Specific β₁ integrin site selectively regulates Akt/protein kinase B signaling via local activation of protein phosphatase 2A

Integrin transmembrane receptors generate multiple signals, but how they mediate specific signaling is not clear. Here we test the hypothesis that particular sequences along the β₁ integrin cytoplasmic domain may exist that are intimately related to specific integrin-mediated signaling pathways. Using systematic alanine mutagenesis of amino acids conserved between different β integrin cytoplasmic domains, we identified the tryptophan residue at position 775 of human β₁ integrin as specific and necessary for integrin-mediated protein kinase B/Akt survival signaling. Stable expression of a β₁ integrin mutated at this amino acid in GD25 β₁-null cells resulted in reduction of Akt phosphorylation at both Ser⁴⁷³ and Thr³⁰⁸ activation sites. As a consequence, the cells were substantially more sensitive to serum starvation-induced apoptosis when compared with cells expressing wild type β₁ integrin. This inactivation of Akt resulted from increased dephosphorylation by a localized active population of protein phosphatase 2A. Both Akt and protein phosphatase 2A were present in β₁ integrin-organized cytoplasmic complexes, but the activity of this phosphatase was 2.5 times higher in the complexes organized by the mutant integrin. The mutation of Trp⁷⁷⁵ specifically affected Akt signaling, without effects on other integrin-activated pathways including phosphoinositide 3-kinase, MAPK, JNK, and p38 nor did it influence activation of the integrin-responsive kinases focal adhesion kinase and Src. The identification of Trp⁷⁷⁵ as a specific site for integrin-mediated Akt signaling supports the concept of specificity of signaling along the integrin cytoplasmic domain.