Molecular Biology of the Human Complement Class III Products of the MHC: Factor I and Its Cofactors

Deletion analysis of the 5′ flanking region of the factor B gene indicates the presence of a region with enhancer-like activity in addition to the promoter element and indicates that both elements are important in the tissue-specific expression of factor B. DNA sequencing of a number of C4 allotypes has allowed definition of the structural basis of the polymorphism found in C4A and C4B types in terms of gene size, reactivity, and antigenicity. It has been established that the regulatory proteins C4bp, CR1, and H as well as the enzymes C2, B, and C1r are structurally related since they contain two or more internal repeating units of 60 amino acids. Thus these complement proteins appear to be members of a new superfamily since this structural feature is widespread, being found in a number of noncomplement proteins.