Modulation of neuronal protein trafficking and function by palmitoylation

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Modification of proteins with the lipid palmitate regulates targeting to specific vesicular compartments and synaptic membranes. Mounting evidence indicates that this lipid modification modulates diverse aspects of neuronal development and synaptic transmission. In particular, palmitoylation regulates the function of proteins that control neuronal differentiation, axonal pathfinding and filopodia formation. In addition, trafficking of numerous proteins associated with synaptic vesicle release machinery requires protein palmitoylation. Remarkably, reversible palmitoylation of specific scaffolding proteins and signaling molecules dynamically regulates ion channel clustering and synaptic strength. The recent discovery of enzymes that palmitoylate specific subsets of synaptic proteins suggests that this process is tightly controlled in neurons.

Introduction

Targeting of many newly synthesized proteins to distinct subcellular compartments involves the Golgi network, where proteins are sorted and segregated into transport vesicles. Protein sorting relies on several modifications that include ubiquitination, phosphorylation, glycosylation and fatty acylation (see glossary). Many types of fatty acylation exist, all of which increase protein hydrophobicity and promote association with the lipid bilayer. One such modification involves the co-translational attachment of a 14-carbon saturated myristic acid to amino-terminal glycine residues [1, 2]. Others involve the post-translational attachment of prenyl groups to C-terminal cysteine-containing motifs, or attachment of palmitate, a 16-carbon saturated fatty acid chain, which is added to cysteine residues in most cases by thioester linkages. Importantly, whereas myristoylation and prenylation are usually stable modifications, palmitoylation is reversible and can be modulated by specific signaling pathways [1, 2, 3]. Addition and removal of palmitate might, therefore, provide an important tool for dynamically regulating responses to diverse cellular stimuli.

Palmitate modifies numerous soluble and integral membrane proteins in neurons [1, 2, 3, 4]. In cytosolic proteins, palmitoylation occurs primarily at cysteine residues found near the amino- and carboxy- termini; however, modification of cysteines located at internal sites or at sites adjacent to other hydrophobic lipids has also been documented. In many cases, the palmitoylated cysteines are found near basic residues that facilitate membrane interactions with the acidic lipid headgroups. Palmitoylation of integral membrane proteins, conversely, usually occurs at residues proximal to a transmembrane domain, indicating that palmitate might cooperate with other membrane targeting signals to control protein sorting.

Palmitoylation also mediates functions beyond modulation of protein targeting to lipid membranes. Recent studies provide clear evidence that this lipid modification facilitates protein sorting into distinct transport vesicles destined for delivery to specific subcellular sites, or enables proteins to associate with lipids enriched at synaptic membranes. Another intriguing observation is that several of the proteins associated with the neurotransmitter release machinery are palmitoylated, hinting at a fundamental role in the dynamic assembly of components that control synaptic vesicle function. Here, we highlight recent discoveries that provide new insights into the diverse roles of palmitoylation in several aspects of protein trafficking and function in neurons.

Section snippets

Modulation of synapse morphology and neuronal protein clustering

Neurons possess an elaborate plasma membrane architecture that consists of highly specialized morphological structures, including axons, dendrites and synapses. Modulation of plasma membrane dynamics and composition regulates the development of these unique structures. Growth associated protein 43 (GAP43) and paralemmin are two palmitoylated neuronal proteins that exert effects on process outgrowth [4, 5, 6]. Gauthier-Campbell et al. found that expression of the palmitoylated forms of these

Acylation regulates sorting and function of numerous presynaptic proteins

In addition to its role at postsynaptic sites, palmitoylation regulates trafficking and function of numerous presynaptic proteins (Figure 2). Prime examples include synaptosome associated protein-25 (SNAP-25) and members of the synaptotagmin family. The palmitoylated cysteines of SNAP-25 are required for efficient SNARE-complex dissociation (see glossary) and the regulation of vesicle exocytosis, but not for membrane targeting [23]. By contrast, palmitoylation of specific members of the

Palmitoylation of neurotransmitter receptors, ion channels and signaling proteins

Neuronal excitability in the brain is modulated by a wide array of receptors, signaling proteins and ion channels. One major group consists of neurotransmitter receptors coupled to G-proteins that control diverse signaling pathways. Interestingly, many components of this protein complex are subject to modification by palmitate attachment. For example, α-subunits of the heteromeric G-proteins are palmitoylated near their amino termini, a process that is thought to regulate localization of

Regulated palmitate turnover

Similar to protein phosphorylation, palmitoylation is a reversible process that is dynamically regulated by specific cellular stimuli. Binding of ligand to the β-adrenoreceptor markedly accelerates the depalmitoylation of the associated Gαs subunit, which dampens G-protein signaling [3]. Depalmitoylation of PSD-95 is regulated by neuronal activity and promotes the removal of PSD-95 from synapses [4]. This process in turn controls activity-induced internalization of GluR1, and, thus, provides a

Enzymes involved in neuronal protein palmitoylation

The diversity of proteins modified by palmitate combined with the absence of common consensus sequences for palmitoylation has hindered the identification of enzymes involved in this process [39]. The first characterization of enzymes involved in palmitoylation came from work done on flies [40, 41, 42•]. These studies revealed that skinny hedgehog (Ski) and porcupine (porc) are palmitoyl acyl-transferases (PATs) that mediate palmitoylation of the secreted factors hedgehog and Wnt-1, and are

Conclusions

New findings provide evidence that palmitoylation plays a central role in regulating the functions of a diverse set of neuronal proteins. In many instances, palmitoylation might cooperate with other signals to regulate protein trafficking or function at a particular subcellular location. Importantly, alterations in protein palmitoylation have been associated with disrupted neurotransmitter receptor clustering and altered synaptic activity. A defect in the association between the DHHC protein,

Update

A recent study by Swarthout et al. [51] revealed that DHHC9 and GCP16 (Golgi complex-associated protein), are both Golgi-localized membrane proteins, and are the human orthologs of the yeast Ras palmitoyl transferase Erf2–Erf4 protein complex. Similar to yeast Erf4, GCP16 is required for DHHC9 palmitoyl transferase activity and stability. Purified DHHC9–GCP16 exhibits substrate specificity, palmitoylating H- and N-Ras, but not myristoylated Gαi1 or GAP43. The localization of the DHHC9–GCP16

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Thanks to S Vincent, S Dakoji, and members of the laboratory for helpful discussions throughout the writing process.

Glossary

Fatty acylation
Primarily refers to the attachment of fatty acid chains to proteins. The most common fatty acylations include myristate to N-terminal glycine through amide linkage (myristoylation) and post-translational addition of palmitate (palmitoylation) or isoprenoid lipid (prenylation) through thioester linkage to cysteine residues.
Multimerization
The process of assembling of two or more different molecules (in this case, PSD-95). Multimers are held together by non-covalent bonds.
Dual

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