Inter-α-inhibitor is a
proteoglycan essential for mammalian reproduction and also plays a less well-characterized role in
inflammation. It comprises two homologous "heavy chains" (HC1 and HC2) covalently attached to
chondroitin sulfate on the bikunin core
protein. Before ovulation, HCs are transferred onto the
polysaccharide hyaluronan (HA) to form covalent HC·HA complexes, thereby stabilizing an extracellular matrix around the oocyte required for fertilization. Additionally, such complexes form during inflammatory processes and mediate leukocyte adhesion in the synovial fluids of
arthritis patients and protect against
sepsis. Here using X-ray crystallography, we show that human HC1 has a structure similar to
integrin β-chains, with a
von Willebrand factor A domain containing a functional
metal ion-dependent adhesion site (MIDAS) and an associated hybrid domain. A comparison of the WT
protein and a variant with an impaired MIDAS (but otherwise structurally identical) by small-angle X-ray scattering and analytical ultracentrifugation revealed that HC1 self-associates in a
cation-dependent manner, providing a mechanism for HC·HA cross-linking and matrix stabilization. Surprisingly, unlike
integrins, HC1 interacted with RGD-containing
ligands, such as
fibronectin,
vitronectin, and the latency-associated
peptides of
transforming growth factor β, in a MIDAS/
cation-independent manner. However, HC1 utilizes its MIDAS motif to bind to and inhibit the cleavage of
complement C3, and small-angle X-ray scattering-based modeling indicates that this occurs through the inhibition of the
alternative pathway C3 convertase. These findings provide detailed structural and functional insights into HC1 as a regulator of innate immunity and further elucidate the role of HC·HA complexes in
inflammation and ovulation.