Impaired secretion of rat mannose-binding protein resulting from mutations in the collagen-like domain

Serum mannose-binding protein (MBP) or mannose-binding lectin initiates the lectin branch of the innate immune response by binding to the surface of potentially pathogenic microorganisms and initiating complement fixation through an N-terminal collagen-like domain. Mutations in this region of human MBP are associated with immunodeficiency resulting from a reduction in the ability of the mutant MBPs to fix complement as well as from reduced serum concentrations. Inefficient secretion of the mutant proteins, which is one possible cause of the reduced serum levels, has been investigated using a mammalian expression system in which each of the naturally occurring human mutations has been recreated in rat serum MBP. The mutations Gly25-->Asp and Gly28-->Glu disrupt the disulfide-bonding arrangement of the protein and cause at least a 5-fold increase in the half-time of secretion of MBP compared with wild-type rat serum MBP. A similar phenotype, including a 3-fold increase in the half-time of secretion, disruption of the disulfide bonding arrangement, and inefficient complement fixation, is observed when nearby glucosylgalactosyl hydroxylysine residues at positions 27 and 30 are replaced with arginine residues. The results suggest that defective secretion resulting from structural changes in the collagen-like domain is likely to be a contributory factor for MBP immunodeficiency.