Abstract
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 TiO(2) 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 (Ser(280) and Ser(283)) 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 Ser(280) and Ser(283) 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 Ser(283) 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 Ser(283) 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.
Highlights
Aquaporins form a family of water and solute channel proteins and are present in most living organisms
A Phosphoproteomics Analysis Reveals Novel Phosphorylation Sites in the C-terminal Tail of AtPIP Aquaporins—A plasma membrane (PM) fraction was purified from Arabidopsis roots by aqueous twophase partitioning and enriched in hydrophobic proteins with a urea and NaOH treatment [31]
The extract was first treated with the endoproteinase Lys-C, which is predicted to release the C-terminal tail of all AtPIP2 aquaporins
Summary
Aquaporins form a family of water and solute channel proteins and are present in most living organisms. Whereas the long term effect of this NaCl stress can be accounted for by an overall transcriptional downregulation of aquaporins, the molecular mechanisms involved in the early inhibition of Lpr by NaCl are not fully understood yet These mechanisms involve a slight decrease in overall abundance of AtPIP1 proteins as soon as 30 min after exposure to NaCl and a trafficking of AtPIP1 and AtPIP2 isoforms between the PM and intracellular compartments that may contribute to reducing the abundance of AtPIPs at the PM and the hydraulic conductivity of salt-stressed root cells [12]. In Arabidopsis a rapid decrease in Lpr can be observed in response to 2 mM H2O2.2 Because of its amplitude (Ͼ70%) and rapidity (half-time, Х8 min) this decrease is undoubtedly due to a down-regulation of root aquaporins
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