Disruption of disulfide bonds is responsible for impaired secretion in human complement factor H deficiency

Factor H, a secretory glycoprotein composed of 20 short consensus repeat mo dules, is an inhibitor of the complement system. Previous studies of inheri ted factor H deficiency revealed single amino acid substitutions at conserv ed cysteine residues, on one allele arginine for cysteine 518 (C518R) and o n the other tyrosine for cysteine 941 (C941Y) (Ault, B. H,, Schmidt, B, Z,, Fowler, N, L., Kashtan, C, E,, Ahmed, A. E,, Vogt, B, A., and Colten, H, R , (1997) J, Biol. Chem, 272, 25168-25175), To ascertain if the phenotype, i mpaired secretion of factor H, is due to the C518R substitution or the C941 Y substitution and to ascertain the mechanism by which secretion is impaire d, we studied COS-1 and HepG2 cells transfected with wild type and several mutant factor H molecules, The results showed markedly impaired secretion o f both C518R and C941Y factor H as well as that of factor H molecules beari ng alanine or arginine substitutions at the Cys(518)-Cys(546) disulfide bon d (C518A, C546A, C546R, C518A-C546A), In each case, mutant factor H was ret ained in the endoplasmic reticulum and degraded relatively slowly as compar ed with most other mutant secretory and membrane proteins that are retained in the endoplasmic reticulum, These data indicate that impaired secretion of the naturally occurring C518R and C941Y mutant factor H proteins is due to disruption of framework-specific disulfide bonds in factor H short conse nsus repeat modules.