Structure-function studies on full-length human complement proteases
The complement system is an essential element of innate immunity. This molecular network of about 40 proteins provides one of the first defense lines against pathogenic microorganisms. In essence, it is a proteolytic cascade system, which activates only, when it encounters dangerous structures (bacteria, viruses, cancerous or damaged host cells) in the organism. Within the complement system, the activation of the lectin pathway relies on supramolecular complexes composed of a pattern recognition molecule (mannose binding lectin = MBL, ficolins, collectins) and associated serine protease (MBL-associated serine protease = MASP) dimers. The structure of the complexes and the exact mechanism of their activation is still not known. There are three known MASP enzymes (MASP-1, MASP-2 and MASP-3) having similar domain structure: a C-terminal catalytic domain preceded by five non-catalytic domains. High-resolution X-ray structure is available only for smaller C- and N-terminal fragments of the MASP 3enzymes. Structural information is available neither for the monomeric, nor for the dimeric state of the full-length (6-domain) molecules. The main goal of our project is to make a first attempt in solving the structure of the full-length dimer MASPs and get closer to the understanding of the activation mechanism. The full-length molecules, in their own, appear to be too flexible for crystallization. We plan to express full-length MASP molecules as recombinant proteins and form dimers from them (RCNS, Institute of Enzymology). In parallel we develop bivalent inhibitors capable of cross-linking two protease heads and form circular (intra-dimer) of filamentous (inter-dimer) complexes (ELTE, Dept. of Biochemistry). In this way, on one hand we can test the complement inhibitory potential of these extraordinary inhibitors, while on the other hand we can make an attempt for the crystallization of these complexes. For the circular complexes, crystallization attempts will be also made in complex with the pattern recognition molecules.
Péter Gál, Gábor Pál