Heterotrimeric guanine nucleotide-binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile, 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.

中文翻译：

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植物异三聚体GTP结合蛋白的核苷酸交换依赖性和核苷酸交换非依赖性功能。

由α，β和γ亚基组成的异三聚体鸟嘌呤核苷酸结合蛋白（G蛋白）是通用的鸟嘌呤核苷酸依赖性分子开-关开关。在动物和真菌中，Gα上GTP的GDP交换控制着G蛋白的活化，对于正常的细胞对多种细胞外信号的反应至关重要。模型植物拟南芥具有单个典型的Gα亚基AtGPA1。我们发现，在植物中，与GTP结合的组成性活性AtGPA1（Q222L）突变体和无核苷酸的AtGPA1（S52C）突变体与具有相似亲和力的Gβγ1和Gβγ2二聚体相互作用，这表明G蛋白异源三聚体的形成独立于核苷酸交换。相反，AtGPA1（Q222L）对Gβγ3的亲和力比AtGPA1（S52C）大，提示AtGPA1（Q222L）的GTP结合构象是独特的，并且与Gβγ3紧密相关。在gpa1-null突变体背景中表达AtGPA1（S52C）或AtGPA1（Q222L）的转基因品系的功能分析显示，各种突变体表型与AtGPA1（S52C）或AtGPA1（Q222L）互补。我们得出的结论是，除了规范的GDP-GTP交换依赖机制外，植物G蛋白还可以独立于核苷酸交换而发挥作用。