This domain family is found in bacteria, and is approximately 70 amino acids in length. There is a conserved VLK sequence motif. It is the C terminal domain of bacterial extracellular fibrinogen binding protein. It contains a helical motif involved in complement regulation. This motif binds to complement and changes its conformation to a form which cannot activate downstream components of the complement cascade.

1: A structural basis for complement inhibition by Staphylococcus aureus. Hammel M, Sfyroera G, Ricklin D, Magotti P, Lambris JD, Geisbrecht BV; Nat Immunol. 2007;8:430-437. PMID:17351618