Ion channel regulation by protein S-acylation

Abstract

Protein S-acylation, the reversible covalent fatty-acid modification of cysteine residues, has emerged as a dynamic posttranslational modification (PTM) that controls the diversity, life cycle, and physiological function of numerous ligand- and voltage-gated ion channels. S-acylation is enzymatically mediated by a diverse family of acyltransferases (zDHHCs) and is reversed by acylthioesterases. However, for most ion channels, the dynamics and subcellular localization at which S-acylation and deacylation cycles occur are not known. S-acylation can control the two fundamental determinants of ion channel function: (1) the number of channels resident in a membrane and (2) the activity of the channel at the membrane. It controls the former by regulating channel trafficking and the latter by controlling channel kinetics and modulation by other PTMs. Ion channel function may be modulated by S-acylation of both pore-forming and regulatory subunits as well as through control of adapter, signaling, and scaffolding proteins in ion channel complexes. Importantly, cross-talk of S-acylation with other PTMs of both cysteine residues by themselves and neighboring sites of phosphorylation is an emerging concept in the control of ion channel physiology. In this review, I discuss the fundamentals of protein S-acylation and the tools available to investigate ion channel S-acylation. The mechanisms and role of S-acylation in controlling diverse stages of the ion channel life cycle and its effect on ion channel function are highlighted. Finally, I discuss future goals and challenges for the field to understand both the mechanistic basis for S-acylation control of ion channels and the functional consequence and implications for understanding the physiological function of ion channel S-acylation in health and disease.

Figure 1.

Protein S-acylation: a reversible lipid posttranslational modification of proteins. (A) Major lipid modifications of proteins. S-acylation is reversible due to the labile thioester bond between the lipid (typically, but not exclusively, palmitate) and the cysteine amino acid of is target protein. Other lipid modifications result from stable bond formation between either the N-terminal amino acid (amide) or the amino acid side chain in the protein (thioether and oxyester). The zDHHC family of palmitoyl acyltransferases mediates S-acylation with other enzyme families controlling other lipid modifications: N-methyltransferase (NMT) controls myristoylation of many proteins such as the src family kinase, Fyn kinase; and amide-linked palmitoylation of the secreted sonic hedgehog protein is mediated by Hedgehog acyltransferase (Hhat), a membrane-bound O-acyl transferase (MBOAT) family. Prenyl transferases catalyze farnesyl (farnesyltransferase, FTase) or geranylgeranyl (geranylgeranyl transferase I [GGTase I] and geranylgeranyl transferase II [GGTase II]) in small GTPase proteins such as RAS and the Rab proteins, respectively. Porcupine (Porcn) is a member of the MBOAT family acylates secreted proteins such as Wnt. (B) zDHHC enzymes typically use coenzyme A (CoA)-palmitate; however, other long chain fatty acids (either saturated or desaturated) can also be used. Deacylation is mediated by several acylthioesterases of the serine hydrolase family. (C) zDHHC acyltransferases (23 in humans) are predicted transmembrane proteins (typically with 4 or 6 transmembrane domains) with the catalytic DHHC domain located in a cytosolic loop.

Figure 2.

Protein S-acylation and regulation of the ion channel lifecycle zDHHCs are found in multiple membrane compartments and regulate multiple steps in the ion channel lifecycle including: (1) assembly and (2) ER exit; (3) maturation and Golgi exit; (4) sorting and trafficking; (5) trafficking and insertion into target membrane; (6) clustering and localization in membrane microdomains; control of properties, activity (7), and regulation by other signaling pathways; and (8) internalization, recycling, and final degradation.

Figure 3.

S-acylation sites in ion channel pore-forming subunits. (A) Schematic illustrating different locations of cysteine S-acylation in transmembrane ion channels subunits. (B) Relative proportion of identified S-acylated cysteine residues: in each location indicated in A (top); in -C-, -CC-, or -Cx_(2–3)C- motifs (middle); or in cytosolic regions of predicted protein disorder (bottom; determined using multiple algorithms on the DisProt server, http://www.disprot.org/metapredictor.php; Sickmeier et al., 2007) for transmembrane ion channel pore-forming subunits.

Figure 4.

Multisite S-acylation in ion channels controls distinct functions. (A–C) Schematic illustrating location of multiple S-acylated domains in AMPA receptor GluA1–4 subunits (A), NMDA receptor GluN2A subunits (B), and BK channel pore-forming α subunits (C), encoded by the Kcnma1 gene. Each domain confers distinct functions/properties on the respective ion channel and is regulated by distinct zDHHCs (see the “Control of ion channel cell surface expression and spatial organization in membranes” section for further details).

Figure 5.

S-acylation controls BK channel trafficking and regulation by AGC family protein kinases via distinct sites. The BK channel STREX splice variant pore-forming α subunit is S-acylated at two sites: the S0-S1 loop and the STREX domain in the large intracellular C terminus. S-acylation of the S0-S1 loop promotes high surface membrane expression of the channel; thus, deacylation of this site decreases the number of channels at the cell surface (see the “Control of ion channel cell surface expression and spatial organization in membranes” section for further details). In contrast, S-acylation of the STREX domain allows inhibition of channel activity by PKA-mediated phosphorylation of a PKA serine motif (closed hexagon) immediately upstream of the palmitoylated cysteine residues in STREX. In the S-acylated state, PKC has no effect on channel activity even though a PKC phosphorylation site serine motif is located immediately downstream of the STREX domain (open triangle). Deacylation of STREX dissociates the STREX domain from the plasma membrane, and exposes the PKC serine motif so that it can now be phosphorylated by PKC (closed triangle), resulting in channel inhibition. In the deacylated state, PKA has no effect on channel activity (open hexagon). Thus, deacylation of the STREX domain switches channel regulation from a PKA-inhibited to a PKC-inhibited phenotype (see the “S-acylation and posttranslational cross-talk controls channel trafficking and activity” section for further details).