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JXB Advance Access published online on August 27, 2004
Journal of Experimental Botany, doi:10.1093/jxb/erh243
© 2004 by Oxford University Press
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Received February 25, 2004
Accepted July 5, 2004

RESEARCH PAPER

Maize ribosome-inactivating proteins (RIPs) with distinct expression patterns have similar requirements for proenzyme activation

Hank W. Bass 1*, Julie E. Krawetz 2, Gregory R. OBrian 3, Christopher Zinselmeier 4, Jeffrey E. Habben 4, Rebecca S. Boston 2

1 Department of Biological Science, Florida State University, Tallahassee, FL 32306-4370, USA
2 Department of Botany, Box 7612, North Carolina State University, Raleigh, NC 27695, USA
3 Department of Botany, Box 7612, North Carolina State University, Raleigh, NC 27695, USA; Present address: Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695, USA
4 Trait and Technology Development, Pioneer Hi-Bred International, Inc., Johnston, IA 50131, USA

* To whom correspondence should be addressed. E-mail: bass{at}bio.fsu.edu.


  Abstract

Ribosome-inactivating proteins (RIPs, EC 3.2.2.22) are potent naturally occurring toxins found in numerous and diverse plant species. The maize RIP is unusual among the plant RIPs because it is synthesized as an inactive precursor (also known as maize proRIP1 or b-32). The proenzyme undergoes proteolytic activation that results in the removal of the NH2-terminal, the COOH-terminal, and internal sequences to form a two-chain holoenzyme capable of irreversibly modifying the large rRNA. The characterization of a second maize RIP (RIP2), encoded by the gene designated Rip3:2 is described here. Low levels of Rip3:2 RNA were detected in roots, shoots, tassels, silks, and leaves, but the Rip3:2 gene, unlike the Rip3:1 gene, is not under the control of the transcriptional activator Opaque-2. Instead, its expression was up-regulated by drought. Rip3:2 encodes a 31.1 kDa polypeptide that is very similar to proRIP1 in regions corresponding to those found in the active protein and the NH2-terminal extension. A 19-amino-acid internal portion of proRIP2 has little similarity to the proRIP1 sequence except that both are very rich in acidic residues. RIP activity assays revealed that Rip3:2 encodes a polypeptide that acquires RNA-specific N-glycosidase activity after proteolytic cleavage. Accumulation as inactive proenzymes may therefore be a general feature of maize RIPs. Differential regulation of the two RIP genes suggests that the corresponding proteins may be involved in defence-related functions with one being regulated developmentally and the other being responsive to an environmental stimulus.

Keywords: Maize; ribosome-inactivating protein; translation; toxin; water stress.
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