Threonine 788 in integrin subunit β1 regulates integrin activation

In the present study, the functional role of suggested phosphorylation of the conserved threonines in the cytoplasmic domain of integrin subunit beta 1 was investigated. Mutants mimicking phosphorylated and unphosphorylated forms of beta 1 were expressed in beta 1 deficient GD25 cells. T788 in beta 1 was identified as a site with major influence on integrin function. The mutation to A788 strongly reduced beta 1-dependent cell attachment and exposure of the extracellular 9EG7 epitope, whereas replacement of T789 with alanine did not interfere with the ligand-binding ability. Talin has been shown to mediate integrin activation, but the talin head domain bound equally well to the wild-type beta 1 and the mutants indicating that the T788A mutation caused defect integrin activation by another mechanism. The phosphorylation-mimicking mutation T788D was fully active in promoting cell adhesion. GD25 cells expressing beta 1T788D accumulated increased number of focal contacts and migrated slowly compared to GD2S beta 1 wild-type. An analogous phenotype is seen when focal adhesion kinase activation is abrogated. However, neither the beta 1T788D nor the beta 1T788A mutation failed to induce tyrosine phosphorylation of focal adhesion kinase. The results suggest that phosphorylation of T788 in integrin beta 1 promotes inside-out receptor activation, as well as focal contact accumulation.