Two mechanisms for mannose-binding protein modulation of the activity of its associated serine proteases.
Serum mannose-binding protein (MBP) neutralizes invading microorganisms by binding to cell surface carbohydrates and activating MBP-associated serine proteases-1, -2, and -3 (MASPs). MASP-2 subsequently cleaves complement components C2 and C4 to activate the complement cascade. To analyze the mechanisms of activation and substrate recognition by MASP-2, zymogen and activated forms have been produced, and MBP.MASP-2 complexes have been created. These preparations have been used to show that MBP modulates MASP-2 activity in two ways. First, MBP stimulates MASP-2 autoactivation by increasing the rate of autocatalysis when MBP.MASP-2 complexes bind to a glycan-coated surface. Second, MBP occludes accessory C4-binding sites on MASP-2 until activation occurs. Once these sites become exposed, MASP-2 binds to C4 while separate structural changes create a functional catalytic site able to cleave C4. Only activated MASP-2 binds to C2, suggesting that this substrate interacts only near the catalytic site and not at accessory sites. MASP-1 cleaves C2 almost as efficiently as MASP-2 does, but it does not cleave C4. Thus MASP-1 probably enhances complement activation triggered by MBP.MASP-2 complexes, but it cannot initiate activation itself.
Chen
Ce-Belle
CB
Glycobiology Institute, Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom.
Wallis
Russell
R
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D002462:Cell Membrane / Q000378:metabolism; D003001:Cloning, Molecular; D003167:Complement Activation; D003175:Complement C2 / Q000378:metabolism; D003181:Complement C4 / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D006868:Hydrolysis; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D037601:Mannose-Binding Lectin / Q000737:chemistry / Q000378:metabolism*; D050606:Mannose-Binding Protein-Associated Serine Proteases; D008954:Models, Biological; D008969:Molecular Sequence Data; D011485:Protein Binding; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D013696:Temperature; D013997:Time Factors; D014461:Ultracentrifugation
D003175:Complement C2; D003181:Complement C4; D018076:DNA, Complementary; D037601:Mannose-Binding Lectin; D011994:Recombinant Proteins; C084067:MASP1 protein, human; C105296:MASP2 protein, human; D050606:Mannose-Binding Protein-Associated Serine Proteases; D012697:Serine Endopeptidases