King Abdullah University of Science and Technology (KAUST), Biological and Environmental Science and Engineering (BESE), Thuwal 23955-6900, Saudi Arabia.
Department of Neuroscience, University of Connecticut School of Medicine, Farmington, CT 06030, USA.
Int J Mol Sci. 2020 May 27;21(11):3771. doi: 10.3390/ijms21113771.
Hyperpolarization-activated calcium channels (HACCs) are found in the plasma membrane and tonoplast of many plant cell types, where they have an important role in Ca-dependent signalling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open in the physiological hyperpolarized resting membrane potential state (the so-called pump or P-state); thus, if not regulated, they would continuously leak Ca into cells. HACCs are permeable to Ca, Ba, and Mg; activated by HO and the plant hormone abscisic acid (ABA); and their activity in guard cells is greatly reduced by increasing amounts of free cytosolic Ca ([Ca]), and hence closes during [Ca] surges. Here, we demonstrate that the presence of the commonly used Mg-ATP inside the guard cell greatly reduces HACC activity, especially at voltages ≤ -200 mV, and that Mg causes this block. Therefore, we firstly conclude that physiological cytosolic Mg levels affect HACC gating and that channel opening requires either high negative voltages (≥ -200 mV) or displacement of Mg away from the immediate vicinity of the channel. Secondly, based on structural comparisons with a Mg-sensitive animal inward-rectifying K channel, we propose that the likely candidate HACCs described here are cyclic nucleotide gated channels (CNGCs), many of which also contain a conserved diacidic Mg binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg, much like in animal cells, is an important component in Ca signalling and homeostasis in plants.
超极化激活钙通道（HACCs）存在于许多植物细胞类型的质膜和液泡膜中，在 Ca 依赖性信号转导中发挥着重要作用。植物 HACCs 的独特门控特性，即通过膜超极化而不是去极化激活，决定了 HACCs 在生理超极化静息膜电位状态（所谓的泵或 P 态）下通常是开放的；因此，如果不受调节，它们会持续将 Ca 漏入细胞。HACCs 对 Ca、Ba 和 Mg 具有通透性；被 HO 和植物激素脱落酸（ABA）激活；细胞溶质 Ca([Ca])增加会大大降低保卫细胞中的 HACC 活性，因此在 [Ca] 激增时关闭。在这里，我们证明了在保卫细胞中普遍存在的 Mg-ATP 大大降低了 HACC 的活性，特别是在 ≤ -200 mV 的电压下，而 Mg 导致了这种阻断。因此，我们首先得出结论，生理细胞溶质 Mg 水平影响 HACC 门控，并且通道开放需要高负电压（≥ -200 mV）或使 Mg 从通道的直接附近移位。其次，基于与 Mg 敏感的动物内向整流 K 通道的结构比较，我们提出这里描述的可能候选 HACCs 是环核苷酸门控通道（CNGCs），其中许多通道的孔内还含有保守的二酸性 Mg 结合基序。这一结论与电生理数据一致。最后，我们提出 Mg 与动物细胞一样，是植物中 Ca 信号转导和稳态的重要组成部分。
