UCSD MOLECULE PAGES Properdin

Properdin is currently the only known positive regulator of complement activation and stabilizes the alternative pathway C3 convertase (C3bBb). It is composed of multiple identical protein subunits, with each subunit carrying a separate ligandbinding site. Previous reports suggest that properdin function depends on multiple interactions between its subunits and its ligands. Properdin recognizes several pathogenor damage/danger-associated molecular patterns (PAMPs and DAMPs, respectively) on foreign and apoptotic cells. Once bound, it initiates and propagates the complement response by attracting fluid-phage C3b to recognize surfaces and fostering de novo convertase assembly, and by stabilizing C3 convertase complexes (C3bBbfP). Therefore, it is central to continuous deposition of the activated complement fragment C3b on the surfaces of the pathogens, which it achieves by preventing the dissociation of the Bb catalytic subunit from the inherently labile C3bBb complexes.