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

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.

The regulation of plasma membrane H^+-ATPases and autoinhibited Ca2^+-ATPases exhibits a complex and dynamic network of posttranslational regulation. The regulation of plasma membrane H⁺-ATPases and autoinhibited Ca²⁺-ATPases exhibits a complex and dynamic network of posttranslational regulation.

P-type ATPases are found in all domains of life and constitute a large superfamily of membrane-bound pumps that share a common machinery, including a reaction cycle that involves catalytic phosphorylation of an Asp, resulting in a phosphorylated intermediate (reviewed in Palmgren and Nissen, 2011; (hence the name P-type; Box 1). The catalytic phosphoryl-aspartate intermediate is not to be confused with regulatory phosphorylation, which occurs on Ser, Thr, and Tyr residues. Five major families of P-type ATPases have been characterized (P1–5), each of which is divided into a number of subfamilies (named with letters). Plasma membrane H⁺-ATPases are classified as P3A ATPases, whereas Ca²⁺ pumps constitute P2A and P2B ATPases. In plants, these pumps are best characterized in the model plant Arabidopsis thaliana (Arabidopsis).Box 1Enzymology of P-type ATPases.P-type ATPases (reviewed in Palmgren and Nissen, 2011) alternate between two extreme conformations during their catalytic cycle: a high-affinity (with respect to ATP and the ion to be exported) Enzyme1 (E1) state, and a low-affinity Enzyme2 (E2) state. Many P-type ATPases are autoinhibited by built-in molecular constraints, namely their C- and N-terminal (for plasma membrane H⁺-ATPases; Palmgren et al., 1999) or N-terminal (for P2B Ca²⁺-ATPases; Malmström et al., 1997) regulatory (R) domains of approximately 100 amino acid residues, which act as brakes by stabilizing the pumps in a low-affinity conformation (Palmgren and Nissen, 2011), most likely E2. Neutralizing the R domain results in a shift in conformational equilibrium towards a high-affinity state, likely E1. In this way, the R domains of plasma membrane H⁺-ATPases and Ca²⁺-ATPases allow posttranslational modification events to control the turnover numbers of these pumps. A structure of a plasma membrane H⁺-ATPase (from the distantly related yeast S. cerevisiae) in its autoinhibited state has been solved (Heit et al., 2021). Its R domain is situated adjacent to the P domain, which would suggest that the R domain functions to restrict the conformational flexibility of the pump. Normally, the hydrolysis of ATP and transport are tightly coupled in P-type ATPases. Therefore, P-type ATPases hydrolyze bound ATP as soon as their ligand-binding site(s) in the membrane region are occupied, but not before. Thus, increasing the ligand affinity of an ATPase simultaneously increases its turnover number, provided that the concentration of ATP is not limiting, which is rarely the case in cells. A specific feature of plasma membrane H⁺-ATPases is that in the autoinhibited state, ATP hydrolysis is only loosely coupled to H⁺ pumping, whereas pump activation results in tight coupling, with one H⁺ pumped per ATP split (Pedersen et al., 2018).In response to internal and/or external cues, plasma membrane H⁺-ATPase and Ca²⁺-ATPase activities are controlled by intracellular concentrations of H⁺ and Ca²⁺, respectively, via interacting proteins, through posttranslational modification by phosphorylation, and by regulated trafficking of the pump to and from the plasma membrane. Their regulation sometimes involves changes in gene expression and turnover, although this is rare, perhaps because both processes are time- and energy-consuming (Haruta et al., 2018).