Stoichiometry of complexes between mannose-binding protein and its associated serine proteases. Defining functional units for complement activation.

Serum mannose-binding protein (MBP) initiates the lectin branch of the complement cascade by binding to sugars on the surfaces of microorganisms and activating two MBP-associated serine proteases (MASP-1 and MASP-2). Rat serum MBP consists of oligomers containing up to four copies of a subunit that is composed of three identical polypeptide chains. Biophysical analysis of intact and truncated MASPs indicates that each MASP is a homodimer that is stabilized through interactions involving an N-terminal CUB domain. The binding sites for MBP are formed from the three N-terminal MASP domains, in which two CUB modules interact with MBP. Each MASP dimer contains binding sites for two MBP subunits. Both sites must be occupied by subunits from a single MBP oligomer to form a stable complex. Thus, the smallest functional unit for complement activation consists of MBP dimers bound to MASP-1 or MASP-2 homodimers. Trimers and tetramers of MBP form complexes containing up to two MASPs. The results reveal how MASP-1 and MASP-2 can function independently to activate the complement cascade.