Amino acid sequences of mouse complement C3 derived from nucleotide sequences of cloned cDNA.

The complement system consists of a group of plasma proteins that play an important role in the defense of higher vertebrates against microbial infections.'. * In addition, complement and complement-derived peptides participate in inflammatory reactions and in the regulation of antibody synthesis by lymphocytes.'-6 C3,d the third component of complement, is an essential element in the functioning of both the classical and the alternative pathways of complement activation. One of the most striking characteristics of C3 is its capacity to interact specifically with a number of other proteins, and all the net effects of activation of both the classical and the alternative pathways of complement depend upon these interactions of C3 and its derivatives: -As a substrate C3 interacts with the C3-convertases of the classical and alternative pathways, which cleave it into the inflammatory peptide C3a (M, = 8300) and the activated protein C3b (Mr = 171,000). -As a constituent C3b participates in the assembly of the C5-convertases of both pathways and the alternative-pathway C3-convertase.