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 2 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 alpha 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(Leu717-Asp723) 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.