Editing Ricin (section)

=== Ribosome inactivation ===
RTA has [[rRNA N-glycosylase|rRNA ''N''-glycosylase]] activity that is responsible for the cleavage of a [[glycosidic bond]] within the large [[ribosomal RNA|rRNA]] of the [[60S]] subunit of eukaryotic ribosomes.<ref name="pmid3036799">{{cite journal | vauthors = Endo Y, Tsurugi K | title = RNA N-glycosidase activity of ricin A-chain. Mechanism of action of the toxic lectin ricin on eukaryotic ribosomes | journal = The Journal of Biological Chemistry | volume = 262 | issue = 17 | pages = 8128–8130 | date = June 1987 | pmid = 3036799 | doi = 10.1016/S0021-9258(18)47538-2 | doi-access = free }}</ref> RTA specifically and irreversibly [[hydrolyses]] the ''N''-glycosidic bond of the [[adenine]] residue at position 4324 (A4324) within the [[28S ribosomal RNA|28S]] rRNA, but leaves the [[phosphodiester]] backbone of the RNA intact.<ref name="pmid3288622">{{cite journal | vauthors = Endo Y, Tsurugi K | title = The RNA N-glycosidase activity of ricin A-chain. The characteristics of the enzymatic activity of ricin A-chain with ribosomes and with rRNA | journal = The Journal of Biological Chemistry | volume = 263 | issue = 18 | pages = 8735–8739 | date = June 1988 | pmid = 3288622 | doi = 10.1016/S0021-9258(18)68367-X | doi-access = free }}</ref> The ricin targets A4324 that is contained in a highly [[conserved sequence]] of 12 [[nucleotide]]s universally found in eukaryotic ribosomes. The sequence, 5'-AGUACGAGAGGA-3', termed the sarcin-ricin loop, is important in binding [[elongation factor]]s during protein synthesis.<ref name="pmid4360718">{{cite journal | vauthors = Sperti S, Montanaro L, Mattioli A, Stirpe F | title = Inhibition by ricin of protein synthesis in vitro: 60 S ribosomal subunit as the target of the toxin | journal = The Biochemical Journal | volume = 136 | issue = 3 | pages = 813–815 | date = November 1973 | pmid = 4360718 | pmc = 1166019 | doi = 10.1042/bj1360813 }}</ref> The depurination event rapidly and completely inactivates the ribosome, resulting in toxicity from inhibited protein synthesis. A single RTA molecule in the [[cytosol]] is capable of depurinating approximately 1500 [[ribosomes]] per minute.