Interaction properties of human mannan-binding lectin (MBL)-associated serine proteases-1 and -2, MBL-associated protein 19, and MBL

The mannan-binding lectin (MBL) activation pathway of complement plays an i mportant role in the innate immune defense against pathogenic microorganism s. In human serum, two MBL-associated serine proteases (MASP-1, MASP-2) and MBL-associated protein 19 (MAp19) were found to be associated with MBL. Wi th a view to investigate the interaction properties of these proteins, huma n MASP-1, MASP-2, MAp19, as well as the N-terminal complement subcomponents C1r/C1s, Uegf, and bone morphogenetic protein-1-epidermal growth factor (C UB-EGF) segments of MASP-1 and MASP-2, were expressed in insect or human ki dney cells, and MBL was isolated from human serum. Sedimentation velocity a nalysis indicated that the MASP-1 and MASP-2 CUB-EGF segments and the homol ogous protein MAp19 all behaved as homodimers (2.8-3.2 S) in the presence o f Ca2+. Although the latter two dimers were not dissociated by EDTA, their physical properties were affected. In contrast, the MASP-1 CUB-EGF homodime r was not sensitive to EDTA. The three proteins and full-length MASP-1 and MASP-2 showed no interaction with each other as judged by gel filtration an d surface plasmon resonance spectroscopy. Using the latter technique, MASP- 1, MASP-2, their CUB-EGF segments, and MAp19 were each shown to bind to imm obilized MBL, with K-D values of 0.8 nM (MASP-2),1.4 nM (MASP-1), 13.0 nM ( MAp19 and MASP-2 CUB-EGF), and 25.7 nM (MASP-1 CUB-EGF). The binding was Ca 2+-dependent and fully sensitive to EDTA in all cases. These data indicate that MASP-1, MASP-2, and MAp19 each associate as homodimers, and individual ly form Ca2+-dependent complexes with MBL through the CUB-EGF pair of each protein. This suggests that distinct MBL/MASP complexes may be involved in the activation or regulation of the MBL pathway. The Journal of Immunology, 2001.