A CONSERVED ELEMENT IN THE SERINE-PROTEASE DOMAIN OF COMPLEMENT FACTOR-B

Factor B and C2 are serine proteases that carry the catalytic sites of the complement C3 and C5 convertases. Their protease domains are acti vated by conformational changes that occur during convertase assembly and are deactivated upon convertase dissociation. Factor B and C2 shar e an 8-amino acid conserved sequence near their serine protease termin i that is not seen in other serine proteases. To determine its importa nce, 24 factor B mutants were generated, each with a single amino acid substitution in this region. Whereas most mutants were functionally n eutral, all five different substitutions of aspartic acid 715 and one phenylalanine 716 substitution severely reduced hemolytic activity. Se veral aspartic acid 715 mutants permitted the steps of convertase asse mbly including C3b-dependent factor D-mediated cleavage and activation of the high affinity C3b-binding site, but the resulting complexes di d not cleave C3. Given that factor B and C2 share the same biological substrates and that part of the trypsin-like substrate specificity reg ion is not apparent in either protein, we propose that the conserved r egion plays a critical role in the conformational regulation of the ca talytic site and could offer a highly specific target for the therapeu tic inhibition of complement.