Targeting of plasmodesmal proteins requires unconventional signals

Abstract

Effective cellular signaling relies on precise spatial localization and dynamic interactions among proteins in specific subcellular compartments or niches, such as cell-to-cell contact sites and junctions. In plants, endogenous and pathogenic proteins gained the ability to target plasmodesmata, membrane-lined cytoplasmic connections, through evolution to regulate or exploit cellular signaling across cell wall boundaries. For example, the receptor-like membrane protein PLASMODESMATA-LOCATED PROTEIN 5 (PDLP5), a potent regulator of plasmodesmal permeability, generates feed-forward or feed-back signals important for plant immunity and root development. However, the molecular features that determine the plasmodesmal association of PDLP5 or other proteins remain largely unknown, and no protein motifs have been identified as plasmodesmal targeting signals. Here, we developed an approach combining custom-built machine-learning algorithms and targeted mutagenesis to examine PDLP5 in Arabidopsis thaliana and Nicotiana benthamiana. We report that PDLP5 and its closely related proteins carry unconventional targeting signals consisting of short stretches of amino acids. PDLP5 contains two divergent, tandemly arranged signals, either of which is sufficient for localization and biological function in regulating viral movement through plasmodesmata. Notably, plasmodesmal targeting signals exhibit little sequence conservation but are located similarly proximal to the membrane. These features appear to be a common theme in plasmodesmal targeting.

Heavy Metal-Associated Isoprenylated Plant Proteins (HIPPs) at Plasmodesmata: Exploring the Link between Localization and Function

“…The mechanisms of protein targeting to PD are still largely unknown. There is no consensus sequence or common mechanism yet known for the localization of natively expressed and pathogenic PD proteins [ 56 , 59 ]. The single transmembrane domain of plasmodesmata localized protein 1 (PDLP1) is sufficient for PD targeting [ 60 ].…”

“…The single transmembrane domain of plasmodesmata localized protein 1 (PDLP1) is sufficient for PD targeting [ 60 ]. More recent work found a novel targeting sequence common to PDLPs and some receptor kinases [ 59 ]. An investigation of PDLP5 with machine learning techniques and molecular experiments found an unconventional targeting signal located near the membrane in the apoplastic region of PDLPs and some receptor kinases.…”

A year at the forefront of plasmodesmal biology

“…Mechanisms targeting proteins to PD have not been fully clarified. Luna et al (2023) highlight the role of unconventional amino acid stretches located on the extracellular portion of PD proteins. Identification of these segments, due to low sequence conservation, was specifically aided by machine-learning algorithms developed for the purpose.…”