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

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

² To whom correspondence should be addressed. E-mail wjlucas{at}ucdavis.edu; fax 530-752-5410.

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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