STRUCTURE OF THE CATALYTIC REGION OF HUMAN-COMPLEMENT PROTEASE C(1)OVER-BAR-S - STUDY BY CHEMICAL CROSS-LINKING AND 3-DIMENSIONAL HOMOLOGY MODELING

C(1) over bars$ is a multidomain serine protease that is responsible for the enzymic activity of C (1) over bar, the first component of the classical pathway of complement. Its catalytic region (gamma-B) comprises two contiguous complement control protein (CCP) modules, IV and V (about 60 residues each), a 15-residue intermediary segment, and the B chain (251 residues), which is the serine protease domain. With a view to identify domain-domain interactions within this region, the gamma-B fragment of C(1) over bars$, obtained by limited proteolysis with plasmin, was chemically cross-linked with the water-soluble carbodiimide 1-ethyl-3-[3(dimethylamino)propyl]carbodiimide; then cross-linked peptides were isolated after CNBr cleavage and thermolytic digestion. N-Terminal sequence and mass spectrometry analyses allowed us to identify two cross-links between Lys 405 of module V and Glu 672 of the B chain and between Glu 418 of the intermediary segment and Lys 608 of the B chain. Three-dimensional modeling of the CCP modules IV and V and of the catalytic B chain was also carried out on the basis of their respective homology with the 16th and 5th CCP modules df complement factor H and type I serine proteases. The information provided by both the chemical cross-linking studies and the homology modeling enabled us to construct a three-dimensional model for the assembly of the C-terminal part of the gamma-B region, comprising module V, the intermediary segment, and the B chain. This model shows that module V interacts with the serine protease B chain on the side opposite to both the activation site and the catalytic site. Functional implications of this interaction are discussed in terms of the possible role of module V in the specific recognition and positioning of C4, one of the two substrates of C(1) over bars$.