Characterization of C-reactive protein and the complement subcomponent Clt as homologous proteins displaying cyclic pentameric symmetry (pentraxins) (amino acid sequence/electron microscopy/amyloid P-component/evolution)

Partial amino acid sequences of rabbit C-re- active protein, a peptide derived from human C-reactive protein by cyanogen bromide cleavage, and the Clt subcomponent of the human complement component Cl have been determined. Extensive sequence homology between these proteins establish their evolutionary relationships. In addition, examination of C-reactive proteins by negative-stain electron microscopy re- vealed that the protein is composed of five subunits arranged in cyclic symmetry. This structure is similar to that reported for both Clt and the amyloid P-component. The extensive structural relationship suggests similar or overlapping functions and the term pentraxin is proposed to describe these homologous pro- teins.