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C1s is the modular serine protease, which executes the catalytic function of the C1 complex: the cleavage of C4 and C2. Like other complement serine proteases C1s has restricted substrate specificity and it is engaged into specific interactions with other subcomponents of the complement system. There has been a rapid progress in determining the 3D structure of complement serine proteases and in revealing the role of the individual domains in the protein-protein interaction properties. In this review we summarize recent findings on the structure of C1s, and on the mechanism of action of this protease. The results obtained by genetic engineering, physico-chemical and functional studies are reviewed. The physiological relevance of the proteolytic action of C1s and its possible implications in health and disease will also be discussed.

C1r, C1s, MBL-associated serine protease (MASP)-1, MASP-2 and MASP-3 are mosaic serine proteases of the classical and lectin pathways of complement. They form a family of enzymes with identical domain organization and similar overall structure, but with different enzymatic properties. MASP-2 of the lectin pathway can autoactivate and cleave C4 and C2 components. In the classical pathway two enzymes mediate these functions: C1r autoactivates and activates C1s, while C1s cleaves C4 and C2. The substrate specificity and the biological function of MASP-1 and MASP-3 have not yet been completely resolved. MASP-1 can autoactivate and the activated MASP-1 has more relaxed substrate specificity than the other members of the family. It was demonstrated that MASP-1 can specifically cleave C2, C3 and fibrinogen, but the physiological relevance of these findings has to be proved. We do not know how MASP-3 becomes activated and its biological function is also not clear. In this review, we will summarize current knowledge about the structure and function of these proteases. Special emphasis will be laid on the specificity, autoactivation and evolution of these enzymes.

C1s is the modular serine protease, which executes the catalytic function of the C1 complex: the cleavage of C4 and C2. Like other complement serine proteases C1s has restricted substrate specificity and it is engaged into specific interactions with other subcomponents of the complement system. There has been a rapid progress in determining the 3D structure of complement serine proteases and in revealing the role of the individual domains in the protein-protein interaction properties. In this review we summarize recent findings on the structure of C1s, and on the mechanism of action of this protease. The results obtained by genetic engineering, physico-chemical and functional studies are reviewed. The physiological relevance of the proteolytic action of C1s and its possible implications in health and disease will also be discussed.

Our protein engineering studies on human C1r and C1s revealed important characteristics of the individual domains of these multidomain serine-proteases, and supplied evidence about the cooperation of the domains to create binding sites, and to control the activation process. We expressed the recombinant subcomponents in the baculovirus-insect cell system and checked the biological activity. Deletions and point mutants of C1r were constructed and C1r-C1s chimeras were also produced. Our deletion mutants demonstrated that the N-terminal CUB domain and the EGF-like domain of C1r together are responsible for the calcium dependent C1r-C1s interaction. It seems very likely that these two modules form the calcium-binding site of the C1r alpha-fragment and participate in the tetramer formation. The deletion mutants also demonstrated that the N-terminal region of the C1r molecule contains essential elements involved in the control of activation of the serine-protease module. The substrate specificity of the serine-protease is also determined by the five N-terminal noncatalytic domain of C1r/C1s chimera, which contains the catalytic domain of C1s preceded by the N-terminal region of C1r, could replace the C1r in the hemolytically active C1 complex. The C1s/C1r chimera, in which the alpha-fragment of the C1r was replaced for that of the C1s exibits both C1r- and C1s-like characteristics. We stabilized the zymogen form of human C1r by mutating the Arg(463)-Ile(464) bond. Using our stable zymogen C1r we showed that one active C1r in the C1 complex is sufficient for the full activity of the entire complex. Further experiment with this mutant could provide us with important information about the structure of the C1 complex.