ABSTRACT
A characteristic of all of the components of the complement pathway is that they are modular proteins, built up either of single copies of several different domains or of multiply repeated copies of a single domain. The occurrence of complement-like repeats (CR) in complement proteins and in low-density lipoprotein receptor family members shows a distinct pattern. The identification of a region of polypeptide as being a CR domain is based on a distinct primary structure pattern of six conserved cysteine residues and certain conserved acidic residues. The small size of the CR domain precludes the existence of a large hydrophobic core, there is nevertheless a small number of hydrophobic residues whose side chains pack together, and which are required for efficient folding and stability of the domain. Nuclear magnetic resonance examination of tandem pairs of CR domains in solution has allowed the examination of the freedom of movement of one domain relative to another.
