De. Hourcade et al., ANALYSIS OF THE SHORT CONSENSUS REPEATS OF HUMAN-COMPLEMENT FACTOR-B BY SITE-DIRECTED MUTAGENESIS, The Journal of biological chemistry, 270(34), 1995, pp. 19716-19722
Human factor B is required for the initiation and propagation of the c
omplement alternative pathway, It also participates in the amplificati
on of the complement classical pathway, Alone, factor B is a zymogen w
ith Little known biochemical activity, but in the context of the alter
native pathway convertases, the factor B serine protease is activated
in a process that first involves the association with C3b and subseque
ntly the cleavage of factor B into two fragments, Ba and Bb, Ba, the N
H2-terminal fragment, is composed mainly of three tandem short consens
us repeats, globular domains found in other complement proteins, It di
ssociates from the convertase during assembly, leaving the active C3 c
onvertase, C3bBb, Previous reports suggest that the Ba region may be i
nstrumental in convertase assembly, This hypothesis was tested using s
ite-directed mutagenesis of recombinant factor B and monoclonal antibo
dy epitope mapping to evaluate the relative importance of specific sho
rt consensus repeat amino acid residues, Three sites of interest were
identified, Site 1 is a stretch of 19 contiguous amino acids in short
consensus repeat 1 that form the epitope of a monoclonal antibody that
effectively blocks factor B function. Site 2, composed of 6 contiguou
s amino acids in short consensus repeat 2, and site 3, consisting of 7
contiguous amino acids in short consensus repeat 3, were defined by m
utations that reduce factor B hemolytic activity to 3% or less, Furthe
r analyses indicated that sites 2 and 3 contribute to factor B-C3b int
eractions.