Molecular Basis for Interaction between Icap1 alpha PTB Domain and beta sub(1) Integrin

Icap1 alpha is a 200-amino acid protein that binds to the COOH-terminal 13 amino acids ( super(786)AVTTVVNPKYEGK super(798)) of the integrin beta sub(1) subunit. Alanine scanning mutagenesis of this region revealed that Val super(787), Val super(790), and super(792)NPKY super(795) are critical for Icap1 alpha binding. The NPXY motif is a known binding substrate for phosphotyrosine binding (PTB) domain proteins. The sequences of Icap1 alpha , residues 58-200, and the beta sub(1) integrin, residues 786-797, were aligned to the available PTB-peptide structures to generate a high quality structural model. Site-directed mutagenesis showed that Leu super(135), Ile super(138), and Ile super(139) of Icap1 alpha , residues predicted by the model to be in close proximity to super(792)NPKY super(795), and Leu super(82) and Tyr super(144), residues expected to form a hydrophobic pocket near Val super(787), are required for the Icap1 alpha - beta sub(1) integrin interaction. These findings indicate that Icap1 alpha is a PTB domain protein, which recognizes the NPXY motif of beta sub(1) integrin. Furthermore, our date suggest that an interaction between Val super(787) and the hydrophobic pocket created by Leu super(82) and Tyr super(144) of Icap1 alpha forms the basis for the specificity of Icap1 alpha for the beta sub(1) integrin subunit.