MASP-1 (mannose/mannan binding lectin associated serine protease-1) is a serum protein (~79kDa poylpeptide) predominantly synthesized by the liver. It is an important player in the innate immune system and is mainly bound to multimeric pathogen recognition receptors such as mannose/mannan-binding lectin (MBL) and the three ficolins (M-ficolin, L-ficolin and H-ficolin). MASP-1 has two CUB, a calcium-binding EGF-like, a trypsin-like serine protease and two complement control protein (CCP) domains. The serine protease domain is auto-activated upon binding of these receptors to their appropriate pathogenic ligands, generally carbohydrate domains or acetylated sugar residues. MASP-1 is therefore a component of the lectin pathway of complement activation. The primary substrate for MASP-1 activity isMASP-2, another serine protease. MASP-2 in turn cleaves and activates complement proteins C4 and C2, thus converging the lectin pathway with the classical pathway of complement activation. MASP-1 activity is negatively regulated by the presence of alternate splice variants, MASP-3 and MAp44. MASP-1 by virtue of its serine protease activity, also plays a role in the coagulation pathway.

MASP-3 (mannose/mannan binding lectin (MBL) associated serine protease-3) is ~82 kDa protein generated through alternative splicing of the MASP1 gene. This gene also generates MASP-1 and MAp44 proteins. MASP-3 is bound to multimeric forms of pathogen receptors, such as MBL and the three ficolins. MASP-3 has two CUB, a calcium-binding EGF-like, a trypsin-like serine protease and two complement control protein (CCP) domains. The serine protease domain however, is not known to be active and does not act on substrates of either MASP-1 or MASP-2. Instead, it competes with MASP-1 and MASP-2 to bind to MBL and therefore plays a regulatory role in the lectin pathway of complement activation. In mice however, MASP-3 can activate the alternative complement pathway, by directly activating complement factor D (fD).