The amino acid sequence of the region of bovine factor H containing the C3b binding site has been derived from sequencing overlapping cDNA clones. A cDNA sequence encoding 669 amino acids was obtained. Like human and mouse factor H the sequence can be arranged into a number of internally 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 factor H, and is therefore likely to be composed of 20 contiguous CPs. Comparisons with human and mouse factor H indicate that the partial bovine sequence encodes CPs 2–12 inclusive of bovine factor H. Bovine factor H binds to human ammonia-treated C3 (causing thiolester cleavage) [C3(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. Multiple sequence alignments of human factor H, mouse factor H (which also interacts with human C3b) and bovine factor H with CP modules whose structures have been determined experimentally, have been used to predict residues in the hypervariable loops of CPs 2–5 and to identify residues of potential importance in human C3 binding and factor I cofactor activity. 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(NH3) directly, whilst CPs 2 and 5 maintain the correct orientation for CPs 3 and 4 to interact.

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