Properdin is a gamma globulinprotein composed of multiple identical protein subunits with a separate ligand-binding site. Native properdin occurs in head-to-tail dimers, trimers and tetramers in the fixed ratio 22:52:28.
The properdin promotes the association of C3b with Factor B and provides a focal point for the assembly of C3bBb on a surface. It binds to preformed alternative pathway C3-convertases. Properdin also inhibits the Factor H – mediated cleavage of C3b by Factor I.
Alternative pathway. Properdin is the "P" in the blue circles. (Some labels are in Polish.)
^Smith C, Pangburn M, Vogel C-W, and Müller-Eberhard H (1984). "Molecular Architecture of Human Properdin, a Positive Regulator of the Alternative Pathway of Complement". J of Biol Chem259: R4582–4588.
^Hourcade D (2006). "The Role of Properdin in the Assembly of the Alternative Pathway C3 Convertases of Complement". J of Biol Chem281: R2128–2132. doi:10.1074/jbc.m508928200.