Protein abundance and localization at the plasma membrane (PM) shapes plant development and mediates adaptation to changing environmental conditions. It is regulated by ubiquitination, a post-translational modification crucial for the proper sorting of endocytosed PM proteins to the vacuole for subsequent degradation. To understand the significance and the variety of roles played by this reversible modification, the function of ubiquitin receptors, which translate the ubiquitin signature into a cellular response, needs to be elucidated. In this study, we show that TOL (TOM1-like) proteins function in plants as multivalent ubiquitin receptors, governing ubiquitinated cargo delivery to the vacuole via the conserved Endosomal Sorting Complex Required for Transport (ESCRT) pathway. TOL2 and TOL6 interact with components of the ESCRT machinery and bind to K63-linked ubiquitin via two tandemly arranged conserved ubiquitin-binding domains. Mutation of these domains results not only in a loss of ubiquitin binding but also altered localization, abolishing TOL6 ubiquitin receptor activity. Function and localization of TOL6 is itself regulated by ubiquitination, whereby TOL6 ubiquitination potentially modulates degradation of PM-localized cargoes, assisting in the fine-tuning of the delicate interplay between protein recycling and downregulation. Taken together, our findings demonstrate the function and regulation of a ubiquitin receptor that mediates vacuolar degradation of PM proteins in higher plants.

质膜 (PM) 的蛋白质丰度和定位决定植物发育并调节对不断变化的环境条件的适应。它受到泛素化的调节，泛素化是一种翻译后修饰，对于将内吞的 PM 蛋白正确分类到液泡以便随后降解至关重要。为了了解这种可逆修饰的重要性和所起的各种作用，需要阐明将泛素特征转化为细胞反应的泛素受体的功能。在这项研究中，我们表明 TOL（TOM1 样）蛋白在植物中作为多价泛素受体发挥作用，通过保守的运输所需内体分选复合物 (ESCRT) 途径控制泛素化货物向液泡的输送。 TOL2 和 TOL6 与 ESCRT 机制的组件相互作用，并通过两个串联排列的保守泛素结合域与 K63 连接的泛素结合。这些结构域的突变不仅会导致泛素结合丧失，还会改变定位，从而消除 TOL6 泛素受体活性。 TOL6 的功能和定位本身受泛素化调节，因此 TOL6 泛素化可能调节 PM 定位货物的降解，有助于微调蛋白质回收和下调之间的微妙相互作用。总而言之，我们的研究结果证明了泛素受体在高等植物中介导 PM 蛋白液泡降解的功能和调节。