Interaction of R139W with MCP and FH. R139 position (A) on the structure of C3b in a complex with FH CCP1-4 or (B) in a model complex with MCP. R139 is close to CCP3 binding site of both FH and MCP. Direct binding of recombinant WT or mutant C3 to (C) FH or (D) MCP, studied by ELISA. FCS-free supernatant, containing recombinant WT or mutant C3 produced by stably transfected CHO cells, was used as a source of C3. SN0 is the supernatant of cells transfected with a plasmid not containing the C3 gene. The real-time binding of recombinant WT or mutant C3 to (E) FH or (F) MCP was studied by SPR, using recombinant C3 proteins purified from serum-free culture supernatants by DEAE-Sepharose column.