Engineered allosteric mutants of the integrin αMβ2 I domain: structural and functional studies

The α-I domain, found in the α-subunit of the leucocyte integrins such as αMβ2 and αLβ2, switches between the open and closed tertiary conformations, reflecting the highand low-affinity ligand-binding states of the integrin that are required for regulated cell adhesion and migration. In the present study we show, by using point mutations and engineered disulphide bonds, that ligand affinity can be reduced or increased allosterically by altering the equilibrium between the closed and open states. We determined equilibrium constants for the binding of two ligands, fibrinogen and intercellular cell-adhesion molecule 1, to the αM-I domain by surface plasmon resonance, and determined crystal structures of a low-affinity mutant. Locking the domain in the open conformation increases affinity by a factor of no greater than 10, consistent with a closely balanced equilibrium between the two conformations in the absence of ligand. This behaviour contrasts with that of the unliganded αL-I domain, for which the equilibrium lies strongly in favour of the closed conformation. These results suggest significant differences in the way the parent integrins regulate I domain conformation and hence ligand affinity.