Molecular basis for interaction between Icap1α PTB domain and β1 integrin

Icap1alpha is a 200-amino acid protein that binds to the COOH-terminal 13 amino acids ((788)AVTTVVNPKYE-GK(798)) of the integrin beta(1) subunit. Alanine scanning mutagenesis of this region revealed that Val(787), Val(790), and (NPKY795)-N-792 are critical for Icap1alpha binding. The NPXY motif is a known binding substrate for phosphotyrosine binding (PTB) domain proteins. The sequences of Icap1alpha, residues 58-200, and the beta(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(135), Ile(138), and Ile(139) of Icap1alpha, residues predicted by the model to be in close proximity to (NPKY795)-N-792, and LeU(82) and Tyr(144), residues expected to form a hydrophobic pocket near Val(787), are required for the Icap1alpha-beta(1), integrin interaction. These findings indicate that Icap1alpha is a PTB domain protein, which recognizes the NPXY motif of beta(1) integrin. Furthermore, our date suggest that an interaction between Val(787) and the hydrophobic pocket created by Leu(82) and Tyr(144) of Icap1alpha forms the basis for the specificity of Icap1alpha for the beta(1) integrin subunit.