ABSTRACT
This chapter reviews the structural data generated by the approach and shows how these provide insights into the architecture and function of the C1 complex at the atomic level. The classical pathway of complement is triggered by the C1 complex, a multimo-lecular protease that combines the ability to bind to pathogens and to convert a recognition signal into specific proteolytic reactions. Structural biology is being used to generate more detailed information about the structure of C1 at the atomic level. A major difficulty lies in the fact that C1 is a noncovalent assembly of three proteins, each exhibiting a modular structure with areas of flexibility. C1r and C1s each contain a single EGF-like module. This module is extremely widespread and has been identified in a variety of extracellular and membrane-bound proteins involved in diverse biological functions. The serine protease domains of C1r and C1s contain 242 and 251 amino acids, respectively, and both belong to the chymotrypsin-like family.
