Proton and calcium pumping P-type ATPases and their regulation of plant responses to the environment

Abstract

Plant plasma membrane H+-ATPases and Ca2+-ATPases maintain low cytoplasmic concentrations of H+ and Ca2+, respectively, and are essential for plant growth and development. These low concentrations allow plasma membrane H+-ATPases to function as electrogenic voltage stats, and Ca2+-ATPases as "off" mechanisms in Ca2+-based signal transduction. Although these pumps are autoregulated by cytoplasmic concentrations of H+ and Ca2+, respectively, they are also subject to exquisite regulation in response to biotic and abiotic events in the environment. A common paradigm for both types of pumps is the presence of terminal regulatory (R) domains that function as autoinhibitors that can be neutralized by multiple means, including phosphorylation. A picture is emerging in which some of the phosphosites in these R domains appear to be highly, nearly constantly phosphorylated, whereas others seem to be subject to dynamic phosphorylation. Thus, some sites might function as major switches, whereas others might simply reduce activity. Here, we provide an overview of the relevant transport systems and discuss recent advances that address their relation to external stimuli and physiological adaptations.

Figure 1

Phosphosites in the C-terminal regions of AHAs. In vivo phosphosites are highlighted in red boxes with white text. Underlined sequences represent phosphopeptides that cannot be associated with a unique isoform (see Table 1). Residues in the autoinhibitory regions (Regions I and II) are marked in bold. Conserved amino acid residues are colored according to side chain properties (brown, charged; green, polar; blue, hydrophobic; pink, no side chain).

Figure 2

Model for the regulation of AHAs. Arabidopsis AHA2 is used as an example. In response to environmental and developmental signals, multiple protein kinases phosphorylate AHA2 at multiple residues, most of which are located in the C-terminal autoinhibitory R domain. Phosphorylation of Thr947, the penultimate residue, generates a binding site for activating 14-3-3 protein. Phosphorylation at other sites can lead to both activation and de-activation of AHA2 activity, e.g. by preventing binding of 14-3-3 protein. Protein kinase-mediated phosphorylation is countered by protein phosphatases, and the combined action of protein kinases and phosphatases dictates the activation status of AHA2. Colors of fonts and arrows: red, negative regulation; green, positive regulation. Solid arrows, known candidates; Dashed arrows, suggested candidates. PSY1R, PSY1 receptor; PKS5, sos2-like protein kinase 5.

Figure 3

Phosphosites in the N-terminal regions of ACAs. Red boxes with white text represent phosphosites observed in vivo, except for the ACA2 phosphosite, which was phosphorylated in vitro. Underlined sequences represent phosphopeptides that cannot be associated with a unique isoform (see Table 2). Hydrophobic anchor points involved in calmodulin binding are marked with asterisks. Residues in CaMBS1 and CaMBS2 are marked in bold. Conserved amino acid residues are colored according to side chain properties (brown, charged; green, polar; blue, hydrophobic; pink, no side chain).

Figure 4

Model for the regulation of ACAs. Arabidopsis ACA8 is used as an example. In Ca²⁺ signaling, an “on” period, which is induced by external signals that provoke a rapid influx and surge in [Ca²⁺]cyt, is always followed by an “off” period, when Ca²⁺ is removed from the cytoplasm again (Berridge et al., 2000, 2003). This results in a Ca²⁺ spike, which can be repeated rhythmically to produce Ca²⁺ oscillations or can proceed throughout the plant as a Ca²⁺ wave. Both the influx and efflux of Ca²⁺ involve multiple transport proteins whose combined activity determines the amplitude and frequency of the Ca²⁺ signal (Kudla et al., 2018; Tian et al., 2020). Furthermore, it has been proposed (Hwang et al., 2000; Sze et al., 2000) that the opposite actions of calmodulin and phosphorylation allow for crosstalk between different Ca²⁺ signaling pathways that modulate the initial calmodulin activation kinetics to shape a specific Ca²⁺ signature. The brown structure interacting with the receptor kinase to the left depicts a bacterium with its flagellum. Colors of fonts and arrows: red, negative regulation; green, positive regulation. Solid arrows, known candidates; Dashed arrows, suggested candidates.CNGC2/4, Cyclic Nucleotide Gated Channel 2 and 4.