Nucleotide exchange–dependent and nucleotide exchange–independent functions of plant heterotrimeric GTP-binding proteins

First author: Natsumi Maruta; Affiliations: University of Queensland (昆士兰大学): Brisbane, Australia

Corresponding author: Jose R. Botella

Heterotrimeric guanine nucleotide–binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile (多功能的), guanine nucleotide–dependent, molecular on-off switches. In animals and fungi, the exchange of GDP for GTP on Gα controls G protein activation and is crucial for normal cellular responses to diverse extracellular signals. The model plant Arabidopsis thaliana has a single canonical Gα subunit, AtGPA1. We found that, in planta, the constitutively active, GTP-bound AtGPA1(Q222L) mutant and the nucleotide-free AtGPA1(S52C) mutant interacted with Gβγ1 and Gβγ2 dimers with similar affinities, suggesting that G protein heterotrimer formation occurred independently of nucleotide exchange. In contrast, AtGPA1(Q222L) had a greater affinity than that of AtGPA1(S52C) for Gβγ3, suggesting that the GTP-bound conformation of AtGPA1(Q222L) is distinct and tightly associated with Gβγ3. Functional analysis of transgenic lines expressing either AtGPA1(S52C) or AtGPA1(Q222L) in the gpa1-null mutant background revealed various mutant phenotypes that were complemented by either AtGPA1(S52C) or AtGPA1(Q222L). We conclude that, in addition to the canonical GDP-GTP exchange–dependent mechanism, plant G proteins can function independently of nucleotide exchange.

异源三聚体鸟嘌呤核苷酸结合蛋白G蛋白由α、β和γ三个亚基组成，是多功能的、鸟嘌呤核苷酸依赖型的分子开关。在动物和真菌中，α亚基上GDP到GTP的转变控制着G蛋白的活性，并且对于细胞对多种胞外信号的正常响应至关重要。在模式植物拟南芥中，只有一个经典的G蛋白α亚基，即AtGPA1。作者发现，在植物体内，组成型活跃的、能够结合GTP的AtGPA1（Q222L）突变体和不能结合核苷酸的AtGPA1（S52C）突变体都能够与Gβγ1和Gβγ2二聚体互作，并且亲和力并无明显差异，说明拟南芥中G蛋白异源三聚体的形成独立于核苷酸的交换。相反，AtGPA1（Q222L）突变体要比AtGPA1（S52C）突变体对于Gβγ3的亲和力更强，说明AtGPA1（Q222L）中结合GTP的特性与其能够与Gβγ3互作紧密关联。gpa1功能缺失突变体背景下AtGPA1（Q222L）和AtGPA1（S52C）的转基因株系功能分析显示gpa1突变体的各种缺陷表型都能够被AtGPA1（Q222L）或AtGPA1（S52C）所互补。因此，作者认为除了经典的GDP-GTP交换依赖型机制，植物的G-蛋白能够独立于核苷酸交换发挥功能。

通讯：Jose R. Botella (https://agriculture.uq.edu.au/profile/209/jimmy-botella)

个人简介：西班牙马德里大学，学士；西班牙马拉加大学，博士；密歇根州立大学、宾夕法尼亚州立大学，博士后。

研究方向：植物异三聚体G蛋白。

doi: 10.1126/scisignal.aav9526

Journal: Science Signaling

Published date: November 05, 2019