Division of Plant and Crop Science, School of Biosciences, University of Nottingham, Nottingham LE12 5RD, UK.
School of Life Sciences, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK.
Plant Physiol. 2023 Dec 30;194(1):422-433. doi: 10.1093/plphys/kiad506.
AUXIN RESISTANCE4 (AXR4) regulates the trafficking of auxin influx carrier AUXIN1 (AUX1), a plasma-membrane protein that predominantly localizes to the endoplasmic reticulum (ER) in the absence of AXR4. In Arabidopsis (Arabidopsis thaliana), AUX1 is a member of a small multigene family comprising 4 highly conserved genes-AUX1, LIKE-AUX1 (LAX1), LAX2, and LAX3. We report here that LAX2 also requires AXR4 for correct localization to the plasma membrane. AXR4 is a plant-specific protein and contains a weakly conserved α/β hydrolase fold domain that is found in several classes of lipid hydrolases and transferases. We have previously proposed that AXR4 may either act as (i) a post-translational modifying enzyme through its α/β hydrolase fold domain or (ii) an ER accessory protein, which is a special class of ER protein that regulates targeting of their cognate partner proteins. Here, we show that AXR4 is unlikely to act as a post-translational modifying enzyme as mutations in several highly conserved amino acids in the α/β hydrolase fold domain can be tolerated and active site residues are missing. We also show that AUX1 and AXR4 physically interact with each other and that AXR4 reduces aggregation of AUX1 in a dose-dependent fashion. Our results suggest that AXR4 acts as an ER accessory protein. A better understanding of AXR4-mediated trafficking of auxin transporters in crop plants will be crucial for improving root traits (designer roots) for better acquisition of water and nutrients for sustainable and resilient agriculture.
生长素转运蛋白 AUX1（AUX1）的内流载体生长素不敏感 4 号（AXR4）调节其运输，AUX1 是一种质膜蛋白，在不存在 AXR4 的情况下主要定位于内质网（ER）。在拟南芥（Arabidopsis thaliana）中，AUX1 是一个由 4 个高度保守基因-AUX1、类生长素 1（LAX1）、LAX2 和 LAX3 组成的小多基因家族的成员。我们在此报告 LAX2 也需要 AXR4 才能正确定位于质膜。AXR4 是一种植物特异性蛋白，含有一个弱保守的α/β水解酶折叠结构域，该结构域存在于几类脂质水解酶和转移酶中。我们之前提出，AXR4 可能（i）通过其α/β水解酶折叠结构域作为一种翻译后修饰酶发挥作用，或（ii）作为内质网辅助蛋白发挥作用，内质网辅助蛋白是一类特殊的内质网蛋白，可调节其同源伴侣蛋白的靶向。在这里，我们表明 AXR4 不太可能作为一种翻译后修饰酶发挥作用，因为其α/β水解酶折叠结构域中的几个高度保守氨基酸的突变是可以耐受的，并且缺少活性位点残基。我们还表明，AUX1 和 AXR4 彼此物理相互作用，并且 AXR4 以剂量依赖的方式减少 AUX1 的聚集。我们的结果表明，AXR4 作为一种内质网辅助蛋白发挥作用。更好地理解 AXR4 介导的作物生长素转运蛋白的运输对于改善根特性（设计根）以更好地获取水和养分，从而实现可持续和有弹性的农业至关重要。
