Interaction of the Integrin β₁ Cytoplasmic Domain with ICAP-1 Protein*

Abstract

In a yeast two-hybrid screen, a protein named ICAP-1 (β₁ integrin cytoplasmic domain associated protein) associated with the integrin β₁ cytoplasmic tail but not with tails from three other integrin β subunits (β₂, β₃, and β₅) or from seven different α subunits. Likewise in human cells, ICAP-1 associated specifically with the β₁ but not β₂, β₃, or β₅ tails. The carboxyl-terminal 14 amino acids of β₁ were critical for ICAP-1 interaction. ICAP-1 is a ubiquitously expressed protein of 27 and 31 kDa, with the smaller form being preferentially solubilized by Triton X-100. Phosphorylation of both 27- and 31-kDa forms was constitutive but was increased by 1.5–2-fold upon cell spreading on fibronectin, compared with poly-l-lysine. Also, ICAP-1 contributes to β₁ integrin-dependent migration because (i) ICAP-1 transfection markedly increased chemotactic migration of COS7 cells through fibronectin-coated but not vitronectin-coated porous filters, and (ii) support of β₁-dependent cell migration (in Chinese hamster ovary cells transfected with various wild type and mutant β₁ forms) correlated with ICAP-1 association. In summary, ICAP-1 (i) associates specifically with β₁ integrins, (ii) is phosphorylated upon β₁ integrin-mediated adhesion, and (iii) may regulate β_1-dependent cell migration.