PKC regulation of ion channels: The involvement of PIP2

Abstract

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.

Keywords: PKC; inwardly rectifying potassium channels; ion channel, phosphorylation; phosphatidylinositol (4,5) bisphosphate; transient receptor potential channels; voltage-gated potassium channel.

Figure 1

Schematic representation of PKC isozyme structures and conventional PKC (cPKC) activation. A, C1 (orange) domains in the N-terminal region bind diacylglycerol (DAG) and phorbol esters like phorbol myristate acetate (PMA). The C2 domain (blue) binds anionic lipids and calcium. The pseudosubstrate region (blue), which is located on the N-terminal end of the C1 domain, is a sequence of amino acids that mimics a substrate; the C1 domain in aPKCs binds PIP₃ or ceramide. The C-terminal catalytic domain contains the ATP-binding domain C3 (magenta) and the substrate binding domain C4 (magenta). B, hydrolysis of PIP₂ by phospholipase C releases two second messengers, DAG and inositol triphosphate (IP₃). IP₃ releases calcium from the endoplasmic reticulum (ER), which binds to the C2 domain of cPKCs while DAG binds to the C1 domain; the pseudosubstrate is released resulting in activation of cPKC and translocation to the cell membrane. (Purple circles, Ca²⁺; green, IP₃ receptor in ER). aPKC, atypical PKC; PIP₂, phosphatidylinositol (4,5) bisphosphate.

Figure 2

PIP₂is a master regulator of the function of ion channels (22, 23, 24, 27, 86, 103, 112, 171, 172). Ten different ion transporting membrane protein families are shown to be dependent on PIP₂ for their activity and the specific direction of the control of the activity of these proteins by PIP₂ is indicated. PIP₂, phosphatidylinositol (4,5) bisphosphate.

Figure 3

PKC can regulate the activity of ion channels in a PIP₂-dependent manner (31, 39, 57, 58, 63, 95, 173). Examples of ion channels where PKC regulation of their activity has been linked to alteration of channel–PIP₂ interactions. PIP₂, phosphatidylinositol (4,5) bisphosphate.

Figure 4

D228 and phosphorylated S196 compete for R201 to gate the GIRK2 channel (55). In the absence of Na⁺ and phosphorylation of S196, the D228 residue (that can bind Na⁺) forms a salt bridge interaction with R201 stabilizing the helix bundle crossing (HBC) gate of the GIRK2 channel in the constricted conformation (top). Phosphorylation of S196 competes away R201 from D228 causing HBC channel gate dilation (bottom). GIRK, G-protein-gated inwardly rectifying K⁺ channel.