Mg²⁺ modulates the activity of hyperpolarization-activated calcium currents in plant cells

ABSTRACT

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 Ca²⁺-dependent signaling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open at physiological hyperpolarized resting membrane potentials (the so called pump or P-state), thus, if not regulated, they would be continuously leaking Ca²⁺ into cells. In guard cells, HACCs are permeable to Ca²⁺, Ba²⁺ and Mg²⁺, activated by H₂O₂ and the plant hormone abscisic acid (ABA) and their activity is greatly reduced by low amounts of free cytosolic Ca²⁺ ([Ca²⁺]_Cyt) and hence will close during [Ca²⁺]_Cyt surges. Here we demonstrate that the presence of the commonly used Mg-ATP inside the cell greatly reduces HACC activity especially at voltages ≤ −200 mV and that Mg²⁺ causes this block. We therefore conclude, firstly, that physiological cytosolic Mg²⁺ levels affect HACCs gating and that channel opening requires either high negative voltages (≥ −200 mV) and/or displacement of Mg²⁺ away from the immediate vicinity of the channel. Secondly, based on structural comparisons with Mg²⁺-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 di-acidic Mg²⁺-binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg²⁺, much like in animal cells, is an important component in Ca²⁺ signalling and homeostasis in plants.