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J. Biol. Chem., Vol. 282, Issue 28, 20435-20446, July 13, 2007

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A Novel Plant Protein-disulfide Isomerase Involved in the Oxidative Folding of Cystine Knot Defense Proteins^*

Christian W. Gruber, Maa emaar¹, Richard J. Clark, Tomohisa Horibe, Rosemary F. Renda, Marilyn A. Anderson, and David J. Craik, An ARC professorial fellow²

From the Institute for Molecular Bioscience, University of Queensland, Brisbane, Queensland 4072, Australia and the Department of Biochemistry, La Trobe University, Bundoora, Victoria 3086, Australia

We have isolated a protein-disulfide isomerase (PDI) from Oldenlandia affinis (OaPDI), a coffee family (Rubiaceae) plant that accumulates knotted circular proteins called cyclotides. The novel plant PDI appears to be involved in the biosynthesis of cyclotides, since it co-expresses and interacts with the cyclotide precursor protein Oak1. OaPDI exhibits similar isomerase activity but greater chaperone activity than human PDI. Since domain c of OaPDI is predicted to have a neutral pI, we conclude that this domain does not have to be acidic in nature for PDI to be a functional chaperone. Its redox potential of -157 ± 4 mV supports a role as a functional oxidoreductase in the plant. The mechanism of enzyme-assisted folding of plant cyclotides was investigated by comparing the folding of kalata B1 derivatives in the presence and absence of OaPDI. OaPDI dramatically enhanced the correct oxidative folding of kalata B1 at physiological pH. A detailed investigation of folding intermediates suggested that disulfide isomerization is an important role of the new plant PDI and is an essential step in the production of insecticidal cyclotides.

Received for publication, January 2, 2007 , and in revised form, May 22, 2007.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) 911777.

^* This work was supported by grants from the Australian Research Council and a University of Queensland confirmation scholarship (to C. W. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ An ARC postdoctoral fellow.

² To whom correspondence should be addressed Tel.: 61-7-3346-2019; Fax: 61-7-3346-2029; E-mail: d.craik{at}imb.uq.edu.au.

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