Structure of the integrin receptor αMβ2 headpiece in complex with a function-modulating nanobody

The integrin receptor αMβ2 mediates phagocytosis of complement-opsonized objects, adhesion to the extracellular matrix and trans-endothelial migration of leukocytes. Here we present the first atomic structure of the human αMβ2 headpiece fragment in complex with the nanobody hCD11bNb1 determined at a resolution of 3.2 Å. The receptor headpiece adopts the closed conformation expected to have low ligand affinity. The crystal structure advocates that in the R77H αM variant associated with systemic lupus erythematosus, the modified allosteric coupling between ligand coupling and integrin outside-inside signalling is due to subtle conformational effects transmitted over 40 Å. The nanobody binds to the αI domain of the αM subunit in an Mg2+ independent manner with low nanomolar affinity. Biochemical and biophysical experiments with purified proteins argue that the nanobody acts as a competitive inhibitor through steric hindrance exerted on the thioester domain of iC3b attempting to bind the αM subunit. Surprisingly, the nanobody stimulates the interaction of cell-bound αMβ2 with iC3b suggesting that it represents a novel high-affinity proteinaceous αMβ2 specific agonist. We propose a model based on the conformational spectrum of the receptor to reconcile these conflicting observations regarding the functional consequences of hCD11bNb1 binding to αMβ2. Furthermore, our data suggest that the iC3b-αMβ2 complex may be more dynamic than predicted from the crystal structure of the core complex.