Oligomeric domain structure of human complement factor H by X-ray and neutron solution scattering.

Factor H is a regulatory component of the complement system. It has a monomer Mr of 150,000. Primary structure analysis shows that the polypeptide is divided into 20 homologous regions, each 60 amino acid residues long. These are independently folding domains and are termed "short consensus repeats" (SCRs) or "complement control protein" (CCP) repeats. High-flux synchrotron X-ray and neutron scattering studies were performed in order to define its solution structure in conditions close to physiological. The Mr of factor H was determined as 250,000-320,000 to show that factor H is dimeric. This structure is maintained at concentrations between 1 and 11 mg/mL in the pH range 5-9. Zn2+ ions are an inhibitor of C3b cleavage by factor I, a reaction in which factor H acts as a cofactor. Additions of Zn2+ to factor H caused it to form oligomers containing 4-10 monomers. The radius of gyration RG of native factor H by X-rays or by neutrons in 0% or 100% 2H2O buffers is not measurable but is greater than 12.5 nm. Two cross-sectional radii of gyration RXS-1 and RXS-2 were determined as 3.0-3.1 and 1.8 nm, respectively. Analyses of the cross-sectional intensities show that factor H is composed of two distinct subunits. The RXS-1 corresponds to the cross-sectional properties of both subunits and exhibits an unusual radiation dependence on the X-ray flux. Since RXS-2 is close to the corresponding RXS of C4b binding protein (91% of which is formed from SCR/CCP domains), it is inferred that the SCR/CCP domains of factor H and C4b binding protein have similar solution structures. The use of hydrodynamic spheres to reproduce literature sedimentation coefficients of 5.5-5.6 S showed that these were compatible with a V-shaped arrangement of two rods (36 spheres each, length 87 +/- 5 nm) joined at an angle of 5 degrees. The use of a similar arrangement of 244 spheres arranged in two rods (length 77 nm) to fit the experimental X-ray and neutron scattering curves showed that the two rods are joined at an angle of 5 degrees. This model corresponds to an actual RG of 21-23 nm. The separation between each SCR/CCP in factor H is close to 4 nm. In the solution structure of factor H, the SCR/CCP domains are in a highly extended conformation.