Two monomers of the complement deregulator FHR5 associate to form a compact dimer to provide new insights into CFHR5 nephropathy

The activation and regulation of complement is a significant requirement for the clearance of pathogens and prevention of complement-mediated host cell damage. The main circulating complement regulator Factor H is antagonized by the human complement Factor H–related 5 protein (FHR5). Usually, FHR5 is made up of nine short complement regulator (SCR) domains. However, a FHR5 mutant has been discovered and it has a duplicated N-terminal SCR-1/2 domain pair that causes complement FHR5 nephropathy. The first time complement FHR5 was identified was in glomerular immune deposits from patients with glomerulonephritis. Complement FHR5 is part of a family of structurally related proteins comprised of five complement Factor H–related proteins and the major serum complement regulator Factor H. Factor H contains 20 SCR domains which have been extensively characterized in terms of function, structure, binding to activated C3b and its fragment C3d and regulating excess C3 activation. The specific physiological function of FHR5 is however not well understood.

In a new study by scientists from University College London: Dr. Nilufar Kadkhodayi-Kholghi, Dr Jayesh Bhatt, Dr. Jayesh Gor, Professor Daniel P. Gale and Professor Stephen J. Perkins elucidated the solution conformation of full-length FHR5 and identified its role in healthy people and its involvement in complement FHR5 nephropathy. The authors demonstrated the involvement of FHR5 in the activation of complement in the kidney and how complement FHR5 nephropathy arises. Dr. Lindsay McDermott from the University of Bedfordshire was also a co-author and contributed to the research. The original research article is now published in the Journal of Biological Chemistry.

The research team first purified full-length FHR5 protein. Then, with the use of SEC-MALLS, the solubility and mass of FHR5 were determined. Ultraviolet and refractive index measurements were used to detect FHR5 following its elution from a size-exclusion column. Multi-angle light scattering was used in parallel to analyze the size distributions of the eluate. In the size-exclusion elution profile, three peaks were observed. The major  peak had lower light scattering but higher ultraviolet and refractive index values than the other two peaks, consistent with non-aggregated, soluble protein The estimated molecular mass of this protein peak corresponded to FHR5 dimer formation.

The presence of FHR5 dimer was confirmed when the authors investigated the shape and oligomerization of FHR5, using sophisticated analytical ultracentrifugation sedimentation velocity experiments and size distribution c(s) analyses to determine its molecular mass and its solution shape from the sedimentation coefficient s_20,w. Furthermore, to explore the solution structure of the FHR5 dimer in concentration series, they also employed small-angle X-ray scattering in three separate buffers, two being physiological and one being low-salt.

At the time of this study, there was no atomic level structural information on FHR5. Therefore, the team created a starting model for the FHR5 monomer with the use of comparative modeling. This enabled them to identify the atomistic-level solution structure for the FHR5 dimer. Based on this model, they were able to show how the mutant FHR5 can form oligomers that have extra binding sites for C3b in FHR5. The duplicated N-terminal SCR-1/2 domains are able to link up with additional FHR5 molecules, clarifying that this is a gain-of-function variant of FHR5.

This study is a landmark research because the FHR5 domain organization was previously unknown. Through the use of a combination of analytical ultracentrifugation and small-angle X-ray scattering, combined with molecular simulations, the authors performed in depth FHR5 analysis. They successfully demonstrated the dimeric structure of FHR5 and its conformation, and explained the major features of the pathology and mechanism of complement FHR5 nephropathy and how FHR5 is involved. The findings will contribute to better understand FHR5 nephropathy and provide new therapeutic options in the future. Professor Perkins commented: “When we extracted the first native FHR5 dimer model from the simulations, we were thrilled to find that it was so easy to add an extra SCR-1/2 domain pair to this model, thereby offering a simple explanation for the molecular basis of the mutant in CFHR5 nephropathy”.