Molecular characterization of human complement factor B subtypes.

While several laboratories have agreed that there are two subtypes of the BFF alleles, no information has been available until now at the molecular level. The region of the BF gene corresponding to the Ba fragment [1.7 kilobases (kb)] of the BFS, BFFA, and BFFB alleles has been sequenced after specific amplification using the polymerase chain reaction (PCR). A point mutation at codon 7 has been revealed converting a cytosine in the BFS allele to a thymidine in BFFB. At the translational level an arginine residue in BFS is substituted for a tryptophan residue in BFFB. The amino-terminal sequencing of factor B immunoprecipitated from serum has been carried out from microquantities of protein blotted onto polyvinylidene fluoride (PVDF) membranes. We have shown that the difference between the BFFA and the BFFB subtypes in characterized by a glutamine at position 7 in BFFA and a tryptophan in BFFB.