This family of proteins represents Sbi domain IV which binds the central complement protein C3. Sbi-IV interacts with Sbi-III to induce a consumption of complement via alternative pathway activation [1]. When not interacting with Sbi-III, Sbi-IV inhibits the alternative pathway without complement consumption. The structure of Sbi-IV consists of a three-helix bundle fold [1].

1: Structure-function analysis of the C3 binding region of Staphylococcus aureus immune subversion protein Sbi. Upadhyay A, Burman JD, Clark EA, Leung E, Isenman DE, van den Elsen JM, Bagby S; J Biol Chem. 2008;283:22113-22120. PMID:18550524