Targeting of plasmodesmal proteins requires unconventional signals

Abstract

Cellular signaling relies on precise spatial localization and dynamic interactions of proteins within the subcellular compartment or niches, including cell-cell contact sites and connections. In plants, both endogenous and pathogenic proteins gained the ability to target plasmodesmata, the membrane-lined cytoplasmic connections, to regulate or exploit cellular signaling across cell wall boundaries. Those include the receptor-like membrane protein PDLP5, a potent regulator of plasmodesmal permeability that generates feed-forward or -back signals vital for plant immunity and root development. However, little is known about what molecular features determine the plasmodesmal association of PDLP5 or other proteins. Notably, although these proteins each have the ability to target plasmodesmata, no protein motifs or sequences have been identified indicative of targeting signals. To address this knowledge gap, we combined machine learning and targeted mutagenesis approaches. Here we report that PDLP5 and its closely related proteins carry novel targeting signals comprising short stretches of amino acid residues. As for PDLP5, it contains two non-redundant, tandemly arranged signals, either of which is sufficient for both localization and biological function regulating viral movement. Strikingly, plasmodesmal targeting signals exhibit little conservation in sequence but are located similarly proximal to the membrane. These novel unconventional features appear to be a common theme in plasmodesmal targeting.