The physiology of protein S-acylation

Abstract

Protein S-acylation, the only fully reversible posttranslational lipid modification of proteins, is emerging as a ubiquitous mechanism to control the properties and function of a diverse array of proteins and consequently physiological processes. S-acylation results from the enzymatic addition of long-chain lipids, most typically palmitate, onto intracellular cysteine residues of soluble and transmembrane proteins via a labile thioester linkage. Addition of lipid results in increases in protein hydrophobicity that can impact on protein structure, assembly, maturation, trafficking, and function. The recent explosion in global S-acylation (palmitoyl) proteomic profiling as a result of improved biochemical tools to assay S-acylation, in conjunction with the recent identification of enzymes that control protein S-acylation and de-acylation, has opened a new vista into the physiological function of S-acylation. This review introduces key features of S-acylation and tools to interrogate this process, and highlights the eclectic array of proteins regulated including membrane receptors, ion channels and transporters, enzymes and kinases, signaling adapters and chaperones, cell adhesion, and structural proteins. We highlight recent findings correlating disruption of S-acylation to pathophysiology and disease and discuss some of the major challenges and opportunities in this rapidly expanding field.

Figure 1.

Major lipid modifications of proteins. S-acylation is reversible due to the labile thioester bond between the lipid (typically, but not exclusively, palmitate) and a cysteine amino acid of a protein. The zDHHC family of palmitoyl acyltransferases mediates S-acylation. Other major lipid modifications result from stable bond formation between either the NH₂-terminal amino acid (amide) or the amino acid side chains in the protein (thioether and oxyester). Distinct enzyme families control these lipid modifications: N-myristoyltransferase (NMT) controls myristoylation of many proteins such as the src-family kinase, Fyn kinase; amide-linked palmitoylation of the secreted sonic hedgehog protein is mediated by Hedgehog acyltransferase (Hhat), a member of the membrane-bound O-acyl transferase (MBOAT) family. Prenyl transferases catalyze farnesylation (farnesyltransferase, FTase) or geranylgeranylation (geranylgeranyl transferase I and II, GGTase I and II) of small GTPase proteins such as RAS and the Rab proteins, respectively. Porcupine (Porcn), a member of the MBOAT family, acylates secreted proteins such as Wnt.

Figure 2.

Protein S-acylation: a reversible lipid posttranslational modification controlled by the zDHHC family of acyltransferases. A: zDHHC enzymes typically utilize coenzyme A (CoA)-palmitate; however, other long-chain fatty acids (either saturated or unsaturated) can also be used. Deacylation is mediated by a number of acyl thioesterases of the serine hydrolase family. B: phylogenetic tree showing the relationships of the DHHC-CR domain of the 23 human zDHHC acyltransferases that are predicted transmembrane proteins (C) (typically with 4, or 6, transmembrane domains) with the catalytic DHHC domain located in a cytosolic loop.

Figure 3.

Location of sites of S-acylation in transmembrane and peripheral-membrane proteins. Schematic illustrating different locations of cysteine S-acylation in transmembrane and peripheral membrane proteins. A: for transmembrane proteins, S-acylation may allow a cytosolic NH₂ or COOH terminus or intracellular (IC) loop of a protein to associate with the membrane interface. S-acylation can also confer structural constraints in particular when located close to, or within, transmembrane (TM) domains. In several cases, S-acylation near a TM domain has been proposed to control transmembrane orientation that may be important for controlling hydrophobic mismatch in different subcellular membrane compartments. B: for peripheral membrane proteins, S-acylation of NH₂ or COOH terminus or soluble loops may control membrane association of the protein. In many cases, membrane interaction is promoted through other membrane affinity domains (such as hydrophopic or polybasic domains) of the protein. C: alternatively, dual lipidation is required for membrane association including prenylation or myristoylation at sites close to the S-acylated cysteine residue.

Figure 4.

S-acylation regulates multiple steps in the life cycle of membrane and peripheral-membrane proteins. S-acylation can occur at multiple membranes including the ER, Golgi apparatus, trafficking and recycling vesicles/endosomes, as well as the plasma membrane. Different zDHHCs have been reported to be resident on these distinct membranes. S-acylation thus controls multiple aspects of membrane and peripheral membrane protein life cycle including assembly and 1) ER exit, 2) maturation and Golgi exit, 3) sorting and trafficking to target membranes, 4) recycling and internalization, 5) clustering and localization in membrane microdomains, 6) control of properties and regulation by other signaling pathways, 7) partitioning of peripheral membrane proteins between the cytosol and membranes, and 8) recycling and final degradation.

Figure 5.

Diversity of S-acylated proteins. Schematic illustrating the diversity of proteins demonstrated to be targets for S-acylation. A large number of proteins identified to date are involved in cellular transport and signaling, although structural, chaperone, cell adhesion, and proteins required for translational/transcription are also S-acylated.