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Plant and Cell Physiology Advance Access originally published online on July 18, 2007
Plant and Cell Physiology 2007 48(8):1182-1191; doi:10.1093/pcp/pcm093
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© The Author 2007 Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Protein Phosphorylation and Binding of a 14-3-3 Protein in Vicia Guard Cells in Response to ABA

Yohei Takahashi1, Toshinori Kinoshita1,2 and Ken-ichiro Shimazaki1,*

1Department of Biology, Faculty of Science, Kyushu University, Ropponmatsu, Fukuoka, 810-8560 Japan
2Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Kawaguchi, Saitama, 332-0012 Japan

*Corresponding author: E-mail, kenrcb{at}mbox.nc.kyushu-u.ac.jp; Fax, 81-92-726-4758.


  Abstract

Under drought stress, ABA promotes stomatal closure to prevent water loss. Although protein phosphorylation plays an important role in ABA signaling, little is known about these processes at the biochemical level. In this study, we searched for substrates of protein kinases in ABA signaling through the binding of a 14-3-3 protein to phosphorylated proteins using Vicia guard cell protoplasts. ABA induced binding of a 14-3-3 protein to proteins with molecular masses of 61, 43 and 39 kDa, with the most remarkable signal for the 61 kDa protein. The ABA-induced binding to the 61 kDa protein occurred only in guard cells, and reached a maximum within 3 min at 1 µM ABA. The 61 kDa protein localized in the cytosol. ABA induced the binding of endogenous vf14-3-3a to the 61 kDa protein in guard cells. Autophosphorylation of ABA-activated protein kinase (AAPK), which mediates anion channel activation, and ABA-induced phosphorylation of the 61 kDa protein showed similar time courses and similar sensitivities to the protein kinase inhibitor K-252a. AAPK elicits the binding of the 14-3-3 protein to the 61 kDa protein in vitro when AAPK in guard cells was activated by ABA. The phosphorylation of the 61 kDa protein by ABA was not affected by the NADPH oxidase inhibitor, H2O2, W-7 or EGTA. From these results, we conclude that the 61 kDa protein may be a substrate for AAPK and that the 61 kDa protein is located upstream of H2O2 and Ca2+, or on Ca2+-independent signaling pathways in guard cells.

Keywords: 14-3-3 protein - ABA - AAPK - Guard cell protoplasts - Protein phosphorylation - Vicia faba

Abbreviations: AAPK, ABA-activated protein kinase; AKIP1, AAPK-interacting protein 1; BAP, bacterial alkaline phosphatase; CDPK, calcium-dependent protein kinase; DPI, diphenylene iodonium chloride; GST, glutathione S-transferase; W-7, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide

(Received April 5, 2007; Accepted July 4, 2007)
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