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Plant Physiology Preview Published on December 22, 2006; 10.1104/pp.106.092965
OPEN ACCESS ARTICLE
Received November 14, 2006 Desiccation and Zinc-binding Induces Transition of Tomato ASR1, a Water-stress and Salt-stress Regulated Plant Specific Protein, from Unfolded to Folded State
Department of Chemistry and Department of Life Sciences and Doris and Bertie Black Center for Bioenergetics in Life Sciences Ben-Gurion University, Beer-Sheva 84105, Israel. Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, MD 20892-0540 USA * Corresponding author; email: barzvi{at}bgu.ac.il.
ASR1 (abscisic acid stress ripening) is a low molecular weight plant specific protein encoded by an abiotic-stress regulated gene. Overexpression of the ASR1 in transgenic plants increases their salt-tolerance. The ASR1 protein possesses a zinc-dependent DNA-binding activity. The DNA binding site was mapped to the central part of the polypeptide using truncated forms of the protein (Rom et al., (2006) Biochimie 88: 621-628). Two additional zinc-binding sites were shown to be localized at the amino terminus of the polypeptide. ASR1 protein is presumed to be an intrinsically unstructured protein using a number of prediction algorithms. The degree of order of ASR1 was determined experimentally using non-tagged recombinant protein expressed in E. coli and purified to homogeneity. Purified ASR1 was shown to be unfolded using dynamic light scattering, gel filtration, microcalorimetry, circular dichroism, and Fourier transform infrared (FTIR) spectrometry. The protein was shown to be monomeric by analytical ultracentrifugation. Addition of zinc ions resulted in a global change in ASR1 structure, from monomer to homodimer. Upon binding of zinc ions, the protein becomes ordered as shown by FTIR and microcalorimetry, concomitant with dimerization. Tomato leaf soluble ASR1 is unstructured in the absence of added zinc and gains structure upon binding of the metal ion. The effect of zinc binding on ASR1 folding and dimerization is discussed.
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