[植物生化] 在ON/OFF都能提供功能的蛋白質。20191204 SCIENCE

Plasmodesmata
Dec 4, 2019
2 min read

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

Natsumi Maruta, Yuri Trusov, David Chakravorty, Daisuke Urano, Sarah M. Assmann, Jose R. Botella

Abstract

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.

植物中形成異質聚合的GTP接合蛋白質，有著與核苷酸交換相關以及與核苷酸交換無關的兩種功能。

摘要

會形成異質聚合（與不同蛋白質結合）的鳥糞嘌呤結合蛋白（G蛋白）是由 α、β和γ等零件蛋白所組成的，具有多樣功能，且功能的開關是依靠著是否與鳥糞嘌呤結合而決定。在動物及真箘中，在Gα蛋白上將GDP取代成GTP是控制G蛋白正常細胞對於多樣的細胞外刺激做出反應的重要調控。模式植物阿拉伯芥中，只有單一個經典的Gα零件蛋白，AtGPA1。我們在植物中研究發現，一直與GTP結合而持續開啟功能的AtGPA1(Q222L)突變蛋白，以及無法與核苷酸結合的AtGPA1(S52C)突變蛋白，和Gβγ1以及Gβγ2聚合蛋白的結合力非常相似。這個結果顯示了G蛋白聚合體的形成是獨立於核苷酸交換這個事件外的。相反的，AtGPA1(Q222L)對於Gβγ3有著較高的結合力，顯示出與GTP結合的時候，ATGPA1(Q222L)在構型上有所不同，所以可以很緊密地與Gβγ3結合。對於轉殖植物做的功能性研究顯示，在gpa1完全不表現的突變植物中，不論表現AtGPA1(S52C)或是表現AtGPA1(Q222L)，都各自有一些不同的突變現象被克服。我們的結論是，除了傳統的GDP-GTP交換所帶來的功能，植物的G蛋白在獨立於核苷酸交換外也有另外的功能性存在。