Biochemical and functional characterization of the factor-H-related protein 4 (FHR-4).

Factor-H-related proteins and the complement regulatory protein factor H represent a family of structurally and immunologically related plasma proteins. The function of the various factor-H-related proteins are currently unclear and under investigation. The newest member of this group of proteins, the factor-H-related protein 4 (FHR-4) has recently been identified as an amphipathic protein, that is present in free form in human plasma and also as a constituent of triglyceride-rich lipoproteins. In plasma FHR-4 occurs exclusively in a dimeric form, that most likely represents a homodimer consisting of two identical FHR-4 monomers. In order to identify the function of the FHR-4 protein we have recombinantly expressed the protein in the baculovirus system. The recombinant protein is detected in the supernatant of infected insect cells, both in its monomeric and dimeric form. Both the native form (86 kDa) and the recombinant (84 kDa) proteins are posttranslationally modified. Lectin staining showed that the differences in the apparent molecular masses are due to distinct types of attached N-carbohydrate side chains. Functional analyses show dose-dependent binding of recombinant FHR-4 to C3b, thus demonstrating a functional relatedness between FHR-4, factor H and FHL-1 and other complement regulators of the RCA gene cluster.