Interactions of phosphatidylinositol 3-kinase Src homology 3 domain with its ligand peptide studied by absorption, circular dichroism, and UV resonance raman spectroscopies.

Absorption, circular dichroism (CD), and UV resonance Raman (UVRR) spectroscopies were applied to selectively examine the environmental and structural changes of Trp and Tyr residues in the phosphatidylinositol 3-kinase (PI3K) SH3 domain induced by ligand association. Comparison of the spectra of PI3K SH3 in the presence or absence of its ligand peptide RLP1 (RKLPPRPSK) indicated that RLP1 binding changed the environment of Trp55 of the SH3 to be more hydrophilic and its H bonding weaker and that of Tyr residues to be more hydrophobic. The D21N mutant (Asp21 --> Asn) of the SH3 yielded a UV CD distinct from that of the wild type, and its spectral changes induced by RLP1 binding were smaller and different from those of the wild type in absorption, CD, and UVRR spectra, suggesting that the mutation of conserved Asp21 affected the conformation of the ligand binding cleft and thus might lead to the decrease in the ligand affinity. These data provide direct evidence for the occurrence of environmental and structural changes of PI3K SH3 by the association of a ligand and the D21N mutation.