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Plant Cell Advance Online Publication
Published on June 29, 2007; 10.1105/tpc.107.052522

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Received April 26, 2007
Returned for revision May 29, 2007
Accepted June 4, 2007

Reciprocal Phosphorylation and Glycosylation Recognition Motifs Control NCAPP1 Interaction with Pumpkin Phloem Proteins and Their Cell-to-Cell Movement

Ken-ichiro Taoka 1, Byung-Kook Ham 1, Beatriz Xoconostle-Cázares 2, Maria R. Rojas 3, and William J. Lucas 1*

1 Section of Plant Biology, College of Biological Sciences, University of California, Davis, California 95616
2 Section of Plant Biology, College of Biological Sciences, University of California, Davis, California 95616; Departmento de Biotecnologia y Bioingenieria, Centrol de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Zacatenco 07360, Mexico
3 Department of Plant Pathology, College of Agriculture and Environmental Sciences, University of California, Davis, California 95616

* To whom correspondence should be addressed. E-mail: wjlucas{at}ucdavis.edu.

In plants, cell-to-cell trafficking of non-cell-autonomous proteins (NCAPs) involves protein-protein interactions, and a role for posttranslational modification has been implicated. In this study, proteins contained in pumpkin (Cucurbita maxima cv Big Max) phloem sap were used as a source of NCAPs to further explore the molecular basis for selective NCAP trafficking. Protein overlay assays and coimmunoprecipitation experiments established that phosphorylation and glycosylation, on both Nicotiana tabacum NON-CELL-AUTONOMOUS PATHWAY PROTEIN1 (Nt-NCAPP1) and the phloem NCAPs, are essential for their interaction. Detailed molecular analysis of a representative phloem NCAP, Cm-PP16-1, identified the specific residues on which glycosylation and phosphorylation must occur for effective binding to NCAPP1. Microinjection studies confirmed that posttranslational modification on these residues is essential for cell-to-cell movement of Cm-PP16-1. Lastly, a glutathione S-transferase (GST)-Cm-PP16-1 fusion protein system was employed to test whether the peptide region spanning these residues was required for cell-to-cell movement. These studies established that a 36-amino acid peptide was sufficient to impart cell-to-cell movement capacity to GST, a normally cell-autonomous protein. These findings are consistent with the hypothesis that a phosphorylation-glycosylation recognition motif functions to control the binding of a specific subset of phloem NCAPs to NCAPP1 and their subsequent transport through plasmodesmata.




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Y. Lee, H.-S. Lee, J.-S. Lee, S.-K. Kim, and S.-H. Kim
Hormone- and light-regulated nucleocytoplasmic transport in plants: current status
J. Exp. Bot., September 1, 2008; 59(12): 3229 - 3245.
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