Fatty acylation of proteins: The long and the short of it

Abstract

Long, short and medium chain fatty acids are covalently attached to hundreds of proteins. Each fatty acid confers distinct biochemical properties, enabling fatty acylation to regulate intracellular trafficking, subcellular localization, protein-protein and protein-lipid interactions. Myristate and palmitate represent the most common fatty acid modifying groups. New insights into how fatty acylation reactions are catalyzed, and how fatty acylation regulates protein structure and function continue to emerge. Myristate is typically linked to an N-terminal glycine, but recent studies reveal that lysines can also be myristoylated. Enzymes that remove N-terminal myristoyl-glycine or myristate from lysines have now been identified. DHHC proteins catalyze S-palmitoylation, but the mechanisms that regulate substrate recognition by individual DHHC family members remain to be determined. New studies continue to reveal thioesterases that remove palmitate from S-acylated proteins. Another area of rapid expansion is fatty acylation of the secreted proteins hedgehog, Wnt and Ghrelin, by Hhat, Porcupine and GOAT, respectively. Understanding how these membrane bound O-acyl transferases recognize their protein and fatty acyl CoA substrates is an active area of investigation, and is punctuated by the finding that these enzymes are potential drug targets in human diseases.

Keywords: Depalmitoylation; Lipid rafts; MBOAT family; Myristoyl switch; Myristoylation; Palmitoylation.

Figure 1. Membrane binding mechanisms for N-myristoylated proteins

Four different scenarios are illustrated where a second signal, in addition to myristate, contributes to membrane binding. (A) A polybasic cluster of amino acids (blue) enhances membrane binding through electrostatic interactions with head groups of negatively charged phospholipids (red). Phosphorylation of residues within the polybasic cluster reduces or neutralizes the positive charge in the protein, resulting in membrane detachment. (B) Hydrophobic amino acids disposed along the membrane proximal surface of the N-myristoylated protein contribute to membrane binding. Conformational changes can lead to sequestration of the myristate (myristoyl switch) and/or reduced surface accessibility of the hydrophobic residues, leading to release of the protein from the membrane. (C) Interaction with another membrane protein can direct N-myristoylated proteins to the membrane. The protein could detach if another binding partner, with higher affinity, interacts with the N-myristoylated protein and releases it into the cytosol. (D) Palmitoylation of an N-myristoylated protein induces membrane binding, which is reversed upon deacylation.

Figure 2. Fatty acylation and secretion of lipidated Sonic Hedgehog

Shh undergoes multiple processing steps: 1) The Shh precursor polypeptide enters the lumen of the ER, where the signal peptide is removed. 2) Autocleavage and cholesterol attachment. 3) Hhat catalyzes palmitoylation of the Shh N-terminus. 4,5) Shh undergoes vesicular trafficking from the ER (step 4) through the Golgi (step 5) and is delivered to the cell surface. 6) Shh release is promoted by interaction with Dispatched and Scube2.

Figure 3. Fatty acylation and secretion of palmitoleoylated Wnt proteins

1) The Wnt precursor enters the lumen of the ER, where the signal peptide is cleaved. 2) SCD1 catalyzes formation of palmitoleoyl-CoA from palmitoyl-CoA. 3) Porcn catalyzes attachment of palmitoleoylate to Wnt. 4) Palmitoleoylated Wnt interacts with Wntless and (5) is packaged into vesicles that transit through the Golgi and then to the plasma membrane (6), where palmitoleoylated Wnt is released. 7) In some cells, Notum releases the fatty acid from Wnt.