Identification of protein C in sera of the frogs, Rana nigromaculata and Rana brevipoda.

When sera from the frogs, Rana (R.) nigromaculata and Rana (R.) brevipoda, were run on starch-gel electrophoresis (SGE), several bands were seen in an electrophoretic pattern of proteins. This pattern on SGE appeared the same at stages XIV, XV and XXI, and in the adult frog, R. niguromaculata. However, the pattern at stage X was different. A protein, designated "protein C", did not appear clearly at this stage, but afterwards. This protein was the second richest among serum proteins of mature frogs. Protein C (Mr = 180 kD, when estimated by SDS-PAGE) was obtained after SGE and then subjected to an NH2-terminal sequence analysis. Sequences of protein C from R. nigromaculata and R. brevipoda were NH2-TDPMYVIFIPQTLXE for the first 15 amino acids and NH2-TDPHYVIFKG for the first 10 amino acids, respectively. Homology search of GenBank sequences indicated no significant similarity with any known proteins. The results suggest that protein C is a new protein, and that it may play an important role(s) in the serum after stage X in these species.