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Biological Research

Print version ISSN 0716-9760

Biol. Res. vol.35 no.2 Santiago  2002

http://dx.doi.org/10.4067/S0716-97602002000200005 

Biol Res 35: 139-150, 2002

Posttranslational protein S-palmitoylation and the
compartmentalization of signaling molecules in neurons

SEAN I. PATTERSON

Department of Morphology and Physiology and the Institute of Histology and Embryology
Faculty of Medical Sciences. Cuyo National University, Mendoza, Argentina

ABSTRACT

Protein domains play a fundamental role in the spatial and temporal organization of intracellular signaling systems. While protein phosphorylation has long been known to modify the interactions that underlie this organization, the dynamic cycling of lipids should now be included amongst the posttranslational processes determining specificity in signal transduction. The characteristics of this process are reminiscent of the properties of protein and lipid phosphorylation in determining compartmentalization through SH2 or PH domains. Recent studies have confirmed the functional importance of protein S-palmitoylation in the compartmentalization of signaling molecules that support normal physiological function in cell division and apoptosis, and synaptic transmission and neurite outgrowth. In neurons, S-palmitoylation and targeting of proteins to rafts are regulated differentially in development by a number of processes, including some related to synaptogenesis and synaptic plasticity. Alterations in the S-palmitoylation state of proteins substantially affect their cellular function, raising the possibility of new therapeutic targets in cancer and nervous system injury and disease.

Key terms: S-palmitoylation, synapse, growth cone, regeneration, phosphorylation

POSTTRANSLATIONAL PROTEIN MODIFICATIONS

It is increasingly being recognized that covalent lipid-modification of proteins plays an important role in cellular signaling. Four major classes of lipidation have been identified - modification with short (N-myristoylation, (Taniguchi 1999) or long chain (S-palmitoylation, (Resh 1999) fatty acids, isoprenoids (prenylation, (Sinensky 2000b), or inositol-glycan phospholipids (glypiation, (Ferguson et al., 1999). Each of these modifications has it's own characteristic enzymology and cellular physiology (see Table I). Nonetheless, S-palmitoylation stands out from the other lipid modifications due to a number of observations: (1) it has a substantial posttranslational component, (2) no enzyme responsible for the lipid transfer has yet been purified, (3) it shows relaxed substrate specificity in vivo, accepting several saturated and unsaturated long-chain (i.e. ³16 carbon) fatty acids, and (4) for the wide variety of peptide substrate sequences so far described.

Given it's large posttranslational component, it's sensitivity to extracellular stimuli and rapid turnover, it is tempting to draw parallels between S-palmitoylation and another protein modification, tyrosine phosphorylation (Table II). This becomes especially relevant when considered in the context of targeting of signal transduction molecules through phosphorylation-dependent interactions mediated by, for example, SH2 (Sawyer 1998), PTB (Shoelson 1997), PH (Rebecchi and Scarlata 1998), or PX (Sato et al., 2001) domains. These posttranslational modification-dependent domains are critical in a diverse array of biological functions, including directional motility (Parent and Devreotes 1999), synaptic function (Kim and Huganir 1999), immune cell activation (Peterson et al., 1998), cellular responses to insulin (Pawson and Scott 1997), apoptosis (Muslin and Xing 2000) and cell growth (Parsons and Parsons 1997), all processes in which S-palmitoylation has been proposed to play a role. Selective association permits the assembly of membrane-associated protein complexes through the regulated activity of protein or lipid kinases and phosphatases (Pawson and Scott 1997), and is fundamental to the specificity of signal transduction pathway recruitment.

Protein compartmentalization through S-palmitoylation

Perhaps the most important aspect of protein lipidation is it's capacity to target proteins to specific cellular locations through selective interactions with membrane compartments. The original studies with protein palmitoylation and myristoylation described different distributions for the modified proteins, with the former being much more strongly membrane-associated than the latter (Olson 1988). This remained the given wisdom for some time, supported by the observations that neither myristoylation nor prenylation conferred the same energetic favourability for strong-membrane association that palmitoylation, particularly on multiple residues, could (Shahinian and Silvius 1995; Silvius and Zuckermann 1993). Then, in 1997, the first paper was published suggesting that palmitoylation might target the tyrosine kinase fyn to a sub-compartment of cellular membranes (van't Hof and Resh 1997), variously referred to as caveolae, rafts or DIGS (Parton and Simons 1995), highly enriched in cellular signal transduction machinery, and which are discussed in more detail in other articles of this issue (Bender et.al., 2002; Magee et al., 2002).

The issue of membrane targeting has long been confounded by the fact that many acylproteins undergo more than one form of lipid modification. Within the well-studied classes of modified proteins, the src-family kinases and heterotrimeric G-proteins are always myristoylated and often palmitoylated (Resh 1999), while the ras-family G-proteins are always prenylated and sometimes palmitoylated (Milligan et al., 1995). Add to these proteins that associate with membranes through protein-protein (Sudol 1998) or protein-lipid (Sato et al., 2001) binding domains, and single (Niethammer et al., 2002) or multiple (Mouillac et al., 1992) transmembrane domains and it is hardly surprising that there is no consistent localization attributable to palmitoylation per se (Prabhakar et al., 2000; Yang et al., 2002).

 

TABLE I

Characteristics of the 4 major classes of protein acylation. Abbreviations: NMT, N-myristoyltransferase; Ftase, farnesyltransferase; GGtase, geranylgeranyltransferase


N-Myristoylation

S-Palmitoylation

Prenylation
Glypiation

Group and bond

Amide-linked fatty acid
Thioester-linked fatty acid

Thioether-linked isoprenoid

Polysaccharide-linked phosphatidylinositol
Location of modification
N-terminal glycine
Predominantly cysteine
anywhere in the protein
C-terminal cysteine

C-terminal amino acid after
signal cleavage

Reversibility

Stably bonded
Fatty acid turns over
Stably bonded
Stably bonded with fatty
acid remodelling

Association with protein synthesis

Cotranslational
Posttranslational
Cotranslational

Cotranslational and posttranslational

Enzymology
One step, NMT
Not known
Two-step, Ftase, GGtase
Complex synthesis

Type of association

With proteins
Membrane insertion
With proteins, maybe
membrane insertion
Membrane insertion

Type of proteins modified

Intracellular proteins

Intracellular or transmembrane proteins

Intracellular proteins
Cell surface proteins

Effect of modification

Reversible inter- or intra-
protein associations
Proteins localize to rafts and
polarized cell compartments
Proteins associate reversibly
with various intracellular
membranes/proteins
Proteins localize to rafts in
apical or axonal membrane
domains

Table II

Comparison of some properties of 3 posttranslational processes involved in the compartmentalization of cellular proteins. Abbreviations: PI phospatidylinositol; PTK, protein tyrosine kinase; NA, not applicable; SH2, src-homolgy 2; PTB phosphotyrosine-binding; PX, Phox homology; PH, pleckstrin homolgy; PY, phosphotyrosine.


S-Palmitoylation

Tyr-Phosphorylation

PI-Phosphorylation

Modifying group

Large, hydrophobic, neutral

Large, hydrophilic, charged

Large, hydrophilic, charged

Modified group

Predominantly cysteine, some
erine or threonine

Tyrosine Phosphatidylinositol

Chemical bond

Thioester, some oxyester

Oxyester Oxyester
Cellular location

Predominantly the plasma membrane

All cellular compartments Various cellular membranes

Precursor

Energetically activated group (acyl-CoA)

Energetically activated group (ATP)

Energetically activated group (ATP)

Enzymes

Controversial

Many PTKs purified
and cloned

Several lipid kinases
purified and cloned

Consensus sequence

Many, poorly defined

Many, well defined

NA

Mechanism of Reversibility

Thioesterase(s)

Phosphatases

Phosphatases

Type of Association

Membrane insertion

Binds SH2, PTB domains

Binds PX, PH domains

Regulation

Receptor and non-receptor activation,
mechanism unknown

Receptor and non-receptor mechanisms,
well understood

Receptor and non-receptor
mechanisms, well understood

Effect of Phosphorylation

Phosphorylation of adjacent residues
may disrupt palmitoylation

Some interactions require PY,
some are inhibited by PY

Different phosphorylated PIs interact
with different domains

Yet it is clear in a number of cases that S-palmitoylation of proteins, even if they undergo other modifications, is both necessary and sufficient to target them to rafts (Arni et al., 1998; Guzzi et al., 2001). The src-family kinase fyn is myristoylated cotranslationally and associates rapidly with membranes, but does not partition into the DIGs until it is palmitoylated some 20 minutes later (van't Hof and Resh 1997). This lag between synthesis and palmitoylation is also seen with the synaptic Q-SNARE protein SNAP-25, which does not undergo the palmitoylation without intracellular transport (Gonzalo and Linder 1998), presumably to the plasma membrane, where it does partition into rafts (see Fig. 1 and Braun and Madison 2000). Substituting unsaturated palmitate analogs in fyn reduces raft targeting and signal transduction through fyn (Liang et al., 2001). From these and other examples, it may generally be concluded that dual fatty acylation, and more specifically multiple palmitoylation, predisposes proteins to localize to rafts/caveolae, especially in comparison with the prenylation motifs alone (Zacharias et al., 2002).

An additional targeting role for fatty acylation has been identified for src and caveolin, the triply palmitoylated resident protein of caveolae whose acylation is neither dynamic (Parat and Fox 2001) nor necessary for localization to caveolae (Dietzen et al., 1995). One of the three palmitate groups (at Cys156) interacts with the myristoyl moiety of src, localizing the kinase to rafts despite the fact that src is itself not palmitoylated (Lee et al., 2001). Curiously, this interaction is disrupted by the coexpression of glypiated proteins that are targeted to the extracellular leaflet of the rafts. This may indicate that the nature of the caveolin-src coupling is a direct hydrophobic interaction between the fatty acids within the membrane, a novel concept, but one that is gaining in popularity for interactions between extracellular GPI- linked proteins and their cognate intracellular effectors (Hakomori 2002).

Fig. 1: Distribution of raft-associated proteins differs between growth cones and synaptosomes. Low-density detergent-resistant fractions were prepared (Bender et al.,2002) from isolated neonatal rat cerebral growth cones and adult synaptosomes (Gordon-Weeks 1987). Equal proportions of the sucrose gradient fractions (1, top of gradient, 12 bottom of gradient, P, pellet) were loaded and the proteins separated by SDS-PAGE and blotted onto nitrocellulose. (A) Ponceau S staining of the blots, showing the different flotation density of the light fraction from growth cones (GC) and synaptosomes (SYN). The positions of molecular weight markers are shown on the left in kDa. (B) Immunoblot analysis of the distribution of the non-palmitoylated adhesion protein L1, dually-palmitoylated GAP-43, myristoylated and palmitoylated Goa, singly palmitoylated synaptobrevin, and quadruply palmitoylated SNAP-25.

An interesting series of papers from David Bredt's laboratory has evinced a relationship between the palmitoylation motifs and intracellular targeting in polarized cells. The growth-associated protein GAP-43 and scaffolding protein PSD-95 are selectively transported to, respectively, the axonal (Goslin et al., 1990) and dendritic (Craven et al., 1999) compartments in hippocampal neurons. Both proteins are palmitoylated near the N-terminus, and this motif appears necessary, although for PSD-95 not sufficient, to determine their respective cellular localizations (El-Husseini Ael et al., 2001; Liu et al., 1991). Splicing the GAP-43 palmitoylation motif onto PSD-95 resulted in a protein that distributed to both compartments, with some preference for axons. Adding new basic residues close to the palmitoylation site of PSD-95 increased it's distribution to axons, while removing existing nearby basic residues from the GAP-43 sequence reduced it's axonal targeting. The two acylated cysteines in GAP-43 are adjacent, while in PSD-95 they are separated by a leucine. Eliminating this leucine reduced dendritic targeting of PSD-95, in a manner co-operative with the basic residue mutation, while converse results were seen adding a spacing amino acid to the GAP-43 sequence. Thus there seems to be a complex interplay between the spatial organization of palmitoylated residues and nearby basic amino acids, and the axo-dendritic targeting of neuronal proteins. Interestingly, the same motifs that conferred axonal targeting also partitioned the mutant proteins into rafts, while the PSD palmitoylation motif seems to target the protein to a distinct somatic and dendritic vesiculotubular compartment (El-Husseini et al., 2000).

In this laboratory, we have examined the distribution of a variety of acylated proteins in rafts prepared from growth cones, the motile tips of extending axons, and their mature form, synaptosomes. Since these structures respond to many of the same extracellular cues - growth factors and adhesion molecules - with very different responses, it seemed reasonable to suppose that the compartmentalization of transduction and effector proteins would be distinct between the two states. When we purified low-density, detergent-resistant fractions from isolated growth cones and synaptosomes, we found a consistent difference in the density of the material on sucrose gradients, prepared as described (Bender et. al. 2002), between the two forms of the axon terminal (Fig. 1A). This is likely due to a different composition of the rafts and, indeed, there seems to be a substantially greater concentration of proteins in the light fraction from synaptosomes. Whether this is the origin of the difference seen, or whether there are also differences in the lipid composition of the rafts, remains to be determined.

We looked at the distribution of three proteins associated with axonal growth (GAP-43, the neural cell adhesion molecule L1, and the heterotrimeric G-protein Goa), and two with synaptic function (the SNAREs SNAP-25 and synaptobrevin), in DIGs from the axon terminals (Fig 1B). Although L1 has previously been reported to be absent from rafts from adult mouse brain (Kleene et al., 2001), we expected it to show up in growth cone rafts due it's important role in axon guidance during development (Kenwrick and Doherty 1998). Surprisingly, we found the contrary case. Although undetectable in growth cone rafts, fully one fifth of the protein distributed to synaptosomal rafts from adult rat cortex, possibly functionally related to it's postulated role in synaptic plasticity (Schachner 1997). The presence of developmental stage-related proteins in the rafts seems to be confirmed by the 4-fold enrichment of both GAP-43 and Goa in growth cone rafts, while synaptosome raft SNAP-25 and synaptobrevin increase 4- and 7-fold respectively compared to growth cones. Therefore, targeting of L1 to rafts through interactions with other acylproteins, such as the glypiated receptors TAG-1/axonin-1 or NCAM and the myristoyl/palmitoyl-tyrosine kinase fyn (Kenwrick and Doherty 1998), may prove to be important for it's function in synaptic plasticity, and perhaps help explain some of the adult pathology associated with mutations in this protein in the CRASH syndrome (Fransen et al., 1997).

Function of S-palmitoylation in neurons

While the role of S-palmitoylation in targeting proteins to membrane domains has received much attention in the past few years, there have been few studies addressing the general importance of this modification in cellular physiology. Mutational studies with selected proteins have contributed much to our understanding of the function of this modification in specific circumstances, but the lack of a general, cell-permeable, specific inhibitor of the process was limiting until the identification of tunicamycin as just such a pharmacological probe (Patterson and Skene 1994). Despite suggestions to the contrary (Dunphy and Linder 1998), tunicamycin has proved to be a generally applicable inhibitor of cellular protein S-palmitoylation (Hurley et al., 2000; Schroeder et al., 1996), and with a few basic precautions in handling and interpretation (Patterson and Skene 1995), is very easy to use. While the use of nonhydrolyzable analogs of palmitoyl-CoA, the long-chain acylsynthase inhibitor cerulenin, or the fatty acid oxidation inhibitor 2-bromopalmitate has been reported to inhibit protein palmitoylation, the authors of these papers generally do not address the profound consequences of inhibiting lipid synthesis (Coleman et al., 1992; Grimaldi et al., 1992; Perez et al., 1991), making interpretation of their results at the cellular level problematic. This is particularly relevant to studies of the glycosphingolipid-enriched rafts since the initial step in sphingolipid synthesis, catalyzed by serine palmitoyltransferase, requires palmitoyl-CoA, and inhibition of this step has drastic cellular consequences (Zweerink et al., 1992).

An important functional role for S-palmitoylation in axon outgrowth was identified using tunicamycin to inhibit S-palmitoylation and consequently neurite extension in PC12 cells and primary cultures of regenerating neurons (Patterson and Skene 1994). Together with evidence that this effect could be modified by physiological factors (Hess et al., 1993; Patterson and Skene 1994), and that axon terminal protein palmitoylation is regulated endogenously during development (Patterson and Skene 1999), this represented the first evidence for a generally important role for S-palmitoylation in normal cellular processes. While it is known that a number of proteins involved in axon outgrowth are palmitoylated, such as GAP-43, SNAP-25, fyn, Goa, rac1, Ha-ras, and NCAM140, it remains to be determined whether it is any or all of these, or others, whose lack of palmitoylation explains the inhibition of growth cone function.

The palmitoylprotein PSD-95 is necessary for the assembly of multiprotein complexes, including glutamate receptors, potassium channels, nNOS (neuronal nitric oxide synthase), synGAP (a ras GTPase activating protein), stargazin (an AMPA receptor trafficking protein) and neuroligins in the post-synaptic density (Harris and Lim 2001; Sheng 2001). PSD-95 clusters these protein through three PDZ domains, that recognize specific C-terminal sequences in the clustered proteins, and SH3 and guanylate kinase domains that allow multimerization through intra- and inter-PSD-95 interactions (Kim and Huganir 1999). Targeted disruption of PSD-95 in mice alters synaptic plasticity and severely disrupts learning (Migaud et al., 1998). In further work from Bredt's laboratory, they used 2-bromopalmitate inhibition of palmitate to provide strong evidence that synaptic strength can be regulated by activity-dependent palmitate cycling on PSD-95 (El-Husseini Ael et al., 2002). In this study the authors overcame the limitations of using 2-bromopalmitate as a specific protein S-palmitoylation inhibitor by showing that a mutant PSD-95, whose membrane targeting was due to a prenylation motif, was resistant to 2-bromopalmitate's effect. Thus, as in axon outgrowth, normal synaptic function also appears to depend on ongoing cycles of dynamic S-palmitoylation.

Studies of the two strongly-membrane associated intracellular forms of lipidation- palmitoylation and prenylation - have shown that they are not equivalent either in their subcellular localization (Zacharias et al., 2002), or in their ability to support function despite the comparable hydrophobicity of the modification (Silvius and l'Heureux 1994). For instance, Ha-ras is both farnesylated and palmitoylated in PC12 cells, and supports neurite outgrowth through a number of pathways including Raf-mediated activation of MAP kinases and PI 3-kinase-mediated recruitment of rac (Kolkova et al., 2000; Mochizuki et al., 2001). Both these pathways seem to involve the farnesyl moiety of Ha-ras, although the interaction with Raf appears to be a direct binding of the isoprenoid to the protein, while that with the p110g subunit of PI 3-kinase does not (Sinensky 2000a). Mutation of the palmitoylated cysteines in Ha-ras abrogates the ability of the protein to support ras-mediated functions (Willumsen et al., 1996). However, attaching a palmitoylated but not farnesylated C-terminus to the protein allows function, but shifts the balance of signaling from activation of MAP kinases to PI 3-kinase (Booden et al., 2000). Disrupting rafts by depleting cholesterol seems to allow crosstalk between the systems, as there is then a PI 3-kinase dependent activation of the MAP kinase (Chen and Resh 2001). This distinction between the lipid modifications may reflect the tendency of palmitoylation to direct the protein towards the appropriate membrane compartment, followed by a requirement for the farnesyl group to efficiently couple activated Ha-ras to the Raf-1 kinase. An additional complication is the sensitivity of the localization of Ha-ras to the nucleotide-bound state of the protein (Prior et al., 2001), an effect that is not seen with the non-palmitoylated K(B)-ras (discussed in more detail in Magee et al.,2002 which is more efficient than Ha-ras at activating rac (Walsh and Bar-Sagi 2001). Finally, the differential targeting of growth factor-activated ras and the prenylated but not palmitoylated rap1 proteins in PC12 cells has been proposed to be responsible for the sustained activation of ras seen in the extending neurites after nerve growth factor treatment (Mochizuki et al., 2001).

Curious aspects of protein S-palmitoylation enzymology

There is some controversy over whether cellular S-palmitoylation occurs via an enzymatic or a non-enzymatic mechanism. A number of palmitoylating activities have been partially purified, including ones for GAP-43 from rat brain growth cone membranes (Patterson and Skene 1994 and further unpublished results), for ras isoforms (Gutierrez and Magee 1991; Liu et al., 1996), and for src-family tyrosine kinases from bovine brain (Berthiaume and Resh 1995). Nonetheless, failure to completely purify a protein palmitoyltransferase that corresponds to the known characteristics of cellular S-palmitoylation has led to the suggestion that the process may occur without intervention of an enzyme in situ. It is clear that chemical acylation of proteins does occur in vitro (Hartel-Schenk and Agre 1992; O'Brien et al., 1987; Patterson and Skene 1997; Quesnel and Silvius 1994) due to the innate chemical reactivity of acyl-CoA's, and can even show some specificity for different peptide substrates (Bizzozero et al., 2001).

In this regard, it is worth bearing in mind that cells carefully regulate processes that can occur spontaneously, particularly when they involve redox reactions (Stamler et al., 2001). Indeed, another highly reactive cysteine thiol-modifying agent, NO, originally thought to diffuse freely and react chemically with proteins, has now been shown to use protein chaperones both for transport and specificity of action (Hess et al., 2001; Pawloski et al., 2001). The sequence selectivity for protein S-nitrosylation does not necessarily lie in the adjacent residues, but may involve the formation of a favourable hydrophobic pocket through protein folding (Hess et al., 2001). This would involve residues that might be far from the modified cysteine in terms of amino acid sequence, comparable to the formation of protein domains by widely separated sequences (Rebecchi and Scarlata 1998), or the situation with the e-determinants of protein domain specificity (Sudol 1998). A requirement for such a favourable chemical environment for protein S-palmitoylation by protein folding, as may be the case for phospholipase D palmitoylation (Xie et al., 2001), might go some way towards explaining the wide variety of sequence requirements found by different authors in different proteins (Milligan et al., 1995), which has been used in support of the non-enzymatic model of S-palmitoylation (Ten Brinke et al., 2002).

Interestingly, there appears to be some functional relationship between cysteine nitrosylation and palmitoylation in proteins. We observed ten years ago that exposing neurons to NO reduced their incorporation of exogenous labelled palmitate into cellular proteins (Hess et al., 1993). Subsequent studies by other groups have indicated that nitrosylation might actually increase the turnover of the palmitate residues in the b-adrenergic receptor (Adam et al., 1999) and Ha-ras (Baker et al., 2000). Given that nNOS localizes to the growth cones of extending axons (Qian et al., 1996), but to the postsynaptic compartment after synaptogenesis through interaction with PSD-95 (Brenman et al., 1996), it occurred to us that there might be differential regulation of axon terminal palmitoylation through NOS in the two developmental states of the axon terminal. Indeed, we found that calcium-stimulated NO production in isolated growth cones effectively inhibited the incorporation of radiolabelled palmitate into a number of proteins, while being substantially less effective in the synaptosomes which represent the presynaptic compartment (Fig. 2). It remains to be determined whether this difference is due to lack of NO generation in response to calcium in the synaptosomes, or whether it reflects mechanisms to select the substrates for nitrosylation that are different between growth cones and synapses (Hess et al., 2001).

Perspectives on S-palmitoylation and signaling networks

The assembly of complex networks of protein interactions in order to provide spatial and temporal specificity to cellular signaling is now paradigmatic (Grant and Blackstock 2001; Pawson 1995; Pawson and Scott 1997; Sudol 1998; Turner 2000). Scaffolding and adaptor proteins with no intrinsic enzymatic activity serve to recruit other proteins to specific cellular locations, usually associated with membranes, through multiple protein/lipid interaction domains. The number of different domains available, and the number of different adaptors, each with it's unique mix of domains, permits both selectivity and variety in the specification of signal transduction. While it seems conceptually obvious that preforming protein complexes would provide specificity to signal transduction pathways, the quantitative modelling of the effect of multimolecular associations shows that they can regulate not only specificity, but also efficiency and amplitude of signal propagation (Levchenko et al., 2000). Regulating the crosstalk between different pathways has been proposed to be the functional basis of signal-dependent cellular memory by allowing several stable states of cellular excitation (Bhalla and Iyengar 1999). In this context, it seems unnecessary to argue that protein S-palmitoylation would be capable of profoundly altering neuronal function, even without the growing evidence in favour of such a role (El-Husseini Ael et al., 2002; Liang et al., 2001; Patterson and Skene 1994; Willumsen et al., 1996). Nevertheless, it is worth emphasizing that the many studies showing little effect of eliminating the palmitoylation sites of a protein on the activity of that protein in no way diminish the potential importance of the process of S-palmitoylation in the physiology of the whole cell.

It is clear that we still have some way to go in explaining how S-palmitoylation exerts it's specific effects in neurons. The studies on PSD-95 and it's palmitoylation-dependent targeting to the post-synaptic compartment, together with the PSD-95 dependent assembly of signalling complexes in the postsynaptic density and the disruption of AMPA receptor cycling, an important factor in modifying synaptic strength (Shi 2001), by inhibiting PSD-95 palmitoylation, comes closest to being a complete story. What remains to be determined is how synaptic activity couples to PSD-95 palmitoylation/depalmitoylation. A hint, perhaps, comes from a recent study showing a form of NO-dependent LTP in the cerebellum that is not mediated through cAMP, cGMP or calcium (Lev-Ram et al., 2002). Could PSD-95 be a target for protein nitrosylation in synapses? The close association of nNOS and PSD-95 in synapses (Brenman et al., 1996) makes this a reasonable possibility, and recent advances in the technology of protein nitrosylation (Gow et al., 2002) make it a straightforward question to answer. There would be a certain conceptual elegance in closing the loop, whereby palmitoylation would regulate nNOS through synaptic targeting, allowing nNOS to regulate palmitoylation through synaptic activity.

  Fig. 2: Calcium inhibits protein palmitoylation in growth cones, but not synaptosomes, through generation of nitric oxide. (A) Addition of calcium ionophore A23187 to isolated neonatal axon terminals metabolically labelled with [3H]-palmitate (Patterson and Skene 1997) inhibited protein palmitoylation. The inhibition was reversed by the addition of the NOS inhibitor L-N-methyl-arginine (N-Me-Arg), but not by the inactive D-isoform. Molecular weight markers are shown on the left. (B) A23187 inhibits protein palmitoylation in growth cones (GC) isolated from neonatal rats but is mostly ineffective in synaptosomes from adult animals (aSYN). The ionophore has an intermediate effect on synaptosomes from immature synaptosomes (14 day old, iSYN), despite the continued high expression of nNOS (Patterson and Skene 1999). Arrow, the prominent growth cone palmitoylprotein, GAP-43, is not depleted from either the GC or SYN preparations by ionophore treatment, as determined by the immunoblot analysis of each sample shown below the autoradiograms. (C) Concurrent analysis of [3H]-palmitate labelled lipids by thin layer chromatography confirms that the NO is acting specifically on protein labelling, and does not affect the incorporation of label into lipids (O, origin, PC, phosphatidylcholine, FFA, free fatty acids).

One of the characteristics of protein domain interactions is their ability to distinguish between different target sequences. Thus the multiple PY residues in activated growth factor receptors selectively recruit different signalling proteins (Pawson 1995), and the different PH and PX domains recognize different inositol phospholipids (Bottomley et al., 1998; Sato et al., 2001). There are obvious similarities in the differential localizations of palmitoylproteins, depending on the presence of nearby basic amino acids or other lipid modifications. This may correspond to evidence that lipid rafts themselves are not a homogeneous population (Iwabuchi et al., 1998), but may coexist in different forms that localize different proteins and thereby subserve different functions (Madore et al., 1999). Our evidence that protein palmitoylation substrates (Patterson and Skene 1999) and resident protein in rafts change between growth cones and synapses is particularly intriguing, given the recent evidence that rafts do mature in neurons (Ledesma et al., 1999) and that synaptogenesis may be limited by cholesterol availability (Mauch et al., 2001). Taken together, these result suggest that the formation of particular S-palmitoylation dependent membrane signalling domains may be the driving force behind synaptogenesis.

ACKNOWLEDGEMENTS

This work was supported by FONCYT grant 02141; CIUNC grant 06/J128 and Fundación Andes grant C-13680/4 to SIP.

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Corresponding author: S. Patterson. IHEM. Facultad de Ciencias Médicas, Universidad Nacional de Cuyo. 5500 Mendoza, Argentina. +54-261-420-5115 int. 2684 (tel). +54-261-449-4117 (fax). e-mail: seanpat@fcm.uncu.edu.ar

Received: May 24, 2002. In revised form: July 11, 2002. Accepted: July 15, 2002

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