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Between 25 and 40% of eukaryotic cellular proteins are membrane associated, whilst an even higher number of intracellular proteins can undergo modifications to localise them to the phospholipid bilayer and increase association with the membrane1. Palmitoylation is one of the most common post-translational modifications, during which a fatty acid is attached to protein2. Specifically, S-palmitoylation is the addition of a saturated 16-carbon fatty acid, e.g palmitic acid, to a specific cysteine thiol side chain of a target protein via a thioester bond (also referred to as S-acylation)3. It is currently considered to be the predominant form of protein acylation, and typically occurs as a follow-up to prenylation/myristoylation, which attaches a lipid to the protein to promote transient membrane attachments. S-palmitoylation subsequently inhibits protein dissociation from the membrane, as the palmitate group acts as a hydrophobic membrane anchor4, 5.
Figure 1: An animation of the formation of a thioester bond between palmitic acid and the sulphur of a cysteine residue of the target protein
Although palmitate is the preferred substrate attached to the proteins during S-palmitoylation, it is by no means the only one: other acyl chains may be used, depending on availability and concentrations present in the cell. Some substitutes include stearate, oleate and arachidonate molecules6. This fatty acid modification is reversible: S-palmitoylation is catalysed by enzymes termed protein palmitoyltransferases (PATs), whilst protein thioesterases (PTTs) catalyse depalmitoylation3. In contrast, N-palmitoylation, which involves the linkage of an amide group, is not reversible7; hence, this resource will focus on S-palmitoylation, since it is the uniqueness of the reversibility of S-palmitoylation that allows for much of protein function to take place. The reversibility also allows for fine-tuning of protein function, in a manner analogous to protein phosphorylation or ubiquitination. Despite this being a relatively new area of research, it is understood that very many membrane proteins undergo S-palmitoylation to aid in a variety of essential cellular function.
Figure 2: Schematic diagram of a DHHC domain-containing palmitoyltransferase. The white ovals ‘DPG’ and ‘TTXE’ refer to other common amino acid sequence motifs that appear across the family as well. (Image credit: Alex Bateman)
PATs (shown in figure 2) are characterised by having a large conserved DHHC (Asp-His-His-Cys) sequence, which is essential for the enzyme’s catalytic activity8, and 23 different DHHC-containing PATs have so far been discovered. These PATs carry out their functions via a 2-step mechanism: PAT first uses acyl-coenzyme A, a palmitate donor, to autoacylate, forming a transient acyl-enzyme intermediate. The palmitoyl moiety is then transferred from the enzyme to the substrate, palmitoylating the target protein at specific cysteine residues9, 10. As mentioned, although there are only 23 known DHHC-containing PATs, there are hundreds of S-palmitoylated proteins found in the human body, hence reinforcing the importance of protein S-palmitoylation in cell function11.
In comparison to PAT activity, much less is known about the depalmitoylating PTTs. Currently, PTTs are thought to be members of the α/β hydrolyase domain-containing (ABHD) family – two enzymes in this family, acyl protein thioesterases (APT) 1 and 2, have so far been found to function as depalmitoylating enzymes in vivo, hydrolysing the thioester bond between target proteins and fatty acids, and are most likely to be palmitoylated proteins themselves12, 13. Another homolog of the α/β hydrolyase family found to have depalmitoylating properties is APTL1, which shares a 33% identity with APT1. Considering that many proteins within the family are still poorly defined, active research is being conducted to elucidate more depalmitoylating enzymes from the α/β hydrolyase family14. More recently, Yokoi et al identified three isoforms of a potential depalmitoylating enzyme, termed ABHD17 A, B and C15, involved in the depalmitoylation of a postsynaptic scaffolding protein, showing that there is still much to be known about the α/β hydrolyase family of depalmitoylating PPTs.
This resource aims to highlight the importance of S-palmitoylation in health and disease, to demonstrate the necessity of both identifying novel palmitoylated proteins, and also further characterising known palmitoylated proteins. It is split into four sections:
Guan, X. & Fierke, C.A. 2011. Understanding Protein Palmitoylation: Biological Significance and Enzymology. Sci China Chem. [Online] 54(12): pp1888-1897. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25419213
Pei, Z., Xiao, Y., Meng, J., Hudmon, A. & Cummins, T. R. 2016. Cardiac sodium channel palitoylation regulates channel availability and myocyte excitability with implications for arrhythmia generation. Nature Communications. [Online]. 7(1): 12035. doi: 10.1038/ncomms12035. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27337590
Han, J., Wu, P., Wang, F. & Chen, J. 2015. S-palmitoylation regulates AMPA receptors trafficking and function: a novel insight into synaptic regulation and therapeutics. Acta Pharmaceutica Sinica B. [Online]. 5(1): pp1–7. [Date Accessed: 3rd February]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4629138/
Yeste-Velasco, M., Linder, M.E., & Lu, Y-J. 2013. Protein S-palmitoylation and Cancer. Biochimica et Physica Acta. [Online]. 1856(1): pp107-120. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26112306
Jennings, B.C. & Linder, M.E. 2012. DHHC Protein S-Acyltransferases Use Similar Ping-Pong Kinetic Mechanisms but Display Different Acyl-CoA Specificities. Journal of Biological Chemistry. [Online]. 287(10): pp7236-7245. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/22247542
Mitchell, D. A., Mitchel, l G., Ling, Y., Budde, C. and Deschenes, R. J. 2010. Mutational analysis of Saccharomyces cerevisiae Erf2 reveals a two-step reaction mechanism for protein palmitoylation by DHHC enzymes. Journal of Biological Chemistry. [Online]. 285(49): pp38104-38114. [Date Accessed: 12th February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/20851885
Blaskovic, S., Blanc, M., & van der Goot, F.G. 2013. What does S-palmitoylation do to membrane proteins?. FEBS Journal. [Online]. 280(12): pp2766-2774. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23551889
Devedjiev, Y., Dauter, Z., Kuznetso, S.R., Teresa, L.Z. Jones, T.L.Z. & Derewenda, Z.S. 2000. Crystal Structure of the Human Acyl Protein Thioesterase I from a Single X-Ray Data Set to 1.5 Å. Cell. [Online]. 8(11): pp1137-1146. [Date Accessed: 08 February 2017]. Available from: http://www.sciencedirect.com/science/article/pii/S0969212600005293
Tian, L., McClafferty, H., Knaus, H.G., Ruth, P. & Shipston, M.J. 2012. Distinct Acyl Protein Transferases and Thioesterases Control Surface Expression of Calcium-activated Potassium Channels. Journal of Biological Chemistry. [Online]. 287(17): pp14718-14725. [Date Accessed: 21st February 2017]. Available from: https://www.ncbi.nlm.nih.gov/pubmed/22399288
Conibear, E., & Davis, N.G. 2010. Palmitoylation and depalmitoylation dynamics at a glance. Journal of Cell Science. [Online]. 123(1): pp4007-4010. [Date Accessed: 21st February 2017]. Available from: http://jcs.biologists.org/content/123/23/4007