Plant Molecular Biology Reporter 12 (2) 1994 pages $60-$62 CPGN Supplement
Working Group on Ribosome-Inactivating Proteins: i [email protected]
I II III I
Genes Encoding Ribosome-Inactivating Proteins
John Mundy, Robert Leah, Rebecca Boston, Yaeta Endo, and Fiorenzo Stirpe
(JM) Molecular Biology Institute, Copenhagen University, Oster Farimagsgade 2A, 1353 Copenhagen K, Denmark
(RL) Carlsberg Research Laboratory, Carlsberg Research Center, DK-2500 Copenhagen, Denmark
(RB) Department of Botany, N.C. State University, Raleigh, N.C. 27695, USA (YE) Department of Biochemistry, Yamanashi Medical College, Tamaho,
Nakakoma, Yamanashi 409-38, Japan (FS) Department of General Pathology, University of Bologna, 1-40126 Bologna,
Italy
R ibosome-inactivating proteins (RIPs) are expressed in many plant species. RIPs inhibit protein synthesis on eukaryotic ribosomes by their specific RNA N-glycosidase activity (E.C. 3.2.2.22) that
hydrolyzes the N-glycosidic bond at A 4324 of 28S rRNA. This modification inhibits the binding of elongation factor 2 and results in an inhibition of cellular protein synthesis. The inhibitory activities of RIPs from several species have been determined in cell-free translation systems at between IDs0 0.04 to 4 nM (Stirpe and Barbierei, 1986). Thus, RIPs are potentially cytotoxic (Barbieri and Stirpe, 1981; Roberts and Selitrennikoff, 1986; Endo and Tsurugi, 1987; Endo et al., 1988).
RIPs are generally thought to function as protective proteins in planta because they preferentially inactivate ribosomes from phylogenetically distant species, including fungi and animals. RIPs have therefore been examined as possible therapeutic agents. For example, they show prom- ise in medical treatments against virally infected cells, although their efficacy has yet to be demonstrated in clinical trials (Zarling et al., 1990). In the same vein, the ectopic expression of barley RIP30 has been correlated with increased fungal protection in transgenic tobacco plants (Logemann et al., 1992).
Common names have been given to some RIPs based upon the species from which they originate (ricin, tritin, abrin, luffin). To date, the se-
Abbreviation: RIP, ribosome-inactivating protein.
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Ribosome-Inactivating Proteins $61
quence databases contain 17 RIP genes that appear to encode three general types of proteins. A provisional nomenclature is therefore pro- posed here to reflect this diversity.
�9 Type I RIPs, typified by wheat tritin, are single-chain proteins with molecular masses of about 30 kDa.
�9 Type 2 RIPs, typified by castor bean ricin, are synthesized as 60- kDa precursors that are processed to a 30-kDa RIP A chain and a 30- kDa B chain lectin. This processing involves removal of a secretory peptide and of a linker peptide between the A and B chains.
�9 Type 3 RIPs, typified by maize b-32, are synthesized as approxi- mately 30-kDa precursors that require proteolytic removal of an internal peptide for activity. This results in an A chain of approxi- mately 17 kDa and a B chain of approximately 9 kDa. In constrast to Type 2 RIPs, both of these chains are required for N-glycosidase activity.
Certain toxins from bacteria (Shiga-like, Hovde et al., 1988) and fungi (R-sarcin, Sacco et al., 1983) also inhibit eukaryotic protein synthesis by mechanisms similar to that of plant RIPs. These RIPs are not included in the nomenclature proposed here or in the ISPMB/Mnemonic/Numeric database.
Proposed Nomenclature
We propose that proteins with specific RNA N-glycosidase activity responsible for eukaryotic ribosome inactivation should be denoted ribosome-inactivating proteins.
We propose three plant-wide families of genes corresponding to the three classes of RIPs. The core mnemonic symbol for genes encoding ribosome-inactivating protein genes should be Rip. The plant-wide designations for the three classes of genes are:
Gene Product Gene Product Mnemonic Number
Type 1: Single-chain RIPs Rip1 2.3.2.2.22:1
Type 2: RIPs processed by endoproteolysis to two Rip2 2.3.2.2.22:2 polypeptide chains, one the RNA N-glycosidase and the other a lectin or other binding chain not essential for RNA N-glycosidase activity
Type 3: RIPs processed by endoproteolysis to at least two polypeptide chains, two of which are necessary for RNA N-glycosidase activity
Rip3 2.3.2.2.22:3
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References
Barbieri, L., and F. Stirpe. 1981. Ribosomes inactivating proteins from plants: Properties and possible uses. Cancer Surveys 1:489-520.
Endo, Y., and K. Tsurugi. 1987. RNA N-Glycosidase Activity of Ricin A-chain. J. Biol. Chem. 262:8128-8130.
Endo, Y., K. Tsurugi, and R.F. Ebert. 1988. The mechanism of action of barley toxin: a type 1 ribosome-inactivating protein with RNA N-glycosidase activity. Biochim. Biophys. Acta 954:224-226.
Hovde, C.J., S.B. Calderwood, J.J. Mekalanos, and R.J. Collier, R.J. 1988. Evidence that glutamic acid 167 is an active-site residue of Shiga-like toxin I. Proc. Natl. Acad. Sci. USA 85:2568-2572.
Logemann, J., G. Jach, H. Tommerup, J. Mundy, and J. Schell. 1992. Expression of a barley ribosome-inactivating protein leads to increasd fungal protection in transgenic tobacco plants. Bio/Technology 10:305-308.
Roberts, W.K., and C.P. Selitrennikoff. 1986. Plant proteins that inactivate foreign ribo- somes. Bioscience Reports 6:19-29.
Sacco, G., K. Drickamer, and I.G. Wool. 1983. The primary structure of the cytotoxin c~- sarcin. J. Biol. Chem. 258:5811-5818.
Stirpe, F., and L. Barbieri. 1986. Ribosome-inactivating proteins up to date. FEBS Letters 195:1-8.
Zarling, J.M., P.A. Moran, O. Haffar, J. Sias, D.D. Richman, C.A. Spina, D.E. Myers, V. Kuebelbeck, J.A. Ledbetter, and F.M. Uckun. 1990. Inhibition of HIV replication by pokeweed antiviral protein targeted to CD4 ~ cells by monoclonal antibodies. Nature 347:92-95.
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