Complement Factor P (Properdin)

Complement factor P, also known as Properdin or CFP, is a 53 kDa glycoprotein that is a key positive regulator of the complement activity. It is produced primarily by leucocytes rather than hepatocytes. It circulates in plasma in the form of cyclic oligomers of head-to-tail associations of monomers, such as dimers, trimers, and tetramers, among which the tetramer is the prevalent form. Each monomer contains six thrombospondin type 1 repeat domains (TSR1-6) and a truncated N-terminal domain (TSR0).

Properdin binds to and stabilizes the C3 and C5 convertase enzyme complexes, promoting alternative pathway activation. It also can increase the formation of the alternative pathway C3 convertase by increasing binding of factor B to P, C3b complexes to promoting activation of the alternative pathway. Besides, properdin is also involved in phagocytosis. Properdin, binding to early apoptotic T cells, promotes the phagocytic uptake of the apoptotic cells by macrophages and dendritic cells. Furthermore, properdin is also indicated to suppress the degradation of the complement C3 beta chain (C3b) mediated by CFI-CFH. Deficiency of properdin leads to impaired alternative pathway function, thereby triggering various diseases such as meningococcal infections, arthritis, asthma, and kidney and cardiovascular diseases.

Fig 1. The illustration of activation mechanisms of the alternative pathway.¹

Reference

1. From Wikipedia: By Rantes, oringinally at pl.wikipedia, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Droga_klasyczna.png.