Factor C acts as a lipopolysaccharide-responsive C3 convertase in horseshoe crab complement activation.

The complement system in vertebrates plays an important role in host defense against and clearance of invading microbes, in which complement component C3 plays an essential role in the opsonization of pathogens, whereas the molecular mechanism underlying C3 activation in invertebrates remains unknown. In an effort to understand the molecular activation mechanism of invertebrate C3, we isolated and characterized an ortholog of C3 (designated TtC3) from the horseshoe crab Tachypleus tridentatus. Flow cytometric analysis using an Ab against TtC3 revealed that the horseshoe crab complement system opsonizes both Gram-negative and Gram-positive bacteria.