Fig. 2

Complement regulatory proteins (CRegPs) in humans. a Membrane bound CRegPs share similar structural characteristics facilitating their correct function. CR1 and CD46 attach on the membrane via a transmembrane domain while CD55 and CD59 are attached via a glycosylphosphatidylinositol-(gpi) anchor. Their extracellular domains consist of multiple short consensus repeats (SCR) as shown for CR1, CD46 and CD55 while CD59 has a simpler extracellular domain consisting only of aminoacids linked by disulphide bonds. CD46 and CD55 also share a common serine-threonine-rich structure between their attachment domain and the short consensus repeats. b Structural characteristics of fluid phase complement regulatory proteins. C1 inhibitor, a serine protease inhibitor (serpin) consists of two domains, a C-terminal serpin domain and a N-terminal non-serpin domain the latter important for functionality and recognition. C4b binding protein, is a spider-like structure which consists of seven identical α-chains and one β-chain. The α-chain is comprised of eight SCRs and a C-terminal oligomerization domain, while the β-chain consists of three short consensus repeats and a C-terminal oligomerization domain. Factor H is a glycoprotein consisting of 20 short consensus repeats and its activity is regulated by the first four short consensus repeats while consensus repeats 19–20 are involved in ligand binding. Vitronectin is a glycoprotein with a complex structure enabling its multifunctional character and in humans it circulates either as a single chain of 75 kDa, or a clipped form of two chains: 65 and 10 kDa, held together by a disulphide bond