Membrane pacman: small steps for the voltage‐sensitive phosphatases

Abstract

The voltage-sensitive phosphatases (VSPs) are a fascinating family of membrane-associated enzymes consisting of a voltage-sensing domain (VSD), similar to that found in canonical voltage-gated channels, and an intracellular catalytic domain. Although found in every eukaryotic organism, their functions are not at all clear. Since many of these phosphatases catalyse the degradation of important signalling lipids such as phosphatidylinositol 4,5-bisphosphate (PIP2), they serve several important functions. For example, they control sperm mobility and play a role in depolarization-induced calcium transients upon egg fertilization. It has been speculated that they are important in the control of cell proliferation and organ differentiation; however, our understanding of the physiological roles of these proteins is still developing (Villalba-Galea, 2012). Voltage-dependent control of enzymatic activity in VSPs was first demonstrated by Murata et al. (2005). It was also shown that changes in voltage produce charge movement mediated by the VSD that can be recorded as transient sensing currents, related to the gating currents of voltage-dependent ion channels. One of the major questions still needing clarification is what is the mechanism by which the VSD controls the enzymatic activity. Progress in this area of research has been rapid and the article by Sakata and Okamura in this issue of The Journal of Physiology (Sakata & Okamura, 2014) represents an important advance into this problem. In the case of the tetrameric voltage-gated ion channels, four VSDs are coupled to the opening of a single pore domain, which can be thought of as the analogue of the catalytic domain in VSPs. It has been known for some time that membrane depolarization causes outward movement of the VSDs and that this is coupled to opening of the channel pore through a specialized linker domain. For these proteins, the electromechanical coupling process is strict, meaning that all four VSDs have to be in an activated position for the pore to be open and this relationship can be evidenced experimentally because charge movement, in the form of gating or sensing currents, precedes channel opening at all voltages. It also means that spontaneous or voltage-independent opening of the channel pore is exceedingly improbable. An important similar question for the VSPs relates to the interplay between VSD movements, as reported by the transient sensing current, and the activity of the phosphatase domain. Does the VSD need to be fully activated upon depolarization for the enzymatic activity to take place? The X-ray structures of the catalytic domain suggest a direct and graded control of the catalytic domain by voltage through activation of the VSD, but this point still requires clarification (Kohout et al. 2010). Sakata & Okamura (2014) now shed light on this matter in their novel studies that make use of a clever combination of diverse mutants of the Danio rerio (zebrafish) phosphatase, known as Dr-VSP. Their present findings demonstrate that the phosphatase activity is linked to VSD movement in a graded fashion. In other words, the voltage sensor domain does not need to be fully activated to trigger the enzymatic activity. The experiments in this work thus are consistent with a coupling between charge movement and enzyme activity that is different from what has been observed in voltage-gated potassium ion channels. These data suggest instead that, in contrast to voltage-gated channels, an allosteric coupling model could explain the relationship between activation of the voltage sensor and the catalytic domain of Dr-VSP (Fig. 1). In such a mechanism, both the resting and the activated state(s) of the voltage sensor can drive the activation of the phosphatase domain, although with different efficiencies. If this is the case, one immediate prediction of an allosteric coupling model is that there should be some phosphatase activity at negative potentials. In reality such spontaneous and voltage-independent activity might be very difficult to detect. Another consequence of the allosteric mechanism is that charge movement should have different voltage dependence when the phosphatase is in the active or the inactive conformation. Sakata & Okamura (2014) have thus opened the gates for such future experiments, which will enhance our understanding of the inner workings of these mesmerizing proteins. The resting states of the voltage-sensing domains, Ri, are occupied at negative voltages, while depolarization shifts the equilibrium towards activated states, Ai. The orange arrows represent the charge-moving transitions that give rise to the sensing current. Canonical voltage-gated channels follow the path enclosed in grey, in which the open state is reached only after all charge-moving transitions take place, while the voltage-sensitive phosphatase Dr-VSP can access the activated (catalytic) states, Ai, from any of the resting states, Ri. None declared.