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Molecular & Cellular Proteomics 7:1019-1030, 2008.
© 2008 by The American Society for Biochemistry and Molecular Biology, Inc.

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Research

Multiple Phosphorylations in the C-terminal Tail of Plant Plasma Membrane Aquaporins

Role in Subcellular Trafficking of AtPIP2;1 in Response to Salt Stress^*,S

Sodana Prak, Sonia Hem^,¶, Julie Boudet, Gaëlle Viennois, Nicolas Sommerer, Michel Rossignol, Christophe Maurel and Véronique Santoni^,||

From the Biochimie et Physiologie Moléculaire des Plantes, SupAgro/Institut National de la Recherche Agronomique (INRA)/CNRS/UM2 UMR 5004, 2 place Viala, F-34060 Montpellier cedex 1, France and Laboratoire de Protéomique Fonctionnelle, INRA UR 1199, 2 place Viala, F-34060 Montpellier cedex 1, France

Aquaporins form a family of water and solute channel proteins and are present in most living organisms. In plants, aquaporins play an important role in the regulation of root water transport in response to abiotic stresses. In this work, we investigated the role of phosphorylation of plasma membrane intrinsic protein (PIP) aquaporins in the Arabidopsis thaliana root by a combination of quantitative mass spectrometry and cellular biology approaches. A novel phosphoproteomics procedure that involves plasma membrane purification, phosphopeptide enrichment with TiO₂ columns, and systematic mass spectrometry sequencing revealed multiple and adjacent phosphorylation sites in the C-terminal tail of several AtPIPs. Six of these sites had not been described previously. The phosphorylation of AtPIP2;1 at two C-terminal sites (Ser²⁸⁰ and Ser²⁸³) was monitored by an absolute quantification method and shown to be altered in response to treatments of plants by salt (NaCl) and hydrogen peroxide. The two treatments are known to strongly decrease the water permeability of Arabidopsis roots. To investigate a putative role of Ser²⁸⁰ and Ser²⁸³ phosphorylation in aquaporin subcellular trafficking, AtPIP2;1 forms mutated at either one of the two sites were fused to the green fluorescent protein and expressed in transgenic plants. Confocal microscopy analysis of these plants revealed that, in resting conditions, phosphorylation of Ser²⁸³ is necessary to target AtPIP2;1 to the plasma membrane. In addition, an NaCl treatment induced an intracellular accumulation of AtPIP2;1 by exerting specific actions onto AtPIP2;1 forms differing in their phosphorylation at Ser²⁸³ to induce their accumulation in distinct intracellular structures. Thus, the present study documents stress-induced quantitative changes in aquaporin phosphorylation and establishes for the first time a link with plant aquaporin subcellular localization.

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