Decoupling of carbohydrate binding and MASP-2 autoactivation in variant mannose-binding lectins associated with immunodeficiency.
Mannan-binding lectin (MBL) initiates complement activation by binding to arrays of carbohydrates on the surfaces of pathogenic microorganisms and activating MBL-associated serine proteases (MASPs). Separate point mutations to the collagenous domain of human MBL are associated with immunodeficiency, caused by reduced complement activation by the variant MBLs as well as by lower serum MBL concentrations. In the work reported here, we have used the well characterized rat lectin pathway to analyze the molecular and functional defects associated with two of the variant proteins. Mutations Gly25 --> Asp and Gly28 --> Glu create comparable structural changes in rat MBL but the G28E variant activates complement >10-fold less efficiently than the G25D variant, which in turn has approximately 7-fold lower activity than wild-type MBL. Analysis of mutant MBL . MASP-2 complexes assembled from recombinant components shows that reduced complement activation by both mutant MBLs is caused by failure to activate MASP-2 efficiently on binding to a mannan-coated surface. Disruption of MBL-MASP-2 interactions as well as to changes in oligomeric structure and reduced binding to carbohydrate ligands compared with wild-type MBL probably account for the intermediate phenotype of the G25D variant. However, carbohydrate binding and MASP-2 activation are ostensibly completely decoupled in complexes assembled from the G28E mutant, such that the rate of MASP-2 activation is no greater than the basal rate of zymogen MASP-2 autoactivation. Analogous molecular defects in human MBL probably combine to create the mutant phenotypes of immunodeficient individuals.
Wallis
Russell
R
Department of Infection, Immunity, and Inflammation, University of Leicester, Leicester, United Kingdom.
Lynch
Nicholas J
NJ
Roscher
Silke
S
Reid
Kenneth B
KB
Schwaeble
Wilhelm J
WJ
D019943:Amino Acid Substitution; D000818:Animals; D050260:Carbohydrate Metabolism; D037582:Complement Pathway, Mannose-Binding Lectin; D006224:Cricetinae; D004789:Enzyme Activation; D006801:Humans; D007153:Immunologic Deficiency Syndromes / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D037241:Mannose-Binding Lectins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D050606:Mannose-Binding Protein-Associated Serine Proteases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D010641:Phenotype; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D037241:Mannose-Binding Lectins; D011994:Recombinant Proteins; D050606:Mannose-Binding Protein-Associated Serine Proteases; C491338:Masp2 protein, rat