STRUCTURAL AND THERMODYNAMIC CHARACTERIZATION OF THE INTERACTION OF THE SH3 DOMAIN FROM FYN WITH THE PROLINE-RICH BINDING-SITE ON THE P85 SUBUNIT OF PI3-KINASE

The interaction of the Fyn SH3 domain with the p85 subunit of PI3-kina se is investigated using structural detail and thermodynamic data. The solution structure complex of the SH3 domain with a proline-rich pept ide mimic of the binding site on the p85 subunit is described. This in dicates that the peptide binds as a poly(L-proline) type II helix. Cir cular dichroism spectroscopic studies reveal that in the unbound state the peptide exhibits no structure. Thermodynamic data for the binding of this peptide to the SH3 domain suggest that the weak binding (appr oximately 31 mu M) of this interaction is, in part, due to the entropi cally unfavorable effect of helix formation (Delta S degrees = -78 J . mol(-1). K-1). Binding of the SH3 domain to the intact p85 subunit (m inus its own SH3 domain) is tighter, and the entropic and enthalpic co ntributions are very different from those given by the peptide interac tion (Delta S degrees = +252 J . mol(-1). K-1; Delta H degrees = +44 k J . mol(-1)). From these dramatically different thermodynamic measurem ents we are able to conclude that the interaction of the proline-rich peptide does not effectively mimic the interaction of the intact p85 s ubunit with the SH3 domain and suggest that other interactions could b e important.