Mutational evidence for control of cell adhesion through integrin diffusion/clustering, independent of ligand binding

Previous studies have shown that integrin α chain tails make strong positive contributions to integrin-mediated cell adhesion. We now show here that integrin α⁴ tail deletion markedly impairs static cell adhesion by a mechanism that does not involve altered binding of soluble vascular cell adhesion molecule 1 ligand. Instead, truncation of the α⁴ cytoplasmic domain caused a severe deficiency in integrin accumulation into cell surface clusters, as induced by ligand and/or antibodies. Furthermore, α⁴ tail deletion also significantly decreased the membrane diffusivity of α⁴β₁, as determined by a single particle tracking technique. Notably, low doses of cytochalasin D partially restored the deficiency in cell adhesion seen upon α⁴ tail deletion. Together, these results suggest that α⁴ tail deletion exposes the β₁ cytoplasmic domain, leading to cytoskeletal associations that apparently restrict integrin lateral diffusion and accumulation into clusters, thus causing reduced static cell adhesion. Our demonstration of integrin adhesive activity regulated through receptor diffusion/clustering (rather than through altered ligand binding affinity) may be highly relevant towards the understanding of inside-out signaling mechanisms for β₁ integrins.