Structural and functional studies of cinnamomin, a new type II ribosome-inactivating protein isolated from the seeds of the camphor tree

Cinnamomin is a new type II ribosome-inactivating protein (RIP). Its A-chai n exhibits RNA N-glycosidase activity to inactivate the ribosome and thus i nhibit protein synthesis, whereas the glycosylated B-chain is a lectin. The primary structure of cinnamomin, which exhibits approximately 55% identity with those of ricin and abrin, was deduced from the nucleotide sequences o f cDNAs of cinnamomin A- and B-chains. It is composed of a total of 549 ami no-acid residues: 271 residues in the A-chain, a 14-residue linker and 264 residues in the B-chain. To explore its biological function, the cinnamomin A-chain was expressed in Escherichia coli with a yield of 100 mg per L of culture, and purified through two-step column chromatography. After renatur ation, the recovery of the enzyme activity of the expressed A-chain was 80% of that of native A-chain. Based on the modeling of the three-dimensional structure of the A-chain, the functional roles of five amino acids and the only cysteine residues were investigated by site-directed mutagenesis or ch emical modification. The conserved single mutation of the five amino-acid r esidues led to 8-50-fold losses of enzymatic activity, suggesting that thes e residues were crucial for maintaining the RNA N-glycosidase activity of t he A-chain. Most interestingly, the strong electric charge introduced at th e position of the single cysteine in A-chain seemed to play a role in enzym e/ substrate binding.