title: MAp19, the alternative splice product of the MASP2 gene.
abstract: The lectin pathway of complement is a central part of innate immunity, but as a powerful inducer of inflammation it needs to be tightly controlled. The MASP2 gene encodes two proteins, MASP-2 and MAp19. MASP-2 is the serine protease responsible for lectin pathway activation. The smaller alternative splice product, MAp19, lacks a catalytic domain but retains two of three domains involved in association with the pattern-recognition molecules (PRMs): mannan-binding lectin (MBL), H-ficolin, L-ficolin and M-ficolin. MAp19 reportedly acts as a competitive inhibitor of MASP-2-mediated complement activation. In light of a ten times lower affinity of MAp19, versus MASP-2, for association with the PRMs, much higher serum concentrations of MAp19 than MASP-2 would be required for MAp19 to exert such an inhibitory activity. Just four amino acid residues distinguish MAp19 from MASP-2, and these are conserved between man, mouse and rat. Nonetheless we generated monoclonal rat anti-MAp19 antibodies and established a quantitative assay. We found the concentration of MAp19 in serum to be 217 ng/ml, i.e., 11nM, comparable to the 7 nM of MASP-2. In serum all MASP-2, but only a minor fraction of MAp19, was associated with PRMs. In contrast to previous reports we found that MAp19 could not compete with MASP-2 for binding to MBL, nor could it inhibit MASP-2-mediated complement activation. Immunohistochemical analyses combined with qRT-PCR revealed that both MAp19 and MASP-2 were mainly expressed in hepatocytes. High levels of MAp19 were found in urine, where MASP-2 was absent.
authors.affiliation: Department of Biomedicine, Aarhus University, Wilhelm Meyers Allé 4, DK-8000 Aarhus C, Denmark. sdegn@microbiology.au.dk
authors.forename: Søren E
authors.initials: SE
authors.lastname: Degn
authors.forename: Steffen
authors.initials: S
authors.lastname: Thiel
authors.forename: Ole
authors.initials: O
authors.lastname: Nielsen
authors.forename: Annette G
authors.initials: AG
authors.lastname: Hansen
authors.forename: Rudi
authors.initials: R
authors.lastname: Steffensen
authors.forename: Jens C
authors.initials: JC
authors.lastname: Jensenius
mesh_terms: D017398:Alternative Splicing; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology*; D000915:Antibody Affinity / Q000276:immunology; D015153:Blotting, Western; D003593:Cytoplasm / Q000378:metabolism; D020869:Gene Expression Profiling; D057809:HEK293 Cells; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D006825:Hybridomas; D007150:Immunohistochemistry; D007668:Kidney / Q000378:metabolism; D037102:Lectins / Q000276:immunology / Q000378:metabolism; D016676:Macrophages, Alveolar / Q000378:metabolism; D037601:Mannose-Binding Lectin / Q000276:immunology / Q000378:metabolism; D050606:Mannose-Binding Protein-Associated Serine Proteases / Q000032:analysis / Q000235:genetics* / Q000276:immunology*; D051379:Mice; D011485:Protein Binding; D020033:Protein Isoforms / Q000097:blood / Q000378:metabolism / Q000652:urine; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000097604:Ficolins
chemical_list: D000911:Antibodies, Monoclonal; D037102:Lectins; D037601:Mannose-Binding Lectin; D050606:Mannose-Binding Protein-Associated Serine Proteases; D020033:Protein Isoforms; D000097604:Ficolins; C105296:MASP2 protein, human