A tyrosine-based sorting signal in the beta 2 integrin cytoplasmic domain mediates its recycling to the plasma membrane and is required for ligand-supported migration

Integrins play pivotal roles in supporting shear- and mechanical-stress-res istant cell adhesion and migration, These functions require the integrity o f the short beta subunit cytoplasmic domains, which contain multiple, highl y conserved tyrosine-based endocytic signals, typically found in receptors undergoing regulated, clathrin-dependent endocytosis. We hypothesized that these sequences may control surface integrin dynamics in statically adheren t and/or locomoting cells via regulated internalization and polarized recyc ling of the receptors, By using site-directed mutagenesis and ectopic expre ssion of the alpha L/beta 2 integrin in Chinese hamster ovary cells, we fou nd that Y735 in the membrane-proximal YRRF sequence is selectively required for recycling of spontaneously internalized receptors to the cell surface and to growth factor-induced membrane ruffles, Disruption of this motif by non-conservative substitutions has no effect on the receptor's adhesive fun ction, but diverts internalized integrins from a recycling compartment into a degradative pathway. Conversely, the non-conservative F754A substitution in the membrane-proximal NPLF sequence abrogates ligand-dependent adhesion and spreading without affecting receptor recycling, Both of these mutants display a severe impairment in ligand-supported migration, suggesting the e xistence in integrin cytoplasmic domains of independent signals regulating apparently unrelated functions that are required to sustain cell migration over specific ligands.