Trypanosoma brucei: calcium-dependent endoribonuclease is associated with inhibitor protein

T. brucei cytoplasmic calcium-dependent alkaline ribonuclease activity from DEAE-cellulose fractionation was separated into endoribonuclease and exoribonuclease activities by hydroxyapatite chromatography. T. brucei cytoplasmic extract markedly decreased the endoribonuclease activity, but slightly potentiated the activities of the exoribonuclease and bovine ribonuclease A. While the endoribonuclease was activated by trypsin, the exoribonuclease and bovine ribonuclease A were partially inactivated by trypsin. The endoribonuclease was activated by p-chloromercuribenzoate or N-ethylmaleimide; the exoribonuclease was not affected by these sulfhydryl group reagents. Free ribonuclease was separated from the latent endoribonuclease by 1 M NaCl-Sephadex G-100 gel filtration. The results demonstrate that T. brucei cytoplasm contains a latent endoribonuclease consisting of ribonuclease and inhibitor protein.