The factor H protein family.

The factor H gene family provides a prime example of a multidomain multifunctional protein family whose individual members are defined by conserved common structural elements and display diverse but often overlapping functions. The six identified members of this protein family represent secreted plasma proteins that are primarily synthesized in the liver. Here, we summarize the current understanding of the function of these proteins and suggest a common role in complement control. Factor H is the best characterized member and acts as a complement regulator. The protein displays cofactor activity for factor I in the degradation of the central complement component C3b, acts as a decay accelerating factor for the C3 convertase, C3bBb and is a competitor for factor B binding to C3b. Factor H is a multifunctional protein and displays functions outside the complement system: it binds to the cellular integrin receptor (CD11b/CD18), interacts with cell surface glycosaminoglycans and also binds to the surface of certain pathogenic microorganisms. In addition, factor H has several binding sites for the C3 protein. The factor H-like protein 1 (FHL-1) or reconectin shares the complement regulatory functions with factor H and interacts with heparin. The protein displays cell spreading activity and binds to the N-terminus of the streptococcal M protein. The function of the factor H-related proteins (FHR-1 to FHR-4) is currently under investigation. These proteins are differently distributed. Three proteins (FHR-1, FHR-2 and FHR-4) are constituents of lipoproteins, while FHR-3 interacts with heparin. Binding to C3b and C3d has been demonstrated for FHR-3 and FHR-4 and the two proteins display a cofactor related activity.