title: Probing the "two-pronged plug two-holed socket" model for the mechanism of binding of the Src SH2 domain to phosphotyrosyl peptides: a thermodynamic study.
abstract: Src homology 2 (SH2) domains are protein modules that specifically bind to tyrosyl phosphorylated peptides on signaling proteins. X-ray crystallographic studies of the SH2 domain of the Src kinase have probed the mechanism of binding, leading to the "two-pronged plug two-holed socket" mechanism whereby binding is hypothesized to resemble a two-pronged plug (the peptide) inserting into a two-holed socket (the SH2 domain). This binding model predicts (1) a hydrophobic basis for high-affinity binding largely determined by the level of insertion of the third residue C-terminal to the phosphotyrosine in the peptide into a primarily hydrophobic pocket (the +3 binding pocket) of the SH2 domain, and (2) a binding mechanism involving no significant conformational changes in the SH2 domain. In this study, we have probed these predictions by using isothermal titration calorimetry to extract complete thermodynamic profiles (Delta G degrees, Delta H degrees, Delta S degrees, Delta Cp degrees) for the binding of the Src SH2 domain to two series of tyrosyl phosphopeptides. One series consisted of peptides that have been determined by X-ray crystallography to have different levels of insertion of the peptide's +3 position into the +3 binding pocket. The other series consisted of peptides with progressively smaller hydrophobic side chains (I, L, V, and A) at the +3 position. Consistent with a binding mechanism that does not involve substantial conformational changes, the Delta Cp degrees values for all peptides were small and, at least for the high-affinity interactions, similar to the Delta Cp degrees values predicted from surface area calculations. However, unexpectedly, this study reveals that high-affinity binding was only partially determined by the interactions between the +3 residue in the peptide and the +3 binding pocket. Furthermore, the Delta Cp degrees values for all peptides studied were similar, implying similar degrees of desolvation of the +3 binding pocket upon binding. These results indicate that the "two-pronged plug two-holed socket" model is an oversimplification of the Src SH2 domain binding mechanism.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Bradshaw
authors.forename: R A
authors.initials: RA
authors.lastname: Grucza
authors.forename: J E
authors.initials: JE
authors.lastname: Ladbury
authors.forename: G
authors.initials: G
authors.lastname: Waksman
mesh_terms: D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000737:chemistry / Q000378:metabolism; D002151:Calorimetry; D006224:Cricetinae; D006801:Humans; D008958:Models, Molecular; D010748:Phosphopeptides / Q000737:chemistry / Q000378:metabolism*; D019000:Phosphotyrosine / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017479:Receptors, Platelet-Derived Growth Factor / Q000737:chemistry / Q000378:metabolism; D013329:Structure-Activity Relationship; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics; D018909:src Homology Domains
chemical_list: D000952:Antigens, Polyomavirus Transforming; D010748:Phosphopeptides; D019000:Phosphotyrosine; D017479:Receptors, Platelet-Derived Growth Factor