A NPxY-independent beta5 integrin activation signal regulates phagocytosis of apoptotic cells.

Integrin receptors are heterodimeric transmembrane receptors with critical functions in cell adhesion and migration, cell cycle progression, differentiation, apoptosis, and phagocytosis of apoptotic cells. Integrins are activated by intracellular signaling that alter the binding affinity for extracellular ligands, so-called inside to outside signaling. A common element for integrin activation involves binding of the cytoskeletal protein talin, via its FERM domain, to a highly conserved NPxY motif in the beta chain cytoplasmic tails, which is involved in long-range conformation changes to the extracellular domain that impinges on ligand affinity. When the human beta-5 (beta5) integrin cDNA was expressed in alphav positive, beta5 and beta3 negative hamster CS-1 cells, it promoted NPxY-dependent adhesion to VTN-coated surfaces, phosphorylation of FAK, and concomitantly, beta5 integrin-EGFP protein was recruited into talin and paxillin-containing focal adhesions. Expression of a NPxY destabilizing beta5 mutant (Y750A) abrogated adhesion and beta5-Y750A-EGFP was excluded from focal adhesions at the tips of stress fibers. Surprisingly, expression of beta5 Y750A integrin had a potent gain-of-function effect on apoptotic cell phagocytosis, and further, a beta5-Y750A-EGFP fusion integrin readily bound MFG-E8-coated 10 microm diameter microspheres developed as apoptotic cell mimetics. The critical sequences in beta5 integrin were mapped to a YEMAS motif just proximal to the NPxY motif. Our studies suggest that the phagocytic function of beta5 integrin is regulated by an unconventional NPxY-talin-independent activation signal and argue for the existence of molecular switches in the beta5 cytoplasmic tail for adhesion and phagocytosis.