Functional domains of the human C1q A-chain

This brief review was inspired by discussions relating to the IIIrd. International C1 Workshop (this volume) and the realization that certain functional properties of the C1q molecule are limited exclusively to the A-chain. The collagen-like region of the A-chain contains a major binding site for non-immunoglobulin substances, which include C-reactive protein, serum amyloid P, LPS and DNA. This binding site is immediately adjacent to, and partially overlapping with, an arthritis-modulating epitope common to the C1q A-chain and various types of collagen, including cartilage type II collagen. At the N-terminal end of the C1q A-chain is a leader peptide sequence that anchors the intact C1q molecule firmly in the membrane of macrophages, the C1q molecule can thus be classified as a type II membrane protein, functioning as an additional receptor for molecules known to react with C1q in fluid phase such as the Fc region of IgG, LPS and polyanionic molecules (e.g. chondroitin sulphate, heparin, dextran sulphate etc.). The various domains within the A-chain, and their respective functions (or potential functions), are presented and discussed in the context of the intact C1 molecule and with regard to any wider functional relevance.