We analyzed the binding of fibronectin to integrin α5β1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd ~ 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of α(IIb)β3 joined to the cytoplasmic domains of α5β1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of α5β1 conferred an energy-dependent high affinity state on α(IIb)β3 in CHO but not K562 cells. Three additional α cytoplasmic domains (α2, α6A, α6B) conferred PAC1 binding in CHO cells, while three others (α(M), α(L), α(v)) did not. In the high affinity α chimeras, cotransfection with a truncated (β3Δ724) or mutated (β3(S752→P)) β3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved α subunit GFFKR motif, resulted in high affinity binding of ligands to α(IIb)β3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the β subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the α subunit cytoplasmic domain maintains the default low affinity state.
ASJC Scopus subject areas
- Cell Biology