Anticipating antiport in P-type ATPases

doi:10.1016/j.tibs.2007.12.005

Trends in Biochemical Sciences

Volume 33, Issue 4, April 2008, Pages 156-160

https://doi.org/10.1016/j.tibs.2007.12.005 Get rights and content

Cation-transporting P-type ATPases show a high degree of structural and functional homology. Nevertheless, for many members of this large family, the molecular mechanism of transport is unclear; namely, whether transport is electrogenic or not and if countertransport is involved remains to be established. In a few well-studied cases such as the Na⁺–K⁺-ATPase, plasma membrane Ca²⁺ ATPase (PMCA) and sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) countertransport has been clearly demonstrated. New data based on the crystal structure of SERCA now strongly indicate that countertransport could be mandatory for all P-type ATPases. This concept should be verified for other known and for all newly characterized P-type ATPases.

Section snippets

Introduction: P-type ATPases – a ubiquitous family of transport proteins

P-type ATPases comprise a large ubiquitous family of transmembrane proteins that mediate ion transport processes in the plasma membrane and intracellular organelles in almost all organisms, including bacteria, archaea and eukaryotes. The phosphorylated (P) intermediate of the transport protein gives the family its name. P-type pumps use ATP to maintain ion gradients across the cell membrane. On the basis of sequence conservation, >260 potential members of the P-type ATPase family were

Analyzing transport stoichiometries and electrogenicity of P-type ATPases

Before discussing the mechanisms used by our model P-type ATPases, it is first necessary to briefly introduce the techniques that have been used to determine the stoichiometries of their transport so that the reader is aware of their benefits and limitations. Although there are pitfalls in providing confident estimates for the number of counterions in an antiporter mechanism, such a mechanism can still be distinguished from a uniporter mechanism.

Transport stoichiometries and charge transfer of

The model P-type ATPases

The following three P-type ATPases will be presented in more detail for two reasons. First, they belong to the best-characterized pumps of this type and, second, they are biologically relevant for either all or most eukaryotic cells. The first enzyme is Na⁺–K⁺-ATPase. This pump builds up the Na and K ion gradients across the plasma membrane. These gradients are vital to all cells. The second enzyme is PMCA, which provides the low intracellular Ca²⁺ concentration observed at resting conditions

Further evidence for counterion transport from the SERCA structure

Obara et al. [27] have provided a 2.4-Å-resolution crystal structure of SERCA in the absence of calcium stabilized by inhibitors. Models based on this structure show that water and protons fill in the spaces and compensate for the severe charge imbalance created by calcium release. The four carboxyl groups clustered in the calcium-binding sites are likely to be deleterious to structural integrity if left ionized (Figure 3). So, in addition to being important for balancing charge across the

Concluding remarks

For many members of the large family of cation transporting P-type ATPases, the molecular mechanism of transport, whether electrogenic or not and whether countertransport is involved, is unclear. This is due to experimental difficulties in the determination of transport properties, which requires purification of functional enzymes and subsequent reconstitution into phospholipids vesicles. However, in the best-studied cases outlined above, countertransport has clearly been demonstrated and the

Note added in proof

After acceptance of this article, Pedersen et al. [33] published the crystal structure of a plant plasma membrane proton pump. Based on this structure, they propose that Arg655 might act as a built-in counterion, neutralizing the deprotonated negatively charged Asp684, thus removing the structural requirement for counter transport. However, full electrogenicity of the proton transport still remains to be demonstrated.

Acknowledgements

Our work is supported by the Swiss National Science Foundation grants No. 3100A0–103708/1 (V.N.) and No. 3100A0–105272/1 (E.S.).

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Trends in Biochemical Sciences

OpinionAnticipating antiport in P-type ATPases

Section snippets

Introduction: P-type ATPases – a ubiquitous family of transport proteins

Analyzing transport stoichiometries and electrogenicity of P-type ATPases

The model P-type ATPases

Further evidence for counterion transport from the SERCA structure

Concluding remarks

Note added in proof

Acknowledgements

FEBS Lett.

J. Biol. Chem.

J. Biol. Chem.

Cell Calcium

J. Biol. Chem.

J. Biol. Chem.

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Biophys. J.

Biochim. Biophys. Acta

Curr. Opin. Chem. Biol.

Biochem. Biophys. Res. Commun.

FEBS Lett.

J. Biol. Chem.

J. Biol. Chem.

Opinion
Anticipating antiport in P-type ATPases