Shiga toxin 2 (Stx2) is believed to be a major virulence factor of enterohemorrhagic Escherichia coli (EHEC) contributing to hemolytic uremic syndrome (HUS). The complement system has recently been found to be involved in the pathogenesis of EHEC-associated HUS. Stx2 was shown to activate complement via the alternative pathway, to bind factor H (FH) at short consensus repeats (SCRs) 6-8 and 18-20 and to delay and reduce FH cofactor activity on the cell surface. We now show that complement factor H-related protein 1 (FHR-1) and factor H-like protein 1 (FHL-1), proteins of the FH protein family that show amino acid sequence and regulatory function similarities with FH, also bind to Stx2. The FHR-1 binding site for Stx2 was located at SCRs 3-5 and the binding capacity of FHR-1*A allotype was higher than that of FHR-1*B. FHR-1 and FHL-1 competed with FH for Stx2 binding, and in the case of FHR-1 this competition resulted in a reduction of FH cofactor activity. FHL-1 retained its cofactor activity in the fluid phase when bound to Stx2. In conclusion, multiple interactions of key complement inhibitors FH, FHR-1 and FHL-1 with Stx2 corroborate our hypothesis of a direct role of complement in EHEC-associated HUS.
Keywords: Complement regulatory proteins; EHEC; FH; FHL-1; FHR-1; FI; Factor H; Factor H-like protein 1; Factor H-related protein 1; HUS; Hemolytic uremic syndrome; MCP; SCRs; Shiga toxin; Stx; TCC; aHUS; atypical HUS; enterohemorrhagic Escherichia coli; factor H; factor H-like protein 1; factor H-related protein 1; factor I; heat-inactivated Stx; hemolytic uremic syndrome; hi-Stx; membrane cofactor protein; short consensus repeats; terminal complement complex.
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