Mannose-binding lectin (MBL) and ficolins (Ficolin-1, Ficolin-2 and Ficolin-3) are Ca-dependent (C-type) lectins, which initiate the complement cascade after binding to specific carbohydrate patterns on the target cell surface. Both MBL and ficolins form oligomers of structural subunits. Each subunit of lectin is composed of three identical polypeptides (Jensen PH et al 2005, Dommett RM et al 2006; Garlatti V et al 2010). MBL and ficolins circulate in plasma in complexes with homodimers of MBL-associated serine proteases (MASP) (Fujita et al. 2004; Hajela et al. 2002). Three types of MASPs, MASP-1, MASP-2 and MASP-3, were reported to mediate the complement activation. Upon binding of human lectin (MBL or ficolins) to the target surface the complex of lectin:MASP undergoes conformational changes, which results in the activation of MASPs by cleavage (Matsushita M et al. 2000; Fujita et al. 2004). Catalytically active MASP-2 cleaves C4 to generate C4a and C4b. C4b binds to the bacterial or foreign cell surface via its thioester bond and then binds circulating C2 (Law and Dodds 1997). Bound C2 is cleaved by MASP-2 to yield the C3 convertase C4b-C2a. Active form of MASP-1 was reported to cleave C2 in a manner similar to MASP-2 (Matsushita M et al. 2000; Chen CB & Wallis R 2004). Besides, MASP-1 was shown to cleave proenzyme MASP-2 leading to the complement activation (Heja D et al. 2012). Moreover, MASP-1 was found to cleave fibrinogen to yield fibrinopeptide B, and cleave and activate factor XIII. MASP-1 may have a role in cleavage of 'dead C3', i.e. C3(H2O) (Hajela et al. 2002).
In addition to MASP1,2 and 3 two truncated forms of MASP (MAp44 and sMAP) have been implicated in the complement cascade signaling. MASP-1, MAp44 and MASP-3 are generated from the MASP1-encoded gene by an alternative splicing, while sMAP also known as Map19 is a truncated alternative splicing product of the MASP2 gene (Takahashi M et al. 1999; Degn et al. 2010). The functions of MASP-3, sMAP, and MAp44 in the lectin pathway remain to be clarified.