Regulation of integrin alphaIIbbeta3 activation by distinct regions of its cytoplasmic tails.

The short cytoplasmic tails regulate activation of integrin adhesion receptors via clasping/unclasping of their membrane-proximal helices. Using integrin alpha(IIb)beta(3) as a model, we show that a previously reported activating mutation alpha(IIb)(R(995)D) that perturbs the electrostatic interface in the clasp only partially activates alpha(IIb)beta(3) and that extensive activation of the receptor is achieved by complete deletion of alpha(IIb) CT or triple mutations in alpha(IIb)(V(990)A/F(992)A/R(995)D) that disrupt both electrostatic and hydrophobic interfaces in the clasp. The results provide quantitative evidence for an equilibrium-based integrin activation process where shifting the equilibrium to the fully activated state requires total unclasping of the cytoplasmic tails. We further demonstrate that while the C-terminal region of the alpha(IIb) tail minimally influences alpha(IIb)beta(3) activation, the C-terminal region of the beta(3) tail is critically involved. A disease-causing mutation of S(752)P in this region, but not S(752)A, suppressed partial activation induced by R(995)D or the talin head domain but did not affect activation induced by alpha(IIb) truncation. NMR spectroscopy revealed that S(752)P but not the S(752)A mutation disrupted a C-terminal helix within the beta(3) tail, suggesting that the C-terminal helix may regulate the equilibrium-based clasping/unclasping process. Together, these data provide molecular insights into how distinct regions of the cytoplasmic tails differentially and cooperatively regulate integrin activation.