Extraction of major acidic Ca2+ dependent phosphoproteins from synaptic membranes

Electroacupuncture Promotes the Differentiation of Transplanted Bone Marrow Mesenchymal Stem Cells Overexpressing TrkC into Neuron-Like Cells in Transected Spinal Cord of Rats

Our previous study indicated that electroacupuncture (EA) could increase neurotrophin-3 (NT-3) levels in the injured spinal cord, stimulate the differentiation of transplanted bone marrow mesenchymal stem cells (MSCs), and improve functional recovery in the injured spinal cord of rats. However, the number of neuron-like cells derived from the MSCs is limited. It is known that NT-3 promotes the survival and differentiation of neurons by preferentially binding to its receptor TrkC. In this study, we attempted to transplant TrkC gene-modified MSCs (TrkC-MSCs) into the spinal cord with transection to investigate whether EA treatment could promote NT-3 secretion in the injured spinal cord and to determine whether increased NT-3 could further enhance transplanted MSCs overexpressing TrkC to differentiate into neuron-like cells, resulting in increased axonal regeneration and functional improvement in the injured spinal cord. Our results showed that EA increased NT-3 levels; furthermore, it promoted neuron-phenotype differentiation, synaptogenesis, and myelin formation of transplanted TrkC-MSCs. In addition, TrkC-MSC transplantation combined with EA (the TrkC-MSCs + EA group) treatment promoted the growth of the descending BDA-labeled corticospinal tracts (CSTs) and 5-HT-positive axonal regeneration across the lesion site into the caudal cord. In addition, the conduction of cortical motor-evoked potentials (MEPs) and hindlimb locomotor function increased as compared to controls (treated with the LacZ-MSCs, TrkC-MSCs, and LacZ-MSCs + EA groups). In the TrkC-MSCs + EA group, the injured spinal cord also showed upregulated expression of the proneurogenic factors laminin and GAP-43 and downregulated GFAP and chondroitin sulfate proteoglycans (CSPGs), major inhibitors of axonal growth. Together, our data suggest that TrkC-MSC transplantation combined with EA treatment spinal cord injury not only increased MSC survival and differentiation into neuron-like cells but also promoted CST regeneration across injured sites to the caudal cord and functional improvement, perhaps due to increase of NT-3 levels, upregulation of laminin and GAP-43, and downregulation of GFAP and CSPG proteins.

Growth-associated protein-43 and ephrin B3 induction in the brain of adult SIV-infected rhesus macaques

Understanding the mechanisms of neuronal regeneration and repair in the adult central nervous system is a vital area of research. Using a rhesus lentiviral encephalitis model, we sought to determine whether recovery of neuronal metabolism after injury coincides with the induction of two important markers of synaptodendritic repair: growth-associated protein-43 (GAP-43) and ephrin B3. We examined whether the improvement of neuronal metabolism with combined anti-retroviral therapy (cART) after simian immunodeficiency virus (SIV) infection in rhesus macaques involved induction of GAP-43, also known as neuromodulin, and ephrin B3, both implicated in axonal pathfinding during neurodevelopment and regulation of synapse formation, neuronal plasticity, and repair in adult brain. We utilized magnetic resonance spectroscopy to demonstrate improved neuronal metabolism in vivo in adult SIV-infected cART animals compared to untreated and uninfected controls. We then assessed levels of GAP-43, ephrin B3, and synaptophysin, a pre-synaptic marker, in three brain regions important for cognitive function, cortex, hippocampus, and putamen, by quantitative real-time RT-PCR and immunohistochemistry. Here we demonstrate that (1) GAP-43 mRNA and protein are induced with SIV infection, (2) GAP-43 protein is higher in the hippocampus outer molecular layer in SIV-infected animals that received cART compared to those that did not, and (3) activated microglia and infiltrating SIV-infected macrophages express abundant ephrin B3, an important axonal guidance molecule. We propose a model whereby SIV infection triggers events that lead to induction of GAP-43 and ephrin B3, and that short-term cART results in increased magnitude of repair mechanisms especially in the hippocampus, a region known for high levels of adult plasticity.

Amphipathic C-terminal region of Escherichia coli NADH dehydrogenase-2 mediates membrane localization

Respiratory NADH dehydrogenase-2 (NDH-2) of Escherichia coli is a membrane-bound flavoprotein. Bioinformatics approaches suggested the involvement of NDH-2 C-terminal region in membrane anchorage. Here, we demonstrated that NDH-2 is a peripheral membrane protein and that its predicted C-terminal amphipathic Arg390-Ala406 helix is sufficient to bind the protein to lipid membranes. Additionally, a cytosolic NDH-2 protein (Trun-3), lacking the last 43 aminoacids, was purified and characterized. FAD cofactor was absent in purified Trun-3. Upon the addition of FAD, Trun-3 maximum velocity was similar to native NDH-2 rate with ferricyanide and MTT acceptors. However, Trun-3 activity was around 5-fold lower with quinones. No significant difference in K(m) values was observed for both enzymes. For the first time, an active and water soluble NDH-2 was obtained, representing a major improvement for structural/functional characterizations.

A p53-CBP/p300 transcription module is required for GAP-43 expression, axon outgrowth, and regeneration

Transcription regulates axon outgrowth and regeneration. However, to date, no transcription complexes have been shown to control axon outgrowth and regeneration by regulating axon growth genes. Here, we report that the tumor suppressor p53 and its acetyltransferases CBP/p300 form a transcriptional complex that regulates the axonal growth-associated protein 43, a well-characterized pro-axon outgrowth and regeneration protein. Acetylated p53 at K372-3-82 drives axon outgrowth, GAP-43 expression, and binds specific elements on the neuronal GAP-43 promoter in a chromatin environment through CBP/p300 signaling. Importantly, in an axon regeneration model, both CBP and p53 K372-3-82 are induced following axotomy in facial motor neurons, where p53 K372-3-82 occupancy of GAP-43 promoter is enhanced as shown by in vivo chromatin immunoprecipitation. Finally, by comparing wild-type and p53 null mice, we demonstrate that the p53/GAP-43 transcriptional module is specifically switched on during axon regeneration in vivo. These data contribute to the understanding of gene regulation in axon outgrowth and may suggest new molecular targets for axon regeneration.

Studies on the anchorage of ATP diphosphohydrolase in synaptic plasma membranes from rat brain

ATP diphosphohydrolases are described as ecto-enzymes in several tissues. In the present study, synaptic plasma membrane (SPM) was exposed to a series of agents used to distinguish between peripheral (hydrophilic), G-PI-anchored and transmembrane-polypeptide-anchored membrane proteins. These procedures included: (a) nondetergent extraction, (b) Triton X-114 phase partitioning, (c) phosphatidylinositol-specific phospholipase C (PI-PLC) extraction and (d) protease incubation. In cases (a), (c) and (d) the SPM was incubated with different agents and the ATPase-ADPase activities and the protein concentration was determined in the original sample, in the pellet and in the supernatant obtained after 100,000 g centrifugation. In procedure (b), the SPM was solubilized in 1% triton X-114 and submitted to phase separation onto a sucrose cushion. The aqueous and detergent rich phases obtained by this treatment were assayed for ATPase-ADPase activities and protein determination. The results obtained suggest an intrinsic behaviour for ATP diphosphohydrolase since none of the nondetergent treatments was efficient in removing the enzyme from SPM. Moreover, ATPase and ADPase activities were recovered predominantly (> 50%) in the detergent-rich phase obtained by Triton X-114 partitioning. The enzyme was not released by PI-PLC or proteases. These results indicate that the enzyme is not a GPI-anchored protein, but is probably deeply anchored on the plasma membrane in agreement with the amino acid sequence of the enzyme recently published.

Altered levels of the synaptosomal associated protein SNAP-25 in schizophrenia

BACKGROUND

Identifying brain changes in schizophrenia has been a major research focus for many years. Although impressive gains have been made in neuroimaging and brain electrophysiology, molecular and cellular markers of schizophrenia have lagged. There are no consistent biochemical markers for schizophrenia pathophysiology and none that reflect treatment course.

METHODS

Samples were obtained from 25 postmortem schizophrenic brains and 31 nonschizophrenic controls. These samples were processed, and the synaptosomal fraction was isolated. Ten micrograms of protein from each of these samples was solubilized in a sodium dodecylsulfate sample buffer and separated on 10% (wt/vol) polyacrylamide gels. Monoclonal antibody (SMI-81) was incubated with the blots and, using quantitative Western blotting, we measured the relative amounts of SNAP-25 in these samples.

RESULTS

We report altered levels of SNAP-25 in both the inferior temporal cortex (Brodmann area 20) and prefrontal association cortex (Brodmann areas 9 and 10) in postmortem brains of patients with schizophrenia relative to nonschizophrenic controls. Normal levels of SNAP-25 are noted in schizophrenics in area 17, decreased levels in areas 10 and 20, and an elevated level in area 9.

CONCLUSIONS

These data support cytoarchitectural observations that the cerebral cortex of schizophrenic patients has extensive pathology. The data presented here, along with data on other brain-specific proteins, indicate a complicated molecular adaptation to the causative factors of schizophrenia.

RC3/neurogranin, a postsynaptic calpacitin for setting the response threshold to calcium influxes

In this review, we attempt to cover the descriptive, biochemical and molecular biological work that has contributed to our current knowledge about RC3/neurogranin function and its role in dendritic spine development, long-term potentiation, long-term depression, learning, and memory. Based on the data reviewed here, we propose that RC3, GAP-43, and the small cerebellum-enriched peptide, PEP-19, belong to a protein family that we have named the calpacitins. Membership in this family is based on sequence homology and, we believe, a common biochemical function. We propose a model wherein RC3 and GAP-43 regulate calmodulin availability in dendritic spines and axons, respectively, and calmodulin regulates their ability to amplify the mobilization of Ca2+ in response to metabotropic glutamate receptor stimulation. PEP-19 may serve a similar function in the cerebellum, although biochemical characterization of this molecule has lagged behind that of RC3 and GAP-43. We suggest that these molecules release CaM rapidly in response to large influxes of Ca2+ and slowly in response to small increases. This nonlinear response is analogous to the behavior of a capacitor, hence the name calpacitin. Since CaM regulates the ability of RC3 to amplify the effects of metabotropic glutamate receptor agonists, this activity must, necessarily, exhibit nonlinear kinetics as well. The capacitance of the system is regulated by phosphorylation by protein kinase C, which abrogates interactions between calmodulin and RC3 or GAP-43. We further propose that the ratio of phosphorylated to unphosphorylated RC3 determines the sliding LTP/LTD threshold in concept with Ca2+/ calmodulin-dependent kinase II. Finally, we suggest that the close association between RC3 and a subset of mitochondria serves to couple energy production with the synthetic events that accompany dendritic spine development and remodeling.

Levels of the growth-associated protein GAP-43 are selectively increased in association cortices in schizophrenia

The pathophysiology of schizophrenia may involve perturbations of synaptic organization during development. The presence of cytoarchitectural abnormalities that may reflect such perturbations in the brains of patients with this disorder has been well-documented. Yet the mechanistic basis for these features of the disorder is still unknown. We hypothesized that altered regulation of the neuronal growth-associated protein GAP-43, a membrane phosphoprotein found at high levels in the developing brain, may play a role in the alterations in brain structure and function observed in schizophrenia. In the mature human brain, GAP-43 remains enriched primarily in association cortices and in the hippocampus, and it has been suggested that this protein marks circuits involved in the acquisition, processing, and/or storage of new information. Because these processes are known to be altered in schizophrenia, we proposed that GAP-43 levels might be altered in this disorder. Quantitative immunoblots revealed that the expression of GAP-43 is increased preferentially in the visual association and frontal cortices of schizophrenic patients, and that these changes are not present in other neuropsychiatric conditions requiring similar treatments. Examination of the levels of additional markers in the brain revealed that the levels of the synaptic vesicle protein synaptophysin are reduced in the same areas, but that the abundance of the astrocytic marker of neurodegeneration, the glial fibrillary acidic protein, is unchanged. In situ hybridization histochemistry was used to show that the laminar pattern of GAP-43 expression appears unaltered in schizophrenia. We propose that schizophrenia is associated with a perturbed organization of synaptic connections in distinct cortical associative areas of the human brain, and that increased levels of GAP-43 are one manifestation of this dysfunctional organization.

Increased levels of GAP-43 protein in schizophrenic brain tissues demonstrated by a novel immunodetection method

Studies on the molecular basis of neurological and psychiatric disorders often rely on the precise determination of specific proteins in brain tissues. In this study, we have developed a method for measuring the levels of the neural-specific growth-associated protein, GAP-43, in human postmortem brain specimens. This rapid and quantitative method is based on immunodetection procedures. Briefly, synaptosomal plasma membranes (SPMs) are deposited onto polyvinylidene difluoride (PVDF) membranes via a dot-blotting apparatus, followed by specific GAP-43 detection using a monospecific polyclonal antibody. Overall, the dot-blot procedure provided several advantages over Western blots and one-dimensional and two-dimensional polyacrylamide gels. The assays were more sensitive, reproducible, and allowed the rapid and simultaneous determination of multiple samples. Using this technique, we examined the levels of the GAP-43 protein in Brodmann's areas 17, 20, and 10 of schizophrenic and age-, sex-, and postmortem interval (PMI) matched controls. These studies revealed an increase in the levels of GAP-43 in visual association and frontal cortices (areas 20 and 10) of schizophrenic brains. Given the relationship of GAP-43 expression with the establishment and remodeling of neural connections, our results support the hypothesis that schizophrenia is associated with a perturbed organization of synaptic connections in associative areas of the human brain.

Localization of RC3 (neurogranin) in rat brain subcellular fractions

Previous studies have shown that RC3 (neurogranin) is a postsynaptic, protein kinase C (PKC)/calmodulin-binding substrate that accumulates throughout the perikaryal and dendritic cytoplasm and is often closely associated with the postsynaptic density (PSD) in dendritic spines of neostriatal neurons. Here Western immunoblotting studies of rat brain subcellular fractions confirm that RC3 is predominantly a cytosolic protein but is found in lower amounts in membrane-enriched microsomes and synaptosomes. Solubilization of synaptosomes suggests that RC3 may only be loosely associated with the PSD.

Stimulation of synaptosomal Na+,K(+)-ATPase by ethanol: possible involvement of an isozyme-specific inhibitor of Na+,K(+)-ATPase

In synaptosomal preparations from rat cerebral cortex, ouabain-sensitive Rb+ uptake was stimulated by ethanol (20-80 mM). Based on differential sensitivity to ouabain, 80% of this Na+,K(+)-ATPase activity represented activity of the alpha 1 isozyme while 20% was due to the alpha 2 and/or alpha 3 isozymes (alpha 2/ alpha 3). Stimulation of Na+,K(+)-ATPase was selective for the activity of alpha 2/alpha 3 which was increased by 167% in the presence of 80 mM ethanol. In this concentration range, ethanol had no effect on alpha 1 activity. Exposure of synaptosomal preparations to EGTA increased basal (no ethanol) alpha 2/alpha 3 activity with no effect on alpha 1 activity. Further, ethanol no longer stimulated alpha 2/alpha 3 activity after EGTA treatment. An EGTA extract was concentrated and desalted to yield a fraction that selectively inhibited alpha 2/alpha 3 activity when reconstituted with EGTA-treated synaptosomal preparations. This inhibition was trypsin-sensitive, suggesting protein involvement, and was prevented by 80 mM ethanol. In the presence of the inhibitory protein fraction, ethanol stimulated Na+, K(+)-ATPase activity in EGTA-treated membranes with a dose-response like that observed with the crude (no EGTA) synaptosomes. We propose that the alpha 2/alpha 3 activity of Na+,K(+)-ATPase is subject to inhibitory regulation and that ethanol stimulates this activity by releasing it from inhibition, an effect that may mimic in vivo deregulation of the enzyme by ethanol.

Purification of B-50 by 2-mercaptoethanol extraction from rat brain synaptosomal plasma membranes

Several methods have been described previously for the purification of the nervous-tissue specific protein kinase C substrate B-50 (GAP-43). In this paper we present a new purification method for B-50 from rat brain which employs 2-mercaptoethanol to release the protein from isolated synaptosomal plasma membranes. Most likely, 2-mercaptoethanol reduces disulfide bonds involved in the linkage of B-50 to the membrane. After washing the membranes with 100 mM NaCl to detach loosely bound proteins, B-50 is the major protein (and the only protein kinase C substrate) released by 0.5% 2-mercaptoethanol treatment. Further purification to apparent homogeneity is achieved by affinity chromatography on calmodulin sepharose. B-50 binds to calmodulin in the absence of calcium and specifically elutes from the column with 3 mM calcium. The procedures described is simple, rapid and highly suitable for large scale purification of B-50 from rat brain.

GAP-43 as a plasticity protein in neuronal form and repair

Neurons exhibit a remarkable plasticity of form, both during neural development and during the subsequent remodelling of synaptic connectivity. Here we review work on GAP-43 and G0, and focus upon the thesis that their interaction may endow neurons with such plasticity. We also present new data on the role of G proteins in neurite growth, and on the interaction of GAP-43 and actin. GAP-43 is a protein induced during periods of axonal extension and highly enriched on the inner surface of the growth cone membrane. Its membrane localization is primarily due to a short amino terminal sequence which is subject to palmitoylation. Binding to actin filaments may also assist in restricting the protein to specific cellular domains. Consistent with its role as a "plasticity protein," there is evidence that GAP-43 can directly alter cell shape and neurite extension, and several theses have been advanced for how it might do so. Two other prominent components of the growth cone membrane are the alpha and beta subunits of G0. GAP-43 regulates their guanine nucleotide exchange, which is an unusual role for an intracellular protein. We speculate that GAP-43 may adjust the "set point" of responsiveness for G0 stimulation by receptors, thereby altering the neuronal propensity to growth, without actually causing growth. To begin to address how G protein activity affects axon growth, we have developed a means to introduce guanine nucleotide analogs into sympathetic neurons. Stimulation of G proteins with GTP-gamma-S retards axon growth, whereas GDP-beta-S enhances it. This is compatible with G protein registration of inhibitory signals.

Role of the growth-associated protein B-50/GAP-43 in neuronal plasticity

The neuronal phosphoprotein B-50/GAP-43 has been implicated in neuritogenesis during developmental stages of the nervous system and in regenerative processes and neuronal plasticity in the adult. The protein appears to be a member of a family of acidic substrates of protein kinase C (PKC) that bind calmodulin at low calcium concentrations. Two of these substrates, B-50 and neurogranin, share the primary sequence coding for the phospho- and calmodulin-binding sites and might exert similar functions in axonal and dendritic processes, respectively. In the adult brain, B-50 is exclusively located at the presynaptic membrane. During neuritogenesis in cell culture, the protein is translocated to the growth cones, i.e., into the filopodia. In view of many positive correlations between B-50 expression and neurite outgrowth and the specific localization of B-50, a role in growth cone function has been proposed. Its phosphorylation state may regulate the local intracellular free calmodulin and calcium concentrations or vice versa. Both views link the B-50 protein to processes of signal transduction and transmitter release.

B-50 (GAP-43): biochemistry and functional neurochemistry of a neuron-specific phosphoprotein

The biochemistry and functional neurochemistry of the synaptosomal plasma membrane phosphoprotein B-50 (GAP-43) are reviewed. The protein is putatively involved in seemingly diverse functions within the nervous system, including neuronal development and regeneration, synaptic plasticity, and formation of memory and other higher cognitive behaviors. There is a considerable amount of information concerning the spatial and temporal localization of B-50 (GAP-43) in adult, fetal, and regenerating nervous tissue but far less is known about the physical chemistry and biochemistry of the protein. Still less information is available about posttranslational modifications of B-50 (GAP-43) that may be the basis of neurochemical mechanisms that could subsequently permit a variety of physiological functions. Hence, consideration is given to several plausible roles for B-50 (GAP-43) in vivo, which are discussed in the context of the cellular localization of the protein, significant posttranslational enzymes, and regulatory proteins, including protein kinases, phosphoinositides, calmodulin, and proteases.

Chicken growth-associated protein GAP-43 is tightly bound to the actin-rich neuronal membrane skeleton

We have identified the chicken equivalent of growth-associated protein GAP-43 in a detergent-resistant membrane skeleton from cultures of chick neurones and embryonic chick brain. Antisera to the membrane skeleton protein, the 3D5 antigen, precipitate the translation product of chick GAP-43 cDNA, and the 3D5 antigen is also detected by antisera against synthetic peptides from the known amino acid sequence of rat GAP-43. The chick protein and the rat GAP-43 are biochemically similar proteins that both serve as major targets of phosphorylation by endogenous protein kinase C. The detergent-resistant complex in which GAP-43 is found also contains actin (approximately 5% of the total protein) and a neurone-specific cell surface glycoprotein. We suggest that the membrane skeleton of neurones may be a primary site of action of GAP-43.

Binding of the neuronal protein B-50, but not the metabolite B-60, to calmodulin

Protein kinase C phosphorylates the neurone-specific protein B-50 at a single Ser41 residue, which is also the point for a major proteolytic cleavage in vitro, and probably in vivo, that produces a B-50 phosphorylation-inhibiting N-terminal fragment and a large C-terminal metabolite B-60 (B-50(41-226]. The intact purified protein will bind to calmodulin in the absence of calcium, but the interaction has an absolute requirement for dephospho-B-50. In an attempt to unify two aspects of B-50 biochemistry, we have examined the interaction of B-50 binding to calmodulin and B-50 proteolysis. HPLC- and affinity-purified B-50 bound to calmodulin, but purified B-60 did not. To ensure that this effect was not due to the phosphorylation state of pure, isolated B-60, the metabolite was generated in vitro using a Triton extract of synaptosomal plasma membranes, which contains the as yet uncharacterized B-50 protease. B-60 derived from dephospho-B-50 also failed to bind calmodulin. The results demonstrate a direct connection between B-50 binding to calmodulin and B-50 proteolysis. The position of the proposed calmodulin-binding domain within intact B-50 is discussed in light of the failure of calmodulin to bind B-60.

A membrane-targeting signal in the amino terminus of the neuronal protein GAP-43

Neurons and other cells, such as those of epithelia, accumulate particular proteins in spatially discrete domains of the plasma membrane. This enrichment is probably important for localization of function, but it is not clear how it is accomplished. One proposal for epithelial cells is that proteins contain targeting signals which guide preferential accumulation in basal or apical membranes. The growth-cone membrane of a neuron serves as a specialized transduction system, which helps to convert cues from its environment into regulated growth. Because it can be physically separated from the cell soma, it has been possible to show that the growth-cone membrane contains a restricted set of total cellular proteins, although, to our knowledge, no proteins are limited to that structure. One of the most prominent proteins in the growth-cone membrane is GAP-43. Basi et al. have suggested that the N-terminus of GAP-43 might be important for the binding of GAP-43 to the growth-cone membrane. Skene and Virag recently found that the cysteines in the N-terminus are fatty-acylated and that this post-translational modification correlates with membrane-binding ability. We investigated the binding of GAP-43 to the growth-cone membrane by mutational analysis and by laser-scanning confocal microscopy of fusion proteins that included regions of GAP-43 and chloramphenicol acetyltransferase (CAT). We found that a short stretch of the GAP-43 N-terminus suffices to direct accumulation in growth-cone membranes, especially in the filopodia. This supports a previous proposal for the importance of this region of GAP-43 in determining the membrane distribution of GAP-43.

Protein fatty acid acylation in developing cortical neurons

Neuron-enriched cultures derived from embryonic day 17 rat cerebral cortex were incubated in the presence of [3H]myristic or [3H]palmitic acid. Analysis of radiolabeled proteins by two-dimensional gel electrophoresis and fluorography revealed extensive incorporation of fatty acids into a small number of neuronal proteins. The major acylated proteins had apparent molecular weights and isoelectric points as follows: 87,000, 4.3; 63,000, 4.4; 45,000, 4.4; and 20,000, 5.3. After labeling with [3H]myristic acid, the radioactivity associated with these proteins was identified as myristic acid, which was attached via an ester linkage. All four of the major acylated neuronal proteins were found to be membrane-bound and enriched in growth cones. By virtue of its molecular weight, isoelectric point, subcellular distribution, and peptide map, the 87-kilodalton polypeptide was shown to be equivalent to pp80, a phosphoprotein that has been described in developing and mature synaptic terminals. The 45-kilodalton acylated protein also appears to coincide with another growth cone phosphoprotein, pp40. Acylation may serve as a mechanism to regulate the function of these proteins, or may play a role in directing them to the nerve terminal membrane.

Posttranslational membrane attachment and dynamic fatty acylation of a neuronal growth cone protein, GAP-43

Growth cones, the motile apparatus at the ends of elongating axons, are sites of extensive and dynamic membrane-cytoskeletal interaction and insertion of new membrane into the growing axon. One of the most abundant proteins in growth cone membranes is a protein designated GAP-43, whose synthesis increases dramatically in most neurons during periods of axon development or regeneration. We have begun to explore the role of GAP-43 in growth cone membrane functions by asking how the protein interacts with those membranes. Membrane-washing experiments indicate that mature GAP-43 is tightly bound to growth cone membranes, and partitioning of Triton X-114-solubilized GAP-43 between detergent-enriched and detergent-depleted phases indicates considerable hydrophobicity. The hydrophobic behavior of the protein is modulated by divalent cations, particularly zinc and calcium. In vivo labeling of GAP-43 in neonatal rat brain with [35S]methionine shows that GAP-43 is initially synthesized as a soluble protein that becomes attached to membranes posttranslationally. In tissue culture, both rat cerebral cortex cells and neuron-like PC12 cells actively incorporate [3H]palmitic acid into GAP-43. Isolated growth cones detached from their cell bodies also incorporate labeled fatty acid into GAP-43, suggesting active turnover of the fatty acid moieties on the mature protein. Hydrolysis of ester-like bonds with neutral hydroxylamine removes the bound fatty acid and exposes new thiol groups on GAP-43, suggesting that fatty acid is attached to the protein's only two cysteine residues, located in a short hydrophobic domain at the amino terminus. Modulation of the protein's hydrophobic behavior by divalent cations suggests that other domains, containing large numbers of negatively charged residues, might also contribute to GAP-43-membrane interactions. Our observations suggest a dynamic and reversible interaction of GAP-43 with growth cone membranes.