Emerging complexities of lipid regulation of potassium channels

The Rockefeller University Press $30.00 J. Gen. Physiol. 2016 Vol. 148 No. 3 201–205 www.jgp.org/cgi/doi/10.1085/jgp.201611671 201 Inwardly rectifying potassium (Kir) channels are important regulators of cellular excitability, enabling cells to alter electrical activity in response to diverse autonomic and metabolic signals (Hibino et al., 2010). Fairly or not, the Kir channel family is sometimes derided as the least complex or simplest of the eukaryotic potassium channels because they lack the widely studied transmembrane architecture of their voltage-gated cousins. However, there are many instances where discoveries about Kir channel regulation have presaged important changes in our understanding of electrical signaling mechanisms. For example, the demonstration that GIRK (G protein–coupled inwardly rectifying K) channels are regulated by Gβγ subunits was among the first evidence that Gβγ can directly target effectors in a GPCR (G protein–coupled receptor)-initiated signaling cascade (Logothetis et al., 1987). Kir channels also led the way toward our current understanding of ion channel regulation by lipid environments and the notion that PIP2 (phosphatidyl-inositol-4,5-bisphosphate) is an essential ion channel modulator. Although PIP2 is now recognized as an important regulator of a large number of ion channel types, its effects on electrical signaling were first recognized in KATP (ATP-sensitive K channel, Kir6) and GIRK (Kir3) channels (Hilgemann and Ball, 1996; Baukrowitz et al., 1998; Huang et al., 1998; Shyng and Nichols, 1998). These discoveries helped to establish what is now a deep literature describing a variety of ion channels targeted by PIP2, the molecular determinants of PIP2 sensitivity, and the physiological regulation of cellular excitability by PIP2. Part of the effectiveness of PIP2 as a signaling molecule arises from its weak and regulated abundance in the plasma membrane, but this also means that ion channels sit in a sea of other phospholipids, and we do not yet have a detailed understanding of how this bulk environment might influence ion channel function. In this issue of The Journal of General Physiology, Lee et al. build on a series of recent papers that have added considerable complexity to our understanding of ion channel regulation by phospholipids other than PIP2 and provide structural insights into the underlying mechanism in Kir2.x channels. Like all Kir channels, Kir2.x channels have what seems to be an absolute functional requirement for the anionic phospholipid PIP2. This important phospholipid binds to Kir2.2 and other Kir channels in a common, crystallographically defined location near the channel gate, where several positively charged residues interact with phosphate groups on the inositol ring (Hansen et al., 2011). We are fortunate to have structural data for multiple eukaryotic Kir channels in both the apoand PIP2-bound states. In some cases, as for the Kir2.2 structures, the apo state exhibits a detachment of the C-terminal domain (CTD) from the transmembrane domain (TMD) of the channel (Fig. 1). However, cocrystallization with PIP2 results in a structure in which the CTD and TMD are fully engaged via the slide helix in a well-defined interface (Tao et al., 2009; Hansen et al., 2011). The structural consequences of PIP2 binding in a series of Kir3.2 structures are less pronounced than in Kir2.2, but these conformational changes around the channel gate highlight the importance of a cluster of positively charged residues that bind PIP2 (Whorton and MacKinnon, 2011, 2013). From a functional perspective, it is important to note that, in most Kir channels, there is a strong preference for PI(4,5)P2. Other combinations of inositol head group phosphorylation are often insufficient for function (Rohács et al., 2003), and other anionic phospholipids are ineffective substitutes for PI(4,5)P2 in this canonical binding site. Despite the fairly strict requirement for PIP2, it has become increasingly clear that PIP2 is not the only determinant of Kir channel activity and that lipid membrane composition can modulate PIP2 effects. In a series of articles investigating the functional consequences of defined lipid environments on Kir2.x channels, a secondary lipid requirement for Kir2 channels was revealed, which is distinct from the canonical PIP2 binding site described in crystal structures to date (Cheng et al., 2011; D’Avanzo et al., 2013; Lee et al., 2013). The initial observation leading to this hypothesis was that reconstituted Kir2.x channels required significantly different PIP2 levels for activity, depending on whether the channel was reconstituted in a neutral lipid (e.g., with phosphatidylethanolamine [PE] or phosphatidylcholine [PC] head groups) versus supplementation with an anionic lipid (e.g., with a phosphatidylglycerol [PG] head Emerging complexities of lipid regulation of potassium channels