Abscisic acid (ABA) is a ubiquitous hormone that regulates plant growth, development and responses to environmental stresses. Its action is mediated by the PYR/PYL/RCAR family of START proteins, but it remains unclear how these receptors bind ABA and, in turn, how hormone binding leads to inhibition of the downstream type 2C protein phosphatase (PP2C) effectors. Here we report crystal structures of apo and ABA-bound receptors as well as a ternary PYL2–ABA–PP2C complex. The apo receptors contain an open ligand-binding pocket flanked by a gate that closes in response to ABA by way of conformational changes in two highly conserved β-loops that serve as a gate and latch. Moreover, ABA-induced closure of the gate creates a surface that enables the receptor to dock into and competitively inhibit the PP2C active site. A conserved tryptophan in the PP2C inserts directly between the gate and latch, which functions to further lock the receptor in a closed conformation. Together, our results identify a conserved gate–latch–lock mechanism underlying ABA signalling.

中文翻译：

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脱落酸受体的激素信号门锁机制

脱落酸 (ABA) 是一种普遍存在的激素，可调节植物生长、发育和对环境胁迫的反应。其作用由 START 蛋白的 PYR/PYL/RCAR 家族介导，但尚不清楚这些受体如何结合 ABA，以及激素结合如何导致下游 2C 型蛋白磷酸酶 (PP2C) 效应器的抑制。在这里，我们报告了 apo 和 ABA 结合受体以及三元 PYL2-ABA-PP2C 复合物的晶体结构。apo 受体包含一个开放的配体结合口袋，两侧是一个门，该门通过两个高度保守的 β 环的构象变化而关闭，作为门和闩锁。此外，ABA 诱导的门关闭产生了一个表面，使受体能够停靠并竞争性抑制 PP2C 活性位点。PP2C 中保守的色氨酸直接插入门和闩锁之间，其功能是进一步将受体锁定在闭合构象中。总之，我们的结果确定了 ABA 信号传递背后的保守门-闩锁机制。