Interaction of mannose-binding protein with associated serine proteases - Effects of naturally occurring mutations

Mannose binding protein (MBP; mannose-binding lectin) forms part of the inn ate immune system. By binding directly to carbohydrates on the surfaces of potential microbial pathogens, MBP and MBP-associated serine proteases (MAS Ps) can replace antibodies and complement components C1q, C1r, and C1s of t he classical complement pathway. In order to investigate the mechanisms of MASP activation by MBP, the cDNAs of rat MASP-1 and -2 have been isolated, and portions encompassing the N-terminal CUB and epidermal growth factor-li ke domains have been expressed and purified. Biophysical characterization o f the purified proteins indicates that each truncated MASP is a Ca2+-indepe ndent homodimer in solution, in which the interacting modules include the N -terminal two domains. Binding studies reveal that both MASPs associate ind ependently with rat MBP in a Ca2+-dependent manner through interactions inv olving the N-terminal three domains. The biophysical properties of the trun cated MASPs indicate that the interactions with MBP leading to complement a ctivation differ significantly from those between components C1q, C1r, and C1s of the classical pathway. Analysis of MASP binding by rat MBP containin g naturally occurring mutations equivalent to those associated with human i mmunodeficiency indicates that binding to both truncated MASP-1 and MASP-2 proteins is defective in such mutants.