Regulation of the plasma membrane proton pump (H+-ATPase) by phosphorylation
Section snippets
Role of H+-ATPase in maintaining the membrane potential and acidic pH at the plasma membrane surface
Plant cell growth is highly influenced by the environment, as well as by predetermined developmental programs. Plasma membrane H+-ATPases (H+-pumps) are the primary active transporters that translocate protons to the outside of each cell, providing the electrical and chemical energy that drives solute transport. In plants, this enzyme also provides an acidic environment in the cell wall that is favorable for cell expansion (Figure 1). In animals, a Na+/K+ ATPase provides similar function as the
Phosphorylation regulates PM H+-ATPases
Regulatory phosphorylation sites are mainly clustered in the 100-amino-acid long carboxy terminus, also referred to as the R (regulatory) or auto-inhibitory domain [10]. AHA2 is subject to in vivo phosphorylation on at least seven residues in this C-terminal tail (Figure 2) and changes in AHA2 phosphopeptide abundance has been analyzed via mass spectrometry, either in an untargeted fashion using ion trap based MS/MS or via targeted methods using selected reaction monitoring (SRM) on a triple
Correlation of PM H+-ATPase phosphorylation, catalytic activity, and plant phenotypes
The timing and direction of the changes in H+-ATPase phosphorylation at each of the sites may provide clues on the molecular mechanism of transport function, as well as the roles of the enzyme in plant life. However, since the protonmotive force (PMF) plays such a fundamental role in many functions (most transporters and channels are coupled to either the delta pH or the membrane potential or both), one needs to carefully interpret a causal direct link between AHA phosphorylation and biological
Auxin regulates PM H+-ATPase phosphorylation to promote cell expansion
Based largely on physiological studies correlating auxin-induced elongation growth and apoplastic acidification, auxin has long been hypothesized to activate the plasma membrane H+-ATPases. Indeed, auxin-mediated activation of H+ efflux is a fundamental tenet of the decades-old acid growth theory [7]. Recent work has provided crucial biochemical and genetic support for this hypothesis, as well as the beginnings of mechanistic insight. In a recent study examining the elongation of auxin depleted
Kinases and phosphatases implicated in H+-ATPase regulation
Identifying protein kinases and phosphatases that modify H+-ATPase activity would help understand the regulatory mechanisms of this enzyme during plant growth and adaptation to changes in the environment. In addition to the above-described PP2C.D1 phosphatase involved in SAUR-regulated cell expansion, to date several additional protein kinases and phosphatases have been proposed to directly or indirectly regulate H+-ATPase activity. Consistent with the plasma membrane localization, H+-ATPase
Summary and the future questions
Since the emergence of sophisticated mass spectrometry-based techniques for the identification and quantitation of protein phosphorylation, considerable information has accumulated concerning the chemical basis for H+-ATPase regulation. Genetic mutation and phenotypic assays provide crucial insights into the context-specific roles of these chemical changes in vivo. However, the key questions that remain to be solved include: (1) how many protein kinases and phosphatase are required for the
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This study is supported by U.S. Department of Energy (Grant No. DEFG02-88ER13938 to MRS) and the National Science Foundation (Grant No. MCB 0929395 to MRS). Work in WMG's lab is supported by National Institutes of Health Grant GM067203. The authors thank many other outstanding researchers in the field who contributed toward understanding how the plasma membrane H+-ATPase is regulated. Due to limited space, we were unable to cite many of these excellent H+-ATPase studies.
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