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Plant and Cell Physiology Advance Access originally published online on August 4, 2008
Plant and Cell Physiology 2008 49(9):1364-1377; doi:10.1093/pcp/pcn112
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Maize Plasma Membrane Aquaporins Belonging to the PIP1 and PIP2 Subgroups are in vivo Phosphorylated

Valérie Van Wilder1,3, Urszula Miecielica1,3, Hervé Degand1, Rita Derua2, Etienne Waelkens2 and François Chaumont1,*

1 Institut des Sciences de la Vie, Université catholique de Louvain, Croix du Sud 5–15, B-1348 Louvain-la-Neuve, Belgium
2 Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium

*Corresponding author: E-mail, francois.chaumont{at}uclouvain.be; Fax, +32-10-473872.


  Abstract

Aquaporins are channel proteins that facilitate transmembrane water movement. In this study, we showed that plasma membrane intrinsic proteins (PIPs) from maize shoots are in vitro and in vivo phosphorylated on serine residues by a calcium-dependent kinase associated with the membrane fraction. Mass spectrometry identified phosphorylated peptides corresponding to the C-terminal region of (i) ZmPIP2;1, ZmPIP2;2 and/or ZmPIP2;7; (ii) ZmPIP2;3 and/or ZmPIP2;4; (iii) ZmPIP2;6; together with (iv) a phosphorylated peptide located in the N-terminal region of ZmPIP1;1, ZmPIP1;2, ZmPIP1;3 and/or ZmPIP1;4. The role of phosphorylation in the water channel activity of wild-type and mutant ZmPIP2;1 was studied in Xenopus laevis oocytes. Activation of endogenous protein kinase A increased the osmotic water permeability coefficient of ZmPIP2;1-expressing oocytes, suggesting that phosphorylation activates its channel activity. Mutation of S126 or S203, putative phosphorylated serine residues conserved in all plant PIPs, to alanine decreased ZmPIP2;1 activity by 30–50%, without affecting its targeting to the plasma membrane. Mutation of S285, which is phosphorylated in planta, to alanine or glutamate did not affect the water channel activity. These results indicate that, in oocytes, S126 and S203 play an important role in ZmPIP2;1 activity and that phosphorylation of S285 is not required for its activity.

Keywords: Aquaporins - Maize - Mass spectrometry - Phosphorylation - Xenopus laevis oocytes - Zea mays

Abbreviations: AQP, aquaporin; 8-Br-cAMP, 8-bromo-cAMP; CaMK, Ca2+-calmodulin-dependent protein kinase; CDPK, calcium-dependent protein kinase; DTT, dithiothreitol; GFP, green fluorescent protein; LC, liquid chromatography; MALDI-TOF, matrix-assisted laser desorption ionization-time of flight; MS, mass spectrometry; PEG, polyethylene glycol; Pf, osmotic water permeability coefficient; PIP, plasma membrane intrinsic protein; PKA, protein kinase A; PKC, protein kinase C; PKG, protein kinase G; PVDF, polyvinylidene difluoride; PVP, polyvinylpyrrolidone; TFA, trifluoroacetic acid; TIP, tonoplast intrinsic protein

3These authors contributed equally to this work.

(Received July 16, 2008; Accepted July 30, 2008)
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