Abstracts (complete list) - Wissenschaft Online

Michael Mihlan, Mario Hebecker, Markus Huber-Lang, Peter F. Zipfel, Mihály Józsi
Characterization of ligand binding of the human Factor Hrelated
protein 4 (FHR-4) provides insight into function
Factor H-related protein 4 (FHR-4) is a unique member of the factor H protein family, as
two isoforms has been described. FHR-4A has nine short consensus repeat (SCR)
domains and includes a duplicated region of four domains, where SCR1-4 and SCR5-8
are closely related. FHR-4B is a short isoform and consists of five SCRs, corresponding
to SCR1 and SCR6-9 of FHR-4A.
In order to analyze the function of FHR-4, the two isoforms and fragments of FHR-4A,
representing SCR1-3, SCR4-9, SCR5-7 and SCR8-9, were recombinantly expressed and
their binding properties to the central complement component C3b and to the acute
phase protein C-reactive protein (CRP) were analyzed. Both FHR-4 isoforms as well as
SCR4-9 and SCR8-9 bound to C3b, suggesting a binding site in the two C-terminal
domains. In fluid phase cofactor assay, both FHR-4 isoforms enhanced the cofactor
activity of factor H for C3b cleavage, but the FHR-4 variants alone lacked cofactor
activity.
Both FHR-4A and FHR-4B bound to CRP in the presence of Ca2+, independent of ionic
strength. SCR1-3 but not the other FHR-4A fragments bound to CRP, indicating that
SCR1 is necessary for CRP binding. CRP-binding was confirmed by surface plasmon
resonance analysis and by coimmunoprecipitation. In addition, complexes of FHR-4A
and CRP were isolated from serum of sepsis patients by immunoprecipitation, thus
demonstrating in vivo interaction between the two proteins.
In summary, we identify complement C3b and CRP binding domains of FHR-4A. The
binding characteristics of the protein fragments give further insight into the function of
the FHR-4 proteins, as the separation of binding sites allows simultaneous binding of
FHR-4A to C3b and CRP. Thus, our data indicate a role of FHR-4 in innate immune
response and inflammation.