Functions in the lectin pathway of complement, which performs a key role in innate immunity by recognizing pathogens through patterns of sugar moieties and neutralizing them. The lectin pathway is triggered upon binding of mannan-binding lectin (MBL) and ficolins to sugar moieties which leads to activation of the associated proteases MASP1 and MASP2. Functions as an endopeptidase and may activate MASP2 or C2 or directly activate C3 the key component of complement reaction. Isoform 2 may have an inhibitory effect on the activation of the lectin pathway of complement or may cleave IGFBP5. Also plays a role in development (PubMed:21258343). Inhibited by SERPING1 and A2M. Homodimer. Interacts with the oligomeric lectins MBL2, FCN2 and FCN3; triggers the lectin pathway of complement through activation of C3. Interacts with SERPING1. Interacts with COLEC11; probably triggers the lectin pathway of complement (PubMed:20956340). Protein of the plasma which is primarily expressed by liver. The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. N-glycosylated. Some N-linked glycan are of the complex-type (By similarity). Autoproteolytic processing of the proenzyme produces the active enzyme composed on the heavy and the light chain held together by a disulfide bond. Isoform 1 but not isoform 2 is activated through autoproteolytic processing. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase S1 family. Extended N-terminus.