Critical roles for the COOH-terminal NITY and RGT sequences of the integrin β3 cytoplasmic domain in inside-out and outside-in signaling

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Abstract
Bidirectional signaling of integrin αIIbβ3 requires the β3 cytoplasmic domain. To determine the sequence in the β3 cytoplasmic domain that is critical to integrin signaling, cell lines were established that coexpress the platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX, integrin αIIb, and mutants of β3 with truncations at sites COOH terminal to T741, Y747, F754, and Y759. Truncation at Y759 did not affect integrin activation, as indicated by vWF-induced fibrinogen binding, but affected cell spreading and stable adhesion. Thus, the COOH-terminal RGT sequence of β3 is important for outside-in signaling but not inside-out signaling. In contrast, truncation at F754, Y747, or T741 completely abolished integrin activation. A point mutation replacing Y759 with alanine also abolished integrin activation. Thus, the T755NITY759 sequence of β3, containing an NXXY motif, is critical to inside-out signaling, whereas the intact COOH terminus is important for outside-in signaling. In addition, we found that the calcium-dependent protease calpain preferentially cleaves at Y759 in a population of β3 during platelet aggregation and adhesion, suggesting that calpain may selectively regulate integrin outside-in signaling.

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