Structure of the integrin receptor M2 headpiece in complex with a function-modulating nanobody

The integrin receptor M{beta}2 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 M{beta}2 headpiece fragment in complex with the nanobody hCD11bNb1 determined at a resolution of 3.2 [A]. The receptor headpiece adopts the closed conformation expected to have low ligand affinity. The crystal structure advocates that in the R77H M 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 [A]. The nanobody binds to the I domain of the M 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 M subunit. Surprisingly, the nanobody stimulates the interaction of cell-bound M{beta}2 with iC3b suggesting that it represents a novel high-affinity proteinaceous M{beta}2 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 M{beta}2. Furthermore, our data suggest that the iC3b-M{beta}2 complex may be more dynamic than predicted from the crystal structure of the core complex.