Sequential assignment and secondary structure determination for the Src homology 2 domain of hematopoietic cellular kinase

Crystal structure of the C-terminal SH2 domain of the p85alpha regulatory subunit of phosphoinositide 3-kinase: an SH2 domain mimicking its own substrate

The binding properties of Src homology-2 (SH2) domains to phosphotyrosine (pY)-containing peptides have been studied in recent years with the elucidation of a large number of crystal and solution structures. Taken together, these structures suggest a general mode of binding of pY-containing peptides, explain the specificities of different SH2 domains, and may be used to design inhibitors of pY binding by SH2 domain-containing proteins. We now report the crystal structure to 1.8 A resolution of the C-terminal SH2 domain (C-SH2) of the P85alpha regulatory subunit of phosphoinositide 3-kinase (PI3 K). Surprisingly, the carboxylate group of Asp2 from a neighbouring molecule occupies the phosphotyrosine binding site and interacts with Arg18 (alphaA2) and Arg36 (betaB5), in a similar manner to the phosphotyrosine-protein interactions seen in structures of other SH2 domains complexed with pY peptides. It is the first example of a non-phosphate-containing, non-aromatic mimetic of phosphotyrosine binding to SH2 domains, and this could have implications for the design of substrate analogues and inhibitors. Overall, the crystal structure closely resembles the solution structure, but a number of loops which demonstrate mobility in solution are well defined by the crystal packing. C-SH2 has adopted a binding conformation reminiscent of the ligand bound N-terminal SH2 domain of PI3K, apparently induced by the substrate mimicking of a neighbouring molecule in the crystal.

Self-association and backbone dynamics of the hck SH2 domain in the free and phosphopeptide-complexed forms

Decreased dynamic motion in the peptide backbone of proteins may accompany ligand binding and influence the thermodynamic and kinetic stability of the resulting complexes. We have investigated the diffusional behavior and backbone dynamics of the free and phosphopeptide (EPQpYEEIPIYL) complexed Hck SH2 domain using NMR spectroscopy. Both the free domain and its phosphopeptide complex self-associate at higher protein concentrations. Diffusional measurements and surface analysis indicate that charged side-chain groups are probably responsible for self-association. Higher order aggregation, such as trimer and tetramer, also occurs at elevated protein concentrations. Dynamic motion in the peptide backbone of Hck SH2 was determined from 15N relaxation data fit using extended model-free parameters. The rotational correlation time (taum) for uncomplexed Hck SH2 was 6.8 ns while taum for peptide-bound Hck SH2 was 7.6 ns. Generalized order parameters (S2) increased for most residues upon binding of the phosphopeptide, consistent with peptide binding restricting motion of the NH bond vectors on the picosecond time scale. These studies suggest that complexation increases internal order in Hck SH2 and that internal dynamic motions contribute to the activation of Src-family kinases in vivo.