Substrate-structure dependence of ribotoxins on cleaving RNA in C. camphora ribosome.

Cinnamomin, a type II ribosome-inactivating protein (RIP) isolated from the seeds of Camphora tree (Cinnamomum camphora), could not inactivate its own (autologous) ribosome. Among five RIPs (Cinnamomin A-chain, ricin A-chain, trichosanthin, gelonin, and soporin-S6) tested, only saporin-S6 could cleave the N-glycosidic bond of RNA in C. camphora ribosome to release a specific RNA fragment (R-fragment) after treatment with aniline, which was shorter than that from rat liver ribosome. The amount of saporin-S6 to inactivate C. camphora ribosome was about 1000 times higher than that required for rat liver ribosome. Extra-ribosomal factors (S-100) in the post-ribosomal supernatant could not promote RNA N-glycosidase activity of cinnamomin and gelonin to C. camphora ribosome. These results indicated that there were some changes in the microenvironments of Sarcin/Ricin domain of C. camphora ribosome that abolished the recognition and catalysis of many RIPs. In addition, the length of C. camphora 5.8S ribosomal RNA was found to be longer than that of rat 5.8S ribosomal RNA.