Palmitoylation alters protein activity: blockade of G(o) stimulation by GAP-43

GAP-43 and BASP1 in Axon Regeneration: Implications for the Treatment of Neurodegenerative Diseases

Growth-associated protein-43 (GAP-43) and brain acid-soluble protein 1 (BASP1) regulate actin dynamics and presynaptic vesicle cycling at axon terminals, thereby facilitating axonal growth, regeneration, and plasticity. These functions highly depend on changes in GAP-43 and BASP1 expression levels and post-translational modifications such as phosphorylation. Interestingly, examinations of GAP-43 and BASP1 in neurodegenerative diseases reveal alterations in their expression and phosphorylation profiles. This review provides an overview of the structural properties, regulations, and functions of GAP-43 and BASP1, highlighting their involvement in neural injury response and regeneration. By discussing GAP-43 and BASP1 in the context of neurodegenerative diseases, we also explore the therapeutic potential of modulating their activities to compensate for neuron loss in neurodegenerative diseases.

Analysis of IFITM-IFITM Interactions by a Flow Cytometry-Based FRET Assay

The interferon-induced transmembrane proteins 1–3 (IFITM1–3) inhibit host cell entry of several viruses. However, it is incompletely understood how IFITM1–3 exert antiviral activity. Two phenylalanine residues, F75 and F78, within the intramembrane domain 1 (IM1) were previously shown to be required for IFITM3/IFITM3 interactions and for inhibition of viral entry, suggesting that IFITM/IFITM interactions might be pivotal to antiviral activity. Here, we employed a fluorescence resonance energy transfer (FRET) assay to analyze IFITM/IFITM interactions. For assay calibration, we equipped two cytosolic, non-interacting proteins, super yellow fluorescent protein (SYFP) and super cyan fluorescent protein (SCFP), with signals that target proteins to membrane rafts and also analyzed a SCFP-SYFP fusion protein. This strategy allowed us to discriminate background signals resulting from colocalization of proteins at membrane subdomains from signals elicited by protein–protein interactions. Coexpression of IFITM1–3 and IFITM5 fused to fluorescent proteins elicited strong FRET signals, and mutation of F75 and F78 in IFITM3 (mutant IFITM3-FF) abrogated antiviral activity, as expected, but did not alter cellular localization and FRET signals. Moreover, IFITM3-FF co-immunoprecipitated efficiently with wild type (wt) IFITM3, lending further support to the finding that lack of antiviral activity of IFITM3-FF was not due to altered membrane targeting or abrogated IFITM3-IFITM3 interactions. Collectively, we report an assay that allows quantifying IFITM/IFITM interactions. Moreover, we confirm residues F75 and F78 as critical for antiviral activity but also show that these residues are dispensable for IFITM3 membrane localization and IFITM3/IFITM3 interactions.

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

Analysis of the brain palmitoyl-proteome using both acyl-biotin exchange and acyl-resin-assisted capture methods

Palmitoylation is a reversible post-translational protein modification in which palmitic acid is added to cysteine residues, allowing association with different cellular membranes and subdomains. Recently, techniques have been developed to identify palmitoylation on a proteome-wide scale in order to reveal the full cellular complement of palmitoylated proteins. However, in the studies reported to date, there is considerable variation between the sets of identified palmitoyl-proteins and so there remains some uncertainty over what constitutes the definitive complement of palmitoylated proteins even in well-studied tissues such as brain. To address this issue, we used both acyl-biotin exchange and acyl-resin-assisted capture approaches using rat brain as a common protein source. The palmitoyl-proteins identified from each method by mass spectrometry were then compared with each other and previously published studies. There was generally good agreement between the two methods, although many identifications were unique to one method, indicating that at least some of the variability in published palmitoyl proteomes is due to methodological differences. By combining our new data with previous publications using mammalian cells/tissues, we propose a high confidence set of bona fide palmitoylated proteins in brain and provide a resource to help researchers prioritise candidate palmitoyl-proteins for investigation.

High-order oligomers of intrinsically disordered brain proteins BASP1 and GAP-43 preserve the structural disorder

Brain acid-soluble protein-1 (BASP1) and growth-associated protein-43 (GAP-43) are presynaptic membrane proteins participating in axon guidance, neuroregeneration and synaptic plasticity. They are presumed to sequester phosphatidylinositol-4,5-bisphosphate (PIP2 ) in lipid rafts. Previously we have shown that the proteins form heterogeneously sized oligomers in the presence of anionic phospholipids or SDS at submicellar concentration. BASP1 and GAP-43 are intrinsically disordered proteins (IDPs). In light of this, we investigated the structure of their oligomers. Using partial cross-linking of the oligomers with glutaraldehyde, the aggregation numbers of BASP1 and GAP-43 were estimated as 10-14 and 6-7 monomer subunits, respectively. The cross-linking pattern indicated that the subunits are circularly arranged. The circular dichroism (CD) spectra of the monomers were characteristic of coil-like IDPs showing unordered structure with a high population of polyproline-II conformation. The oligomerization was accompanied by a minor CD spectral change attributable to formation of a small amount of α-helix. The number of residues in the α-helical conformation was estimated as 13 in BASP1 and 18 in GAP-43. However, the overall structure of the oligomers remained disordered, indicating a high degree of 'fuzziness'. This was confirmed by measuring the hydrodynamic dimensions of the oligomers using polyacrylamide gradient gel electrophoresis and size-exclusion chromatography, and by assaying their sensitivity to proteolytic digestion. There is evidence that the observed α-helical folding occurs within the basic effector domains, which are presumably tethered together via anionic molecules of SDS or PIP2 . We conclude that BASP1 and GAP-43 oligomers preserve a mostly disordered structure, which may be of great importance for their function in PIP2 signaling pathway.

Posttranslational Modifications Regulate the Postsynaptic Localization of PSD-95

The postsynaptic density (PSD) consists of a lattice-like array of interacting proteins that organizes and stabilizes synaptic receptors, ion channels, structural proteins, and signaling molecules required for normal synaptic transmission and synaptic function. The scaffolding and hub protein postsynaptic density protein-95 (PSD-95) is a major element of central chemical synapses and interacts with glutamate receptors, cell adhesion molecules, and cytoskeletal elements. In fact, PSD-95 can regulate basal synaptic stability as well as the activity-dependent structural plasticity of the PSD and, therefore, of the excitatory chemical synapse. Several studies have shown that PSD-95 is highly enriched at excitatory synapses and have identified multiple protein structural domains and protein-protein interactions that mediate PSD-95 function and trafficking to the postsynaptic region. PSD-95 is also a target of several signaling pathways that induce posttranslational modifications, including palmitoylation, phosphorylation, ubiquitination, nitrosylation, and neddylation; these modifications determine the synaptic stability and function of PSD-95 and thus regulate the fates of individual dendritic spines in the nervous system. In the present work, we review the posttranslational modifications that regulate the synaptic localization of PSD-95 and describe their functional consequences. We also explore the signaling pathways that induce such changes.

Distinct palmitoylation events at the amino-terminal conserved cysteines of Env7 direct its stability, localization, and vacuolar fusion regulation in S. cerevisiae

Palmitoylation at cysteine residues is the only known reversible form of lipidation and has been implicated in protein membrane association as well as function. Many palmitoylated proteins have regulatory roles in dynamic cellular processes, including membrane fusion. Recently, we identified Env7 as a conserved and palmitoylated protein kinase involved in negative regulation of membrane fusion at the lysosomal vacuole. Env7 contains a palmitoylation consensus sequence, and substitution of its three consecutive cysteines (Cys(13)-Cys(15)) results in a non-palmitoylated and cytoplasmic Env7. In this study, we further dissect and define the role(s) of individual cysteines of the consensus sequence in various properties of Env7 in vivo. Our results indicate that more than one of the cysteines serve as palmitoylation substrates, and any pairwise combination is essential and sufficient for near wild type levels of Env7 palmitoylation, membrane localization, and phosphorylation. Furthermore, individually, each cysteine can serve as a minimum requirement for distinct aspects of Env7 behavior and function in cells. Cys(13) is sufficient for membrane association, Cys(15) is essential for the fusion regulatory function of membrane-bound Env7, and Cys(14) and Cys(15) are redundantly essential for protection of membrane-bound Env7 from proteasomal degradation. A role for Cys(14) and Cys(15) in correct sorting at the membrane is also discussed. Thus, palmitoylation at the N-terminal cysteines of Env7 directs not only its membrane association but also its stability, phosphorylation, and cellular function.

Palmitoylcarnitine affects localization of growth associated protein GAP-43 in plasma membrane subdomains and its interaction with Gα(o) in neuroblastoma NB-2a cells

Palmitoylcarnitine was observed previously to promote differentiation of neuroblastoma NB-2a cells, and to affect protein kinase C (PKC). Palmitoylcarnitine was also observed to increase palmitoylation of several proteins, including a PKC substrate, whose expression augments during differentiation of neural cells-a growth associated protein GAP-43, known to bind phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. Since palmitoylated proteins are preferentially localized in sphingolipid- and cholesterol-rich microdomains of plasma membrane, the present study has been focused on a possible effect of palmitoylcarnitine on GAP-43 localization in these microdomains. Palmitoylcarnitine treatment resulted in GAP-43 appearance in floating fractions (rafts) in sucrose gradient and increased co-localization with cholesterol and with PI(4,5)P(2), although co-localization of both lipids decreased. GAP-43 disappeared from raft fraction upon treatment with 2-bromopalmitate (an inhibitor of palmitoylating enzymes) and after treatment with etomoxir (carnitine palmitoyltransferase I inhibitor). Raft localization of GAP-43 was completely abolished by treatment with methyl-β-cyclodextrin, a cholesterol binding agent, while there was no change upon sequestration of PI(4,5)P(2) with neomycin. GAP-43 co-precipitated with a monomeric form of Gα(o), a phenomenon diminished after palmitoylcarnitine treatment and paralleled by a decrease of Gα(o) in the raft fraction. These observations point to palmitoylation of GAP-43 as a mechanism leading to an increased localization of this protein in microdomains of plasma membrane rich in cholesterol, in majority different, however, from microdomains in which PI(4,5)P(2) is present. This localization correlates with decreased interaction with Gα(o) and suppression of its activity-an important step regulating neural cell differentiation.

Site-specific analysis of protein S-acylation by resin-assisted capture

Protein S-acylation is a major posttranslational modification whereby a cysteine thiol is converted to a thioester. A prototype is S-palmitoylation (fatty acylation), in which a protein undergoes acylation with a hydrophobic 16 carbon lipid chain. Although this modification is a well-recognized determinant of protein function and localization, current techniques to study cellular S-acylation are cumbersome and/or technically demanding. We recently described a simple and robust methodology to rapidly identify S-nitrosylation sites in proteins via resin-assisted capture (RAC) and provided an initial description of the applicability of the technique to S-acylated proteins (acyl-RAC). Here we expand on the acyl-RAC assay, coupled with mass spectrometry-based proteomics, to characterize both previously reported and novel sites of endogenous S-acylation. Acyl-RAC should therefore find general applicability in studies of both global and individual protein S-acylation in mammalian cells.

Palmitoylation modification of Galpha(o) depresses its susceptibility to GAP-43 activation

Interaction between GAP-43 (growth associated protein-43) and Galpha(o) (alpha subunit of Go protein) influences the signal transduction pathways leading to differentiation of neural cells. GAP-43 is known to increase guanine nucleotide exchange by Galpha(o), which is a major component of neuronal growth cone membranes. However, it is not clear whether GAP-43 stimulation is related to the Galpha(o) palmitoylation or the conversion of Galpha(o) from oligmers to monomers, which was shown to be a necessary regulatory factor in GDP/GTP exchange of Galpha(o). Here we expressed and purified GAP-43, GST-GAP-43 and Galpha(o) proteins, detected their stimulatory effect on [(35)S]-GTPgammaS binding of Galpha(o). It was found that the EC(50) of both GAP-43 and GST-GAP-43 activation were tenfold lower in case of depalmitoylated Galpha(o) than palmitoylated Galpha(o). Non-denaturing gel electrophoresis and p-PDM cross-linking analysis revealed that addition of GST-GAP-43 induced disassociation of depalmitoylated Galpha(o) from oligomers to monomers, but did not influence the oligomeric state of palmitoylated Galpha(o), which suggests that palmitoylation is a key regulatory factor in GAP-43 stimulation on Galpha(o). These results indicated the interaction of GAP-43 and Galpha(o) could accelerate conversion of depalmitoylated Galpha(o) but not palmitoylated Galpha(o) from oligomers to monomers, so as to increase the GTPgammaS binding activity of Galpha(o). Results here provide new evidence about how signaling protein palmitoylation is involved in the G-protein-coupled signal transduction cascade, and give a useful clue on the participation of GAP-43 in G-protein cycle by its preferential activation of depalmitoylated Galpha(o).

The role of post-translational modifications for learning and memory formation

Learning and memory depend on molecular mechanisms involving the protein machinery. Recent evidence proposes that post-translational modifications (PTMs) play a major role in these cognitive processes. PTMs including phosphorylation of serine, threonine, and tyrosine are already well-documented to play a role for synaptic plasticity of the brain, neurotransmitter release, vesicle trafficking and synaptosomal or synaptosomal-associated proteins are substrates of a series of specific protein kinases and their counterparts, protein phosphatases. But protein phosphorylation is only one out of many possible PTMs and first work shows a role of palmitoylation as well as glycosylation for proteins involved in memory formation. Recent technology may now allow reliable detection and even quantification of PTMs of proteins involved in the cognitive system. This will contribute to the understanding of mechanisms for learning and memory formation at the chemical level and has to complement determination of protein levels and indeed determination of protein expression per se generates limited information. The many other PTMs expected including protein nitrosylation and alkylation will even represent targets for pharmacological interventions but in turn increase the complexity of the system. Nevertheless, determination of the presence and the function of PTMs is mandatory and promising cognitive research at the protein chemical level.

M-calpain-mediated cleavage of GAP-43 near Ser41 is negatively regulated by protein kinase C, calmodulin and calpain-inhibiting fragment GAP-43-3

Neuronal protein GAP-43 performs multiple functions in axon guidance, synaptic plasticity and regulation of neuronal death and survival. However, the molecular mechanisms of its action in these processes are poorly understood. We have shown that in axon terminals GAP-43 is a substrate for calcium-activated cysteine protease m-calpain, which participates in repulsion of axonal growth cones and induction of neuronal death. In pre-synaptic terminals in vivo, in synaptosomes, and in vitro, m-calpain cleaved GAP-43 in a small region near Ser41, on either side of this residue. In contrast, micro-calpain cleaved GAP-43 in vitro at several other sites, besides Ser41. Phosphorylation of Ser41 by protein kinase C or GAP-43 binding to calmodulin strongly suppressed GAP-43 proteolysis by m-calpain. A GAP-43 fragment, lacking about forty N-terminal residues (named GAP-43-3), was produced by m-calpain-mediated cleavage of GAP-43 and inhibited m-calpain, but not micro-calpain. This fragment prevented complete cleavage of intact GAP-43 by m-calpain as a negative feedback. GAP-43-3 also blocked m-calpain activity against casein, a model calpain substrate. This implies that GAP-43-3, which is present in axon terminals in high amount, can play important role in regulation of m-calpain activity in neurons. We suggest that GAP-43-3 and another (N-terminal) GAP-43 fragment produced by m-calpain participate in modulation of neuronal response to repulsive and apoptotic signals.

BV/ODV-E26: a palmitoylated, multifunctional structural protein of Autographa californica nucleopolyhedrovirus

Autographa californica nucleopolyhedrovirus Ac16 is 1 of 17 genes conserved within Type 1 nucleopolyhedroviruses. This report demonstrates that multiple isoforms of the protein encoded by Ac16, BV/ODV-E26 (E26), are present in the infected cell. One form of E26 associates with viral DNA or DNA-binding proteins, while a second form associates with intracellular membranes and this is likely due to palmitoylation. The different forms of E26 present unique epitopes that can be discriminated by antiserum produced to bacterially or virally produced antigen. A summation of the data now available on E26 suggests that it is a multifunctional protein and the functional states assume unique conformations that can be discriminated by differing antisera.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

CSS-Palm: palmitoylation site prediction with a clustering and scoring strategy (CSS)

Palmitoylation is an important post-translational lipid modification of proteins. Unlike prenylation and myristoylation, palmitoylation is a reversible covalent modification, allowing for dynamic regulation of multiple complex cellular systems. However, in vivo or in vitro identification of palmitoylation sites is usually time-consuming and labor-intensive. So in silico predictions could help to narrow down the possible palmitoylation sites, which can be used to guide further experimental design. Previous studies suggested that there is no unique canonical motif for palmitoylation sites, so we hypothesize that the bona fide pattern might be compromised by heterogeneity of multiple structural determinants with different features. Based on this hypothesis, we partition the known palmitoylation sites into three clusters and score the similarity between the query peptide and the training ones based on BLOSUM62 matrix. We have implemented a computer program for palmitoylation site prediction, Clustering and Scoring Strategy for Palmitoylation Sites Prediction (CSS-Palm) system, and found that the program's prediction performance is encouraging with highly positive Jack-Knife validation results (sensitivity 82.16% and specificity 83.17% for cut-off score 2.6). Our analyses indicate that CSS-Palm could provide a powerful and effective tool to studies of palmitoylation sites.

AVAILABILITY

CSS-Palm is implemented in PHP/PERL+MySQL and can be freely accessed at http://bioinformatics.lcd-ustc.org/css_palm/

CONTACT

yaoxb@ustc.edu.cn; xuyn@bmb.uga.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bionformatics online.

Palmitoyl-protein thioesterase-1 deficiency mediates the activation of the unfolded protein response and neuronal apoptosis in INCL

Numerous proteins undergo modification by palmitic acid (S-acylation) for their biological functions including signal transduction, vesicular transport and maintenance of cellular architecture. Although palmitoylation is an essential modification, these proteins must also undergo depalmitoylation for their degradation by lysosomal proteases. Palmitoyl-protein thioesterase-1 (PPT1), a lysosomal enzyme, cleaves thioester linkages in S-acylated proteins and removes palmitate residues facilitating the degradation of these proteins. Thus, inactivating mutations in the PPT1 gene cause infantile neuronal ceroid lipofuscinosis (INCL), a devastating neurodegenerative storage disorder of childhood. Although rapidly progressing brain atrophy is the most dramatic pathological manifestation of INCL, the molecular mechanism(s) remains unclear. Using PPT1-knockout (PPT1-KO) mice that mimic human INCL, we report here that the endoplasmic reticulum (ER) in the brain cells of these mice is structurally abnormal. Further, we demonstrate that the level of growth-associated protein-43 (GAP-43), a palmitoylated neuronal protein, is elevated in the brains of PPT1-KO mice. Moreover, forced expression of GAP-43 in PPT1-deficient cells results in the abnormal accumulation of this protein in the ER. Consistent with these results, we found evidence for the activation of unfolded protein response (UPR) marked by elevated levels of phosphorylated translation initiation factor, eIF2alpha, increased expression of chaperone proteins such as glucose-regulated protein-78 and activation of caspase-12, a cysteine proteinase in the ER, mediating caspase-3 activation and apoptosis. Our results, for the first time, link PPT1 deficiency with the activation of UPR, apoptosis and neurodegeneration in INCL and identify potential targets for therapeutic intervention in this uniformly fatal disease.

Specific Proteolysis of Neuronal Protein GAP-43 by Calpain: Characterization, Regulation, and Physiological Role

The mechanism of specific proteolysis of the neuronal protein GAP-43 in axonal terminals has been investigated. In synaptic terminals in vivo and in synaptosomes in vitro GAP-43 is cleaved only at the single peptide bond formed by Ser41; this is within the main effector domain of GAP-43. Proteolysis at this site involves the cysteine calcium-dependent neutral protease calpain. The following experimental evidences support this conclusion: 1) calcium-dependent proteolysis of GAP-43 in synaptosomes is insensitive to selective inhibitor of micro-calpain (PD151746), but it is completely blocked by micro- and m-calpain inhibitor PD150606; 2) GAP-43 proteolysis in the calcium ionophore A23187-treated synaptosomes is activated by millimolar concentration of calcium ions; 3) the pattern of fragmentation of purified GAP-43 by m-calpain (but not by micro-calpain) is identical to that observed in synaptic terminals in vivo. GAP-43 phosphorylated at Ser41 by protein kinase C (PKC) is resistant to the cleavage by calpain. In addition, calmodulin binding to GAP-43 decreases the rate of calpain-mediated GAP-43 proteolysis. Our results indicate that m-calpain-mediated GAP-43 proteolysis regulated by PKC and calmodulin is of physiological relevance, particularly in axonal growth cone guidance. We suggest that the function of the N-terminal fragment of GAP-43 (residues 1-40) formed during cleavage by m-calpain consists in activation of neuronal heterotrimeric GTP-binding protein G(o); this results in growth cone turning in response to repulsive signals.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Regulation of dendritic branching and filopodia formation in hippocampal neurons by specific acylated protein motifs

Although neuronal axons and dendrites with their associated filopodia and spines exhibit a profound cell polarity, the mechanism by which they develop is largely unknown. Here, we demonstrate that specific palmitoylated protein motifs, characterized by two adjacent cysteines and nearby basic residues, are sufficient to induce filopodial extensions in heterologous cells and to increase the number of filopodia and the branching of dendrites and axons in neurons. Such motifs are present at the N-terminus of GAP-43 and the C-terminus of paralemmin, two neuronal proteins implicated in cytoskeletal organization and filopodial outgrowth. Filopodia induction is blocked by mutations of the palmitoylated sites or by treatment with 2-bromopalmitate, an agent that inhibits protein palmitoylation. Moreover, overexpression of a constitutively active form of ARF6, a GTPase that regulates membrane cycling and dendritic branching reversed the effects of the acylated protein motifs. Filopodia induction by the specific palmitoylated motifs was also reduced upon overexpression of a dominant negative form of the GTPase cdc42. These results demonstrate that select dually lipidated protein motifs trigger changes in the development and growth of neuronal processes.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Mass spectrometric analysis of GAP-43/neuromodulin reveals the presence of a variety of fatty acylated species

GAP-43 (neuromodulin) is a protein kinase C substrate that is abundant in developing and regenerating neurons. Thioester-linked palmitoylation at two cysteines near the GAP-43 N terminus has been implicated in directing membrane binding. Here, we use mass spectrometry to examine the stoichiometry of palmitoylation and the molecular identity of the fatty acid(s) attached to GAP-43 in vivo. GAP-43 expressed in either PC12 or COS-1 cells was acetylated at the N-terminal methionine. Approximately 35% of the N-terminal GAP-43 peptides were also modified by palmitate and/or stearate on Cys residues. Interestingly, a variety of acylated species was detected, in which one of the Cys residues was acylated by either palmitate or stearate, or both Cys residues were acylated by palmitates or stearates or a combination of palmitate and stearate. Depalmitoylation of membrane-bound GAP-43 did not release the protein from the membrane, implying that additional forces function to maintain membrane binding. Indeed, mutation of four basic residues within the N-terminal domain of GAP-43 dramatically reduced membrane localization of GAP-43 without affecting palmitoylation. These data reveal the heterogeneous nature of S-acylation in vivo and illustrate the power of mass spectrometry for identification of key regulatory protein modifications.

Cellular and subcellular localization, N-terminal acylation, and calcium binding of Caenorhabditis elegans protein phosphatase with EF-hands

The RdgC/PPEF family of serine/threonine protein phosphatases is distinguished by the presence of C-terminal EF-hands and neuron-specific expression, including frequent expression in primary sensory neurons. Here we report that the sole Caenorhabditis elegans PPEF (CePPEF) homolog is also highly expressed in primary sensory neurons and is not found outside the nervous system. Neurons expressing CePPEF include the ciliary chemosensory neurons AWB and AWC; and within these neurons, CePPEF is highly enriched in the sensory cilia. In transgenic C. elegans and in transfected 293 cells, CePPEF is membrane-associated, and the N terminus of CePPEF is necessary and sufficient for this membrane association. [(3)H]Myristate and [(3)H]palmitate labeling studies in 293 cells demonstrated that this association was mediated by myristoylation at Gly(2) and palmitoylation at Cys(3). Introducing the G2A or C3S mutation into CePPEF greatly reduced membrane association in 293 cells and in transgenic nematodes. A recombinant C-terminal fragment of CePPEF containing two putative EF-hands bound between one and two Ca(2+) ions/protein, and mutation of residues presumed to ligand calcium in the two putative EF-hands led to diminished calcium binding. These results establish the first direct evidence for fatty acylation and calcium binding of a PPEF family member and demonstrate a remarkable conservation of sensory neuron expression among the members of this distinctive family of protein phosphatases.

Involvement of fatty acid synthase in axonal development in mouse embryos

BACKGROUND

Dynamic protein palmitoylation of signalling proteins is thought to be an important step in the regulation of signal transduction in eukaryotic cells. The enzyme responsible for protein palmitoylation in vertebrates, however, has not been identified. I have previously reported that p260/270, which is expressed in embryos of the silkworm Bombyx mori, has protein palmitoylase activity.

RESULTS

A homologue of Bombyx p260/270, mouse fatty-acid synthase (FAS), was shown to be expressed specifically throughout most of the central and peripheral nervous system in mouse embryos. Mouse FAS was expressed specifically in cultured primary neurones in which growth-associated protein (GAP)-43 was expressed. GAP-43, by protein palmitoylation, regulates Go signal transduction and neural axonal growth. In a cell-free system, purified FAS from mouse embryos transferred palmitate to GAP-43 through cysteine residues. Furthermore, cerulenin, an inhibitor of FAS, reduced axonal growth and in vivo palmitoylation of GAP-43 in cultured neurones.

CONCLUSIONS

Mouse FAS was hypothesized to be responsible for the palmitoylation of GAP-43 and subsequent regulation of axonal growth in mouse embryos.

Autopalmitoylation of tubulin

Pure rat brain tubulin is readily palmitoylated in vitro using [3H]palmitoyl CoA but no added enzymes. A maximum of approximately six palmitic acids are added per dimer in 2-3 h at 36-37 degrees C under native conditions. Both alpha and beta tubulin are labeled, and 63-73% of the label was hydroxylamine-labile, presumed thioesters. Labeling increases with increasing pH and temperature, and with low concentrations of guanidine HCl or KCl (but not with urea) to a maximum of approximately 13 palmitates/dimer. High SDS and guanidine HCl concentrations are inhibitory. At no time could all 20 cysteine residues of the dimer be palmitoylated. Polymerization to microtubules, or use of tubulin S, markedly decreases the accessibility of the palmitoylation sites. Palmitoylation increases the electrophoretic mobility of a portion of alpha tubulin toward the beta band. Palmitoylated tubulin binds a colchicine analogue normally, but during three warm/cold polymerization/depolymerization cycles there is a progressive loss of palmitoylated tubulin, indicating decreased polymerization competence. We postulate that local electrostatic factors are major regulators of reactivity of tubulin cysteine residues toward palmitoyl CoA, and that the negative charges surrounding a number of the cysteines are sensitive to negative charges on palmitoyl CoA.

Palmitoylation of a conserved cysteine in the regulator of G protein signaling (RGS) domain modulates the GTPase-activating activity of RGS4 and RGS10

RGS4 and RGS10 expressed in Sf9 cells are palmitoylated at a conserved Cys residue (Cys(95) in RGS4, Cys(66) in RGS10) in the regulator of G protein signaling (RGS) domain that is also autopalmitoylated when the purified proteins are incubated with palmitoyl-CoA. RGS4 also autopalmitoylates at a previously identified cellular palmitoylation site, either Cys(2) or Cys(12). The C2A/C12A mutation essentially eliminates both autopalmitoylation and cellular [(3)H]palmitate labeling of Cys(95). Membrane-bound RGS4 is palmitoylated both at Cys(95) and Cys(2/12), but cytosolic RGS4 is not palmitoylated. RGS4 and RGS10 are GTPase-activating proteins (GAPs) for the G(i) and G(q) families of G proteins. Palmitoylation of Cys(95) on RGS4 or Cys(66) on RGS10 inhibits GAP activity 80-100% toward either Galpha(i) or Galpha(z) in a single-turnover, solution-based assay. In contrast, when GAP activity was assayed as acceleration of steady-state GTPase in receptor-G protein proteoliposomes, palmitoylation of RGS10 potentiated GAP activity >/=20-fold. Palmitoylation near the N terminus of C95V RGS4 did not alter GAP activity toward soluble Galpha(z) and increased G(z) GAP activity about 2-fold in the vesicle-based assay. Dual palmitoylation of wild-type RGS4 remained inhibitory. RGS protein palmitoylation is thus multi-site, complex in its control, and either inhibitory or stimulatory depending on the RGS protein and its sites of palmitoylation.

Activation of Go-proteins by membrane depolarization traced by in situ photoaffinity labeling of galphao-proteins with [alpha32P]GTP-azidoanilide

Evidence for depolarization-induced activation of G-proteins in membranes of rat brain synaptoneurosomes has been previously reported (Cohen-Armon, M., and Sokolovsky, M. (1991) J. Biol. Chem. 266, 2595-2605; Cohen-Armon, M., and Sokolovsky, M. (1993) J. Biol. Chem. 268, 9824-9838). In the present work we identify the activated G-proteins as Go-proteins by tracing their depolarization-induced in situ photoaffinity labeling with [alpha32P]GTP-azidoanilide (GTPAA). Labeled GTPAA was introduced into transiently permeabilized rat brain-stem synaptoneurosomes. The resealed synaptoneurosomes, while being UV-irradiated, were depolarized. Relative to synaptoneurosomes at resting potential, the covalent binding of [alpha32P]GTPAA to Galphao1- and Galphao3-proteins, but not to Galphao2- isoforms, was enhanced by 5- to 7-fold in depolarized synaptoneurosomes, thereby implying an accelerated exchange of GDP for [alpha32P]GTPAA. Their depolarization-induced photoaffinity labeling was independent of stimulation of Go-protein-coupled receptors and could be reversed by membrane repolarization, thus excluding induction by transmitters release. It was, however, dependent on depolarization-induced activation of the voltage-gated sodium channels (VGSC), regardless of Na+ current. The alpha subunit of VGSC was cross-linked and co-immunoprecipitated with Galphao-proteins in depolarized brain-stem and cortical synaptoneurosomes. VGSC alpha subunit most efficiently cross-linked with guanosine 5'-O-2-thiodiphosphate-bound rather than to guanosine 5'-O-(3-thiotriphosphate)-bound Galphao-proteins in isolated synaptoneurosomal membranes. These findings support a possible involvement of VGSC in depolarization-induced activation of Go-proteins.

Gel electrophoretic quantitation of protein carbonyls derivatized with tritiated sodium borohydride

A method for the quantitation of protein carbonyls, which have been widely employed as markers of protein oxidative damage, is described. Protein carbonyls were derivatized with tritiated sodium borohydride and the tritiated proteins were separated on SDS-PAGE. Protein bands, visualized by Coomassie blue staining, were then excised and incubated in 30% H2O2 at 60 degrees C for 48 h. Tritium, incorporated into the proteins, was quantitated by liquid scintillation counting after gel solubilization by H2O2. This method can be applied to the measurement of carbonylation of specific proteins as it employs SDS-PAGE and has the advantage that unreacted NaB3H4 in the labeling reaction mixture need not be removed. The present method, when combined with immunochemical detection of protein carbonyls, should be very useful in the quantitation of oxidative damage to individual proteins.

Palmitoylation increases the kinase activity of the G protein-coupled receptor kinase, GRK6

The G protein-coupled receptor kinase GRK6 undergoes posttranslational modification by palmitoylation. Palmitoylated GRK6 is associated with the membrane, while nonpalmitoylated GRK6 remains cytosolic. We have separated palmitoylated from nonpalmitoylated GRK6 to assess their relative kinase activity. Palmitoylated GRK6 is 10-fold more active at phosphorylating beta2-adrenergic receptor than nonpalmitoylated wild-type GRK6 or a nonpalmitoylatable mutant GRK6. A nonpalmitoylatable mutant GRK6 which has been further mutated to undergo posttranslational geranylgeranylation is also more active, recovering most of the activity of the palmitoylated enzyme. This activity increase by lipid modification is expected, as the lipid helps GRK6 localize to cellular membranes where its receptor substrates are found. However, when assayed using a soluble protein (casein) as a substrate, both palmitoylated and prenylated GRK6 display significantly higher activity than nonpalmitoylated wild-type or nonpalmitoylatable mutant GRK6 kinases. This increased activity is not altered by addition of exogenous palmitate or phosphatidycholine vesicles, arguing that it is not due to direct activation of GRK6 by binding palmitate, nor to nonspecific association of the GRK6 with casein. Further, chemical depalmitoylation reduces the casein phosphorylation activity of the palmitoylated, but not prenylated, GRK6 kinase. Thus, palmitoylation of GRK6 appears to play a dual role in increasing the activity of GRK6: it increases the hydrophobicity and membrane association of the GRK6 protein, which helps bring the GRK6 to its membrane-bound substrates, and it increases the kinase catalytic activity of GRK6.

Identification of a neuronal calmodulin-binding peptide, CAP-19, containing an IQ motif

Neurons produce polypeptides which can bind the calcium-poor or pre-activated form of calmodulin. It is expected that this class of peptide will serve an important role in maintaining cellular homeostasis since it would modulate calcium-dependent target regulation and redirect intracellular signaling. The lack of conserved sequence has made the identification of these peptides difficult, consequently leading us to exploit their property of binding calcium-poor calmodulin as a means of finding new species. A new peptide termed Calmodulin-Associated Peptide-19 (CAP-19) was purified and characterized. The protein-sequence information was employed in order to recover a cDNA clone from rat which included the entire reading frame for the peptide. Like its counterparts, neuromodulin (GAP-43), neurogranin (RC3) and PEP-19, it contains an IQ motif although the remainder of the peptide is quite different. Northern blot analysis of ribonucleic acid (RNA) from animals of differing ages indicated that the message appears at birth and then persists into adulthood. Antibodies to synthetic peptide were employed for localizing CAP-19. The results indicated that the peptide was localized to neurons in several brain regions. CAP-19 is similar to other calmodulin-binding proteins in that the domain spanning the IQ motif was demonstrated to participate in binding to calmodulin. Database searching showed CAP-19 to be homologous to the silkworm protein, multiprotein bridging factor 1 (MBF1). This homology suggests a potential new role for calmodulin-associated proteins in cellular homeostasis.

B-50/GAP-43-induced formation of filopodia depends on Rho-GTPase

In the present study we show that expression of the neural PKC-substrate B-50 (growth-associated protein [GAP-43]) in Rat-1 fibroblasts induced the formation of filopodial extensions during spreading. This morphological change was accompanied by an enhanced formation of peripheral actin filaments and by accumulation of vinculin immunoreactivity in filopodial focal adhesions, colocalizing with B-50. In time lapse experiments, the B-50-induced filopodial extensions were shown to stay in close contact with the substratum and appeared remarkably stable, resulting in a delayed lamellar spreading of the fibroblasts. The morphogenetic effects of the B-50 protein were entirely dependent on the integrity of the two N-terminal cysteines involved in membrane association (C3C4), but were not significantly affected by mutations of the PKC-phosphorylation site (S41) or deletion of the C terminus (177-226). Cotransfection of B-50 with dominant negative Cdc42 or Rac did not prevent B-50-induced formation of filopodial cells, whereas this process could be completely blocked by cotransfection with dominant negative Rho or Clostridium botulinum C3-transferase. Conversely, constitutively active Rho induced a similar filopodial phenotype as B-50. We therefore propose that the induction of surface extensions by B-50 in spreading Rat-1 fibroblasts depends on Rho-guanosine triphosphatase function.

Plasma membrane localization is required for RGS4 function in Saccharomyces cerevisiae

RGS4, a mammalian GTPase activating protein for G protein alpha subunits, was identified by its ability to inhibit the pheromone response pathway in Saccharomyces cerevisiae. To define regions of RGS4 necessary for its function in vivo, we assayed mutants for activity in this system. Deletion of the N-terminal 33 aa of RGS4 (Delta1-33) yielded a nonfunctional protein and loss of plasma membrane localization. These functions were restored by addition of a C-terminal membrane-targeting sequence to RGS4 (Delta1-33). Thus, plasma membrane localization is tightly coupled with the ability of RGS4 to inhibit signaling. Fusion of the N-terminal 33 aa of RGS4 to green fluorescent protein was sufficient to localize an otherwise soluble protein to the plasma membrane, defining this N-terminal region as a plasma membrane anchorage domain. RGS4 is palmitoylated, with Cys-2 and Cys-12 the likely sites of palmitoylation. Surprisingly, mutation of the cysteine residues within the N-terminal domain of RGS4 did not affect plasma membrane localization in yeast or the ability to inhibit signaling. Features of the N-terminal domain other than palmitoylation are responsible for the plasma membrane association of RGS4 and its ability to inhibit pheromone response in yeast.

Adenovirus death protein, a transmembrane protein encoded in the E3 region, is palmitoylated at the cytoplasmic tail

The 11.6-K protein of human adenovirus 2 (Ad2), which was recently renamed as adenovirus death protein (ADP), is a type III membrane glycoprotein that ultimately localizes to the nuclear membrane. ADP is encoded in the E3 transcription unit of Ad2 and migrates as a set of multiple bands in SDS-PAGE with three major forms. The corresponding gene product of adenovirus 5 (Ad5) has a slightly lower molecular weight and shows the same pattern in SDS-PAGE. We report here the covalent attachment of fatty acids to cysteine residues of ADP. In the case of Ad5-ADP all three major forms of this protein can be labeled by [3H]palmitic acid, but not by [3H]myristic acid, whereas only two [3H]palmitic acid-labeled Ad2-ADP species could be detected. The label is sensitive to treatment with 1 M hydroxylamine at pH 7 and with 20% beta-mercaptoethanol indicating that the fatty acids are linked via a thioester bond. By thin layer chromatography, the vast majority of the incorporated label was identified as palmitic acid. Two cysteine residues at the boundary between transmembrane domain and cytoplasmic tail which could serve as acceptor sites were mutated to alanine residues by site-directed mutagenesis of the cloned Ad5-ADP gene. Expression of wild-type Ad5-ADP and the resulting mutants was performed in HeLa cells using the vaccinia virus T7 expression system. As demonstrated by labeling with [3H]palmitic acid, only the mutants with one remaining cysteine residue in the cytoplasmic tail were able to incorporate [3H]palmitic acid, indicating that either could serve as acceptor site. In contrast the double cysteine mutant could not be labeled by [3H]palmitic acid, clearly demonstrating that cysteines 53 and 54 are required for palmitoylation and probably represent the palmitoylation sites in Ad5-ADP.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

GAP-43: an intrinsic determinant of neuronal development and plasticity

Several lines of investigation have helped clarify the role of GAP-43 (FI, B-50 or neuromodulin) in regulating the growth state of axon terminals. In transgenic mice, overexpression of GAP-43 leads to the spontaneous formation of new synapses and enhanced sprouting after injury. Null mutation of the GAP-43 gene disrupts axonal pathfinding and is generally lethal shortly after birth. Manipulations of GAP-43 expression likewise have profound effects on neurite outgrowth for cells in culture. GAP-43 appears to be involved in transducing intra- and extracellular signals to regulate cytoskeletal organization in the nerve ending. Phosphorylation by protein kinase C is particularly significant in this regard, and is linked with both nerve-terminal sprouting and long-term potentiation. In the brains of humans and other primates, high levels of GAP-43 persist in neocortical association areas and in the limbic system throughout life, where the protein might play an important role in mediating experience-dependent plasticity.

GAIP is membrane-anchored by palmitoylation and interacts with the activated (GTP-bound) form of G alpha i subunits

GAIP (G Alpha Interacting Protein) is a member of the recently described RGS (Regulators of G-protein Signaling) family that was isolated by interaction cloning with the heterotrimeric G-protein G alpha i3 and was recently shown to be a GTPase-activating protein (GAP). In AtT-20 cells stably expressing GAIP, we found that GAIP is membrane-anchored and faces the cytoplasm, because it was not released by sodium carbonate treatment but was digested by proteinase K. When Cos cells were transiently transfected with GAIP and metabolically labeled with [35S]methionine, two pools of GAIP--a soluble and a membrane-anchored pool--were found. Since the N terminus of GAIP contains a cysteine string motif and cysteine string proteins are heavily palmitoylated, we investigated the possibility that membrane-anchored GAIP might be palmitoylated. We found that after labeling with [3H]palmitic acid, the membrane-anchored pool but not the soluble pool was palmitoylated. In the yeast two-hybrid system, GAIP was found to interact specifically with members of the G alpha i subfamily, G alpha i1, G alpha i2, G alpha i3, G alpha z, and G alpha o, but not with members of other G alpha subfamilies, G alpha s, G alpha q, and G alpha 12/13. The C terminus of G alpha i3 is important for binding because a 10-aa C-terminal truncation and a point mutant of G alpha i3 showed significantly diminished interaction. GAIP interacted preferentially with the activated (GTP) form of G alpha i3, which is in keeping with its GAP activity. We conclude that GAIP is a membrane-anchored GAP with a cysteine string motif. This motif, present in cysteine string proteins found on synaptic vesicles, pancreatic zymogen granules, and chromaffin granules, suggests GAIP's possible involvement in membrane trafficking.

The effects of palmitoylation on membrane association of Semliki forest virus RNA capping enzyme

The nonstructural protein Nsp1 of Semliki Forest virus has guanine-7-methyltransferase and guanylyltransferase-like activities, required in the capping of viral mRNAs. It is palmitoylated and tightly associated with the cytoplasmic surface of the plasma membrane, endosomes, and lysosomes. To localize the acylation site(s) and the putative membrane-targeting domain, a number of deletions were made in the nsp1 gene. Most deletions resulted in the expression of nonpalmitoylated, enzymatically inactive, cytoplasmic protein. Palmitate could be released from Nsp1 with neutral hydroxylamine, indicating a thioester linkage to a cysteine residue. Therefore we mutated the conserved cysteine residues of Nsp1 to alanine. Triple mutation of Cys418, Cys419, and Cys420 resulted in nonpalmitoylated Nsp1, which was enzymatically active and still associated with membranes. However, it could be released from the membranes with 1 M NaCl, whereas 50 mM sodium carbonate (pH 12) was required to release wild type Nsp1, suggesting a conversion from an integral to a peripheral membrane protein. Indirect confocal immunofluorescence microscopy showed that the nonpalmitoylated Nsp1 colocalized with the plasma membrane marker, concanavalin A. However, it was not detected in filopodia, which were heavily stained in cells expressing wild type Nsp1. These results indicate that the acylation of Nsp1 was not needed for its targeting to the plasma membrane, but it was necessary for the migration to the filopodial extensions of the plasma membrane.

Human palmitoyl protein thioesterase: evidence for lysosomal targeting of the enzyme and disturbed cellular routing in infantile neuronal ceroid lipofuscinosis

Palmitoyl protein thioesterase (PPT) is an enzyme that removes palmitate residues from various S-acylated proteins in vitro. We recently identified mutations in the human PPT gene in patients suffering from a neurodegenerative disease in childhood, infantile neuronal ceroid lipofuscinosis (INCL), with dramatic manifestations limited to the neurons of neocortical origin. Here we have expressed the human PPT cDNA in COS-1 cells and demonstrate the lysosomal targeting of the enzyme via the mannose 6-phosphate receptor-mediated pathway. The enzyme was also secreted into the growth medium and could be endocytosed by recipient cells. We further demonstrate the disturbed intracellular routing of PPT carrying the worldwide most common INCL mutation, Arg122Trp, to lysosomes. The results provide evidence that INCL represents a novel lysosomal enzyme deficiency. Further, the defect in the PPT gene causing a neurodegenerative disorder suggests that depalmitoylation of the still uncharacterized substrate(s) for PPT is critical for postnatal development or maintenance of cortical neurons.

Regulation of cellular signalling by fatty acid acylation and prenylation of signal transduction proteins

Covalent modification by fatty acylation and prenylation occurs on a wide variety of cellular signalling proteins. The enzymes that catalyze attachment of these lipophilic moieties to proteins have recently been identified and characterized. Each lipophilic group confers unique properties to the modified protein, resulting in alterations in protein/protein interactions, membrane binding and targeting, and intracellular signalling. The biochemistry and cell biology of protein myristoylation, farnesylation and geranylgeranylation is reviewed here, with emphasis on the Src family of tyrosine kinases, Ras proteins and G protein coupled signalling systems.

Use of a two-hybrid system to investigate molecular interactions of GAP-43

We used the 'interaction trap' (two-hybrid system) to identify polypeptides that interact with the neuronal phosphoprotein, GAP-43, in an intracellular environment. GAP-43 (neuromodulin, B-50, F1), a protein kinase C (PKC) substrate important for the growth and plasticity of neuronal connections, has been implicated in vitro in several signal transduction pathways. In the yeast-based cloning system, the only strong interaction that was detected between GAP-43 and the calcium effector protein, calmodulin (CaM). PKC phosphorylates GAP-43 on serine 41. When we changed this serine to an aspartate residue to mimic constitutive phosphorylation, the interaction with CaM was blocked. Surprisingly, the N-terminal third of GAP-43 alone bound CaM more strongly than did intact GAP-43, suggesting that the protein's C-terminus may play a role in modulating the interaction with CaM. These results, along with other recent findings, suggest a novel role for the interaction between GAP-43 and CaM.

Palmitoylated cysteine 341 modulates phosphorylation of the beta2-adrenergic receptor by the cAMP-dependent protein kinase

We previously showed that substitution of a glycine residue for the palmitoylated cysteine 341 of the human beta2-adrenergic receptor (Gly341beta2AR), increases the basal level of the receptor phosphorylation and reduces its ability to functionally interact with Gs. In the present study, we show that additional mutation of serines 345 and 346 (Ala345,346Gly341beta2AR) restored normal phosphorylation and receptor-Gs coupling, thus suggesting that the increased phosphorylation of this site, rather than the lack of palmitoylation per se, is responsible for the poor coupling of the unpalmitoylated receptor. This is supported by the observation that chemical depalmitoylation of purified beta2AR did not affect the ability of the receptor to stimulate adenylyl cyclase in reconstitution assays. Furthermore, mutation of Ser345,346 in a wild type receptor background (Ala345,346beta2AR) significantly decreased the rate of agonist-promoted desensitization of the receptor-stimulated adenylyl cyclase activity, supporting a role for this phosphorylation site in regulating the functional coupling of the receptor. Since serines 345 and 346 are located in a putative cyclic AMP-dependent protein kinase (PKA) phosphorylation site immediately downstream of the palmitoylated cysteine 341, the hypothesis that the accessibility of this site may be regulated by the receptor palmitoylation state was further assessed in vitro. In membrane phosphorylation assays, Gly341beta2AR was found to be a better substrate for PKA than the wild type receptor, thus supporting the notion that palmitoylation restrains access of the phosphorylation site to the enzyme. Taken together, the data demonstrate that palmitoylation of cysteine 341 controls the phosphorylation state of the PKA site located in the carboxyl tail of the beta2AR and by doing so modulates the responsiveness of the receptor.

Distribution of GAP-43 mRNA in the immature and adult cerebellum: a role for GAP-43 in cerebellar development and neuroplasticity

Expression of GAP-43 mRNA in the rat cerebellum and inferior olivary nucleus was examined at birth, during postnatal development and in the adult by both Northern and in situ hybridization. Northern blot analysis revealed that cerebellar GAP-43 mRNA expression increases from birth to postnatal day (PD) 7 and then declines to a lower level in the adult. At birth, in situ hybridization experiments showed intense labeling of GAP-43 mRNA in the premigratory, but not the germinal, zone of the cerebellar external granule cell layer. Localization of GAP-43 within the premigratory zone, a layer containing post-mitotic granule cells, indicates that granule cells begin expressing GAP-43 mRNA after final mitosis and during axonal outgrowth of the parallel fibers. The deep cerebellar nuclei and the inferior olive were also intensely labeled at birth. GAP-43 mRNA was localized in granule cells during their migration through the molecular layer of the developing cerebellum and after their arrival in the internal granule cell layer. By PD 21, the pattern of GAP-43 expression was similar to that observed in the adult; GAP-43 mRNA was localized to the internal granule layer and the inferior olive with minimal to no hybridization in the deep cerebellar nuclei and none in the molecular layer. Purkinje cells were devoid of GAP-43 mRNA throughout the postnatal and adult periods. In light of our observations, we propose that GAP-43 is a critical factor in granule cell differentiation/migration, as well as in the parallel and climbing fiber axonal outgrowth and synaptogenesis during development. Localization of GAP-43 mRNA within granule and inferior olivary cells of adult animals indicates that GAP-43 protein observed in the molecular layer is transported from these cells to their terminals in the molecular layer suggesting that GAP-43 is also an intrinsic presynaptic determinant in cerebellar neuroplasticity.