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

Integrin affinities for ligands can change markedly via a process term ed inside-out signaling. We expressed several truncations of the beta( 3) cytoplasmic domain in conjunction with an ''activating'' alpha subu nit chimera, alpha(IIb)alpha(6B). Deletion of the 4 C-terminal residue s of the beta(3) tail blocked inside-out signaling as assessed by the binding of an activation specific antibody, PAC1. Several additional t runcations remained in the low affinity state, but complete truncation (beta(3) Delta 717) caused PAC1 binding, Activation by this truncatio n mutant did not depend on the cr subunit cytoplasmic domain and was r esistant to inhibitors of cellular metabolism and the over-expression of an isolated beta(3) cytoplasmic domain. Since deletion of beta 3(Le u(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 st ate. The amino acid sequence of this region is conserved among integri ns. Moreover, the conserved membrane-proximal sequence in alpha subuni t tails seems to serve a similar function. Consequently, the conserved membrane-proximal regions of both integrin cytoplasmic domains contro l the ligand binding affinity of the extracellular domain.