Molecular modeling and mechanism of action of human decay-accelerating factor

A model of the regulatory region of human decay accelerating factor (DAF) was built based on the known coordinates of a fragment of the structurally and functionally homologous serum protein, factor H. According to this model, the four short consensus repeats (SCRs) in DAF are arranged in a helical fashion. A positively charged surface area on SCRs 2 and 3, two of the three repeating units essential for function, is postulated to be the primary recognition site for the C3 convertases C4b2a and C3bBb. This area encompasses a cavity on SCR 2, as well as part of the groove on the SCR 2-SCR 3 interface. Two additional surface depressions are centered around the C-terminal disulfide bridges of SCRs 3 and 4. These are likely to provide additional ligand binding sites. Based on this model in conjunction with sequence homology to the Ba fragment of factor B, a mechanism of DAF's accelerated convertase decay action is postulated.