Substrate specificities of recombinant mannan-binding lectin-associated serine proteases-1 and-2

Mannan-binding lectin (MBL)-associated serine proteases-1 and 2 (MASP-1 and MASP-2) are homologous modular proteases that each interact with MBL, an o ligomeric serum lectin involved in innate immunity. To precisely determine their substrate specificity, human MASP-1 and MASP-2, and fragments from th eir catalytic regions were expressed using a baculovirus/insect cells syste m. Recombinant MASP-2 displayed a rather wide, Cls-like esterolytic activit y, and specifically cleaved complement proteins C2 and C4, with relative ef ficiencies 3- and 23-fold higher, respectively, than human Cls. MASP-2 also showed very weak C3 cleaving activity. Recombinant MASP-1 had a lower and more restricted esterolytic activity. It showed marginal activity toward C2 and C3, and no activity on C4. The enzymic activity of both MASP-1 and MAS P-2 was specifically titrated by C1 inhibitor, and abolished at a 1:1 C1 in hibitor:protease ratio. Taken together with previous findings, these and ot her data strongly support the hypothesis that MASP-2 is the protease that, in association with MBL, triggers complement activation via the AML pathway , through combined self-activation and proteolytic properties devoted to C1 r and Cls in the C1 complex. In view of the very low activity of MASP-1 on C3 and C2, our data raise questions about the implication of this protease in complement activation.