[Production and stabilization of an integrin-binding moiety of complement component 3]

The third component of complement, C3, plays a central role in human innate immunity. The subsequent proteolysis product of native C3, iC3b, is the primary ligand of complement receptors (CRs) CR3 and CR4. CR3 and CR4 are β2-family integrins, and their binding to iC3b contributes to phagocytosis. How iC3b binds to its receptors and transmits signals into the cells is not clear. To perform structural and functional studies on the interaction between iC3b and its receptors CR3/CR4, we isolated the integrin-binding fragment of iC3b, MG3-4. Low temperature is required for its soluble expression in Escherichia coli. Purified MG3-4 existed as a dimer in solution and was easy to aggregate. We tried different agents and found glycerol could efficiently stabilize the MG3-4 fragment to avoid aggregation. Using surface plasmon resonance (SPR) analysis, we confirmed MG3-4 could bind I domain, the iC3b-binding domain of CR3. Here, we report the successful production of a soluble, stable, and biologically active integrin-binding moiety of human iC3b for further studies.