THE CONSERVED MEMBRANE-PROXIMAL REGION OF AN INTEGRIN CYTOPLASMIC DOMAIN SPECIFIES LIGAND-BINDING AFFINITY

Integrin affinities for ligands can change markedly via a process termed inside-out signaling. We expressed several truncations of the beta(3) cytoplasmic domain in conjunction with an ''activating'' alpha subunit chimera, alpha(IIb)alpha(6B). Deletion of the 4 C-terminal residues of the beta(3) tail blocked inside-out signaling as assessed by the binding of an activation specific antibody, PAC1. Several additional truncations remained in the low affinity state, but complete truncation (beta(3) Delta 717) caused PAC1 binding, Activation by this truncation mutant did not depend on the cr subunit cytoplasmic domain and was resistant to inhibitors of cellular metabolism and the over-expression of an isolated beta(3) cytoplasmic domain. Since deletion of beta 3(Leu(717)-Asp(723)) results in a constitutively activated integrin, this membrane-proximal seven amino acids of the beta(3) cytoplasmic domain is required to maintain alpha(IIb)beta(3) in a default low affinity state. The amino acid sequence of this region is conserved among integrins. Moreover, the conserved membrane-proximal sequence in alpha subunit tails seems to serve a similar function. Consequently, the conserved membrane-proximal regions of both integrin cytoplasmic domains control the ligand binding affinity of the extracellular domain.