Peptide antagonists of the plasmodesmal macromolecular trafficking pathway

In plants, cell-to-cell transport of endogenous and viral proteins and ribo nucleoprotein complexes (RNPCs) occurs via plasmodesmata. Specificity of th is transport pathway appears to involve interaction between such proteins/R NPCs and plasmodesmal chaperones/receptors. Here, KN1 and the cucumber mosa ic virus movement protein (CMV-MP) were used, in a modified phage-display s creening system, to identify peptides capable of interacting with proteins present in a plasmodesmal-enriched cell wall fraction. Binding/competition assays and microinjection experiments revealed that these phage-displayed p eptides and homologous synthetic oligopeptides function as ligand-specific antagonists of macromolecular trafficking through plasmodesmata. A KN1 pept ide antagonist had the capacity to interact with a motif involved in the di lation of plasmodesmal microchannels. Although KN1 could still achieve limi ted movement through plasmodesmata when this SEL motif was blocked, KN1-med iated transport of KN1-sense RNA was fully inhibited, These findings provid e direct support for the hypothesis that KN1 requires, minimally, two physi cally separated signal motifs involved in the dilation of, and protein tran slocation through, plasmodesmal microchannels, and provide direct proof tha t plasmodesmal dilation is a prerequisite for the cell-to-cell transport of an RNPC.