J Physiol 592.5 (2014) pp 823–824 823

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Membrane pacman: small stepsfor the voltage-sensitivephosphatases

Leon D. IslasDepartamento de Fisiologıa, Facultad deMedicina, UNAM, Mexico City, Mexico

Email: islas@liceaga.facmed.unam.mx

The voltage-sensitive phosphatases (VSPs)are a fascinating family of membrane-associated enzymes consisting of a voltage-sensing domain (VSD), similar to that foundin canonical voltage-gated channels, andan intracellular catalytic domain. Althoughfound in every eukaryotic organism,their functions are not at all clear.Since many of these phosphatases catalysethe degradation of important signallinglipids such as phosphatidylinositol 4,5-bisphosphate (PIP2), they serve severalimportant functions. For example, theycontrol sperm mobility and play a rolein depolarization-induced calcium trans-ients upon egg fertilization. It has beenspeculated that they are important in thecontrol of cell proliferation and organdifferentiation; however, our understanding

Figure 1. Schematic representation of the subunits and gating paths of VSPs and ionchannelsThe resting states of the voltage-sensing domains, Ri, are occupied at negative voltages, whiledepolarization shifts the equilibrium towards activated states, Ai. The orange arrows representthe charge-moving transitions that give rise to the sensing current. Canonical voltage-gatedchannels follow the path enclosed in grey, in which the open state is reached only after allcharge-moving transitions take place, while the voltage-sensitive phosphatase Dr-VSP can accessthe activated (catalytic) states, Ai, from any of the resting states, Ri.

of the physiological roles of these proteins isstill developing (Villalba-Galea, 2012).

Voltage-dependent control of enzymaticactivity in VSPs was first demonstratedby Murata et al. (2005). It was alsoshown that changes in voltage producecharge movement mediated by the VSDthat can be recorded as transient sensingcurrents, related to the gating currents ofvoltage-dependent ion channels. One of themajor questions still needing clarification iswhat is the mechanism by which the VSDcontrols the enzymatic activity. Progress inthis area of research has been rapid andthe article by Sakata and Okamura in thisissue of The Journal of Physiology (Sakata& Okamura, 2014) represents an importantadvance into this problem.

In the case of the tetrameric voltage-gatedion channels, four VSDs are coupled tothe opening of a single pore domain,which can be thought of as the analogueof the catalytic domain in VSPs. It hasbeen known for some time that membranedepolarization causes outward movementof the VSDs and that this is coupledto opening of the channel pore througha specialized linker domain. For theseproteins, the electromechanical couplingprocess is strict, meaning that all four VSDs

have to be in an activated position for thepore to be open and this relationship canbe evidenced experimentally because chargemovement, in the form of gating or sensingcurrents, precedes channel opening at allvoltages. It also means that spontaneous orvoltage-independent opening of the channelpore is exceedingly improbable.

An important similar question for theVSPs relates to the interplay between VSDmovements, as reported by the transientsensing current, and the activity of thephosphatase domain. Does the VSD needto be fully activated upon depolarizationfor the enzymatic activity to take place?The X-ray structures of the catalytic domainsuggest a direct and graded control ofthe catalytic domain by voltage throughactivation of the VSD, but this point stillrequires clarification (Kohout et al. 2010).

Sakata & Okamura (2014) now shedlight on this matter in their novel studiesthat make use of a clever combinationof diverse mutants of the Danio rerio(zebrafish) phosphatase, known as Dr-VSP.Their present findings demonstrate thatthe phosphatase activity is linked to VSDmovement in a graded fashion. In otherwords, the voltage sensor domain does notneed to be fully activated to trigger theenzymatic activity. The experiments in thiswork thus are consistent with a couplingbetween charge movement and enzymeactivity that is different from what has beenobserved in voltage-gated potassium ionchannels.

These data suggest instead that, incontrast to voltage-gated channels, anallosteric coupling model could explainthe relationship between activation of thevoltage sensor and the catalytic domain ofDr-VSP (Fig. 1). In such a mechanism,both the resting and the activated state(s) ofthe voltage sensor can drive the activationof the phosphatase domain, although withdifferent efficiencies. If this is the case,one immediate prediction of an allostericcoupling model is that there should besome phosphatase activity at negativepotentials. In reality such spontaneous andvoltage-independent activity might be verydifficult to detect. Another consequenceof the allosteric mechanism is that chargemovement should have different voltagedependence when the phosphatase is in theactive or the inactive conformation. Sakata& Okamura (2014) have thus opened the

C© 2014 The Author. The Journal of Physiology C© 2014 The Physiological Society DOI: 10.1113/jphysiol.2013.269423

824 Perspectives J Physiol 592.5

gates for such future experiments, whichwill enhance our understanding of the innerworkings of these mesmerizing proteins.

References

Kohout SC, Bell SC, Liu L, Xu Q, Minor DL Jr &Isacoff EY (2010). Nat Chem Biol 6, 369–375.

Murata Y, Iwasaki H, Sasaki M, Inaba K &Okamura Y (2005). Nature 435, 1239–1243.

Sakata S & Okamura Y (2014). J Physiol 592,899–914.

Villalba-Galea CA (2012). Front Pharmacol 3,161.

Additional information

Competing interests

None declared.

C© 2014 The Author. The Journal of Physiology C© 2014 The Physiological Society