Structural and functional studies on C1r and C1s: New insights into the mechanisms involved in C1 activity and assembly

C1r and C1s, the enzymes responsible for the activation and proteolytic activity of the C1 complex of complement, are modular serine proteases featuring similar overall structural organizations, yet expressing very distinct functional properties within C1. This review will initially summarize available information on the structure and function of the: protein modules and serine protease domains of C1r and C1s. It will then focus on the regions of both proteases involved in: (i) assembly of C1s-C1r-C1r-C1s, the Ca2+-dependent tetrameric catalytic subunit of C1; (ii) expression of C1 catalytic activities. Particular emphasis will be laid on recent structural and functional studies that provide new insights into the complex mechanisms involved in the assembly, activation, and proteolytic activity of C1.