PREDICTION FROM SEQUENCE COMPARISONS OF RESIDUES OF FACTOR-H INVOLVEDIN THE INTERACTION WITH COMPLEMENT COMPONENT C3B

The amino acid sequence of the region of bovine factor H containing th e C3b binding site has been derived from sequencing overlapping cDNA c lones, A cDNA sequence encoding 669 amino acids was obtained. Like hum an and mouse factor H the sequence can be arranged into a number of in ternally homologous units (CPs), each of which is about 60 amino acids long and is based on a framework of four conserved cysteine residues. Bovine factor H is of the same molecular mass as human and mouse fact or H, and is therefore likely to be composed of 20 contiguous CPs. Com parisons with human and mouse factor H indicate that the partial bovin e sequence encodes CPs 2-12 inclusive of bovine factor H. Bovine facto r H binds to human ammonia-treated C3 (causing thiolester cleavage) [C 3(NH3)] and promotes the cleavage of human C3(NH3) in the presence of bovine factor I. Other studies indicate that CPs 2-5 of human factor H encompass the C3b binding and factor I cofactor activity site. Multip le sequence alignments of human factor H, mouse factor H (which also i nteracts with human C3b) and bovine factor H with CP modules whose str uctures have been determined experimentally, have been used to predict residues in the hypervariable loops of CPs 2-5 and to identify residu es of potential importance in human C3 binding and factor I cofactor a ctivity. Leu-17 and Gly-20 of CP 2, Ser-17, Ala-19, Glu-21, Asp-23 and Glu-25 of CP 3 and Lys-18 of CP 4 are all conserved between the three species. It may be that CPs 3 and 4 interact with C3(NH,) directly, w hilst CPs 2 and 5 maintain the correct orientation for CPs 3 and 4 to interact.