Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, USA.
Department of Computer and Information Sciences, University of Delaware, Newark, DE 19716, USA.
Plant Cell. 2023 Aug 2;35(8):3035-3052. doi: 10.1093/plcell/koad152.
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 2 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.
有效的细胞信号依赖于蛋白质在特定亚细胞区室或龛位（如细胞间接触位点和连接点）中的精确空间定位和动态相互作用。在植物中，内源性和致病性蛋白质通过进化获得了靶向胞间连丝（膜衬细胞质连接）的能力，以调节或利用跨细胞壁边界的细胞信号。例如，受体样膜蛋白 PLASMODESMATA-LOCATED PROTEIN 5（PDLP5）是胞间连丝通透性的有力调节剂，它产生了对植物免疫和根系发育很重要的正向或反馈信号。然而，决定 PDLP5 或其他蛋白质与胞间连丝结合的分子特征在很大程度上仍是未知的，也没有鉴定出任何蛋白基序作为胞间连丝靶向信号。在这里，我们开发了一种结合定制机器学习算法和靶向突变的方法来研究拟南芥和本氏烟中的 PDLP5。我们报告称，PDLP5 及其密切相关的蛋白质带有非传统的靶向信号，由短的氨基酸序列组成。PDLP5 包含 2 个发散的、串联排列的信号，其中任何一个信号都足以定位和发挥生物学功能，从而通过胞间连丝调节病毒的运动。值得注意的是，胞间连丝靶向信号的序列保守性很小，但在膜附近的位置相似。这些特征似乎是胞间连丝靶向的一个共同主题。
