consensus sequence bound with highest affinity and in the predicted orientation. Conclusions: The Fyn SH3 domain has a well-defined structure in solution. The relat ive binding affinities of the three ligand peptides and their orientat ion within the Fyn SH3 complex were consistent with recently proposed models for the binding of 'consensus' polyproline sequences. Although the affinities of consensus and non-consensus peptides are different, the degree of difference is not very large, suggesting that SH3 domain s bind to polyproline peptides in a promiscuous manner.