Low molecular weight GTP-binding proteins are associated with neuronal organelles involved in rapid axonal transport and exocytosis

Localisation of functional muscarinic receptors in the rat cochlea: evidence for efferent presynaptic autoreceptors

In the rat cochlea, the activation of muscarinic receptors stimulates the hydrolysis of phosphoinositides but the importance of this muscarinic effect is still unknown. In order to find out about the role of the muscarinic receptors in the cochlea, we examined their functional distribution within this organ. This was achieved by measuring the formation of [3H]inositol phosphates induced by carbachol (1 mM) in two regions of the cochlea: the modiolus and the organ of Corti. At both sites, carbachol enhanced the accumulation of inositol phosphates in an atropine-sensitive way. These stimulations were completely antagonised by 4-diphenylacetoxy-N-methyl piperidine methiodide (1 microM) but unchanged by pirenzepine (1 microM). In cochleas depleted of outer hair cells by a treatment with amikacin, the carbachol-induced formation of inositol phosphates is not altered with respect to control, undamaged cochleas. Conversely, when the medial cholinergic axons which form synapses with the outer hair cells are destroyed by the section of the crossed olivocochlear bundle the carbachol-stimulated inositol phosphates response is reduced by 35% in the organ of Corti. This section has no effect in the modiolus, despite the degeneration of some modiolar fibers. Our results show that functional muscarinic receptors are distributed both in the organ of Corti and in the modiolus. These two structures contain presumably the same class of cholinoceptor. The effects of selective destruction clearly demonstrate that a population of muscarinic receptors is located on presynaptic membranes at the level of the medial axon-outer hair cell contacts. They also point to spiral ganglion neurons and/or the Schwann cells as sites for the functional cholinoceptors in the modiolus.

Ral and Rab3a are major GTP-binding proteins of axonal rapid transport and synaptic vesicles and do not redistribute following depolarization stimulated synaptosomal exocytosis

We have employed high-resolution SDS polyacrylamide gels to demonstrate that there are two major low-molecular-weight GTP-binding proteins associated with axonal membranes including synaptic vesicles, rapid transported membranes and clathrin-coated vesicles. We demonstrate that one of the major proteins is Ral and that the other is Rab3A. Following the depolarization of synaptosomes resulting in increased neurotransmitter release, we see no significant dissociation of either Ral or Rab3a from synaptic vesicle derived membranes in contrast to results reported previously.

Identification of G protein subtypes in peripheral nerve and cultured Schwann cells

In this study, we investigated the expression of various G proteins in whole sciatic nerves, in myelin and nonmyelin fractions from these nerves, and in membranes of immortalized Schwann cells. In myelin, nonmyelin, and Schwann cell membranes we detected two 39-40-kDa pertussis toxin substrates that were resolved on separation on urea-gradient gels. Two cholera toxin substrates with apparent molecular masses of 42 and 47 kDa were present in nerve and brain myelin and in Schwann cell membranes. In these membranes, a third 45-kDa cholera toxin substrate, which displayed the highest labeling, was also present. Immunoblotting with specific antisera allowed the identification of G(o) alpha, Gi1 alpha, Gi2 alpha, Gi3 alpha, Gq/G11 alpha, and the two isoforms of Gs alpha in nerve homogenates, nerve, and brain myelin fractions. In Schwann cell membranes we identified G(o) alpha, Gi2 alpha, Gi3 alpha, and proteins from the Gq family, but no immunoreactivity toward anti-Gi1 alpha antiserum was detected. In these membranes, anti-Gs alpha antibody recognized the three cholera toxin substrates mentioned above, with the 45-kDa band displaying the highest immunoreactivity. Relative to sciatic nerve myelin, the Schwann cell membranes revealed a significantly higher expression of Gi3 alpha and the absence of Gi1 alpha. The different distribution of G proteins among the different nerve compartments might reflect the very specialized function of Schwann cells and myelin within the nerve.

Localization of the ras-like rab3A protein in the adult rat brain

Rab3A is a small GTP-binding synaptic vesicle protein, shown to dissociate from synaptic vesicle membranes upon depolarization-induced exocytosis. Using an antiserum raised against rab3A, we found that the antigen was localized to the neuropil of specific brain regions, but was not present in major fiber tracts or most cell bodies. For example, the neuropil of several thalamic nuclei (i.e., dorsal lateral geniculate nucleus, lateral posterior nucleus, ventroposterior nucleus), cerebral cortex, upper layers of the superior colliculus and matrix zones of the neostriatum, were strongly immunoreactive, while the anterior commissure, corpus callosum, optic tract and internal capsule were devoid of staining. The hippocampus, regions of cerebral cortex and the cerebellum exhibited striking laminar distributions of rab3A immunoreactivity. In the hippocampus, dark staining was observed in the stratum oriens, stratum radiatum and molecular layer of the dentate gyrus, while the pyramidal, stratum lacunosum moleculare and dentate granule layers were not stained. In cerebellum the molecular layer and to a lesser extent, the underlying granule cell layer showed enhanced immunoreactivity. Seven days after excitotoxic lesions of the cerebral cortex, rab3A immunoreactivity was diminished in the mirror locus in the contralateral cortical hemisphere and in certain thalamic nuclei ipsilateral to the injection site. These results show that rab3A is localized to a number of specific regions. Its absence from other areas suggests that this synaptic vesicle protein is not universal to all neuronal terminals and pathways. In addition, our lesion studies indicate that for some brain regions, much of the antigen originates in cortical neurons and is distributed within specific axonal projections.

Synthetic peptides of the effector-binding domain of rab enhance secretion from digitonin-permeabilized chromaffin cells

There is evidence that the rab class of low molecular weight GTP-binding proteins is involved in vesicular transfer from endoplasmic reticulum to Golgi and between Golgi cisternae. To determine whether similar proteins play a role in regulated exocytosis, the effects of synthetic peptides derived from low molecular weight GTP-binding proteins on catecholamine secretion from digitonin-permeabilized chromaffin cells were investigated. The synthetic peptides represent the putative effector-binding domains of the rab, ras and ral classes of low molecular weight GTP-binding proteins and correspond to ras(33-48). Two rab peptides but neither a ras nor a ral peptide enhanced Ca(2+)-dependent secretion by approximately 30%. Maximal secretion in response to Ca2+ was increased. The enhancement was not blocked by the pseudosubstrate inhibitor of protein kinase C, PKC(19-31), thus indicating that activation of protein kinase C was not responsible for the enhancement of secretion. Similarly a rab peptide but neither a ras nor a ral peptide enhanced GppNHp-induced secretion 30-70%. The peptides had little or no effect in the absence of Ca2+ or GppNHp. The data are consistent with a protein of the rab class playing a role in regulated exocytosis.

Identification of some of the brain Gn27 as the ral gene product. Comparison between the brain and platelet Gn-proteins

Two major Gn-proteins, Gn27 and Gn26, were detected in the 100,000 x gav particulate fraction of rabbit and bovine brain. The Gn26 protein was also present in significant amounts (approximately 50% of total) in the brain supernatant fraction. An antiserum raised against recombinant simian ralA recognized a 27-kDa brain protein with the same apparent molecular mass as the Gn27 protein. In further analysis by two-dimensional polyacrylamide gel electrophoresis, the brain particulate Gn-proteins were resolved into 6 major forms, four of 27 kDa (Gn27a-d) and two of 26 kDa (Gn26a and Gn26b). Minor GTP-binding components were also observed at 25 kDa and 24 kDa. The ralA antibody reacted strongly with the brain Gn27b form and weakly with the Gn27a and Gn27c but not with Gn27d or any of the other Gn-proteins. In addition, comparison of human platelet and bovine brain particulate Gn-proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated a tissue/cell-type specific expression of the various forms of Gn-proteins.

Purification of squid synaptic vesicles and characterization of the vesicle-associated proteins synaptobrevin and Rab3A

Two proteins associated with mammalian synaptic vesicles, the integral membrane protein synaptobrevin and the GTP-binding protein rab3A, are identified in squid nervous tissue using Western blotting and subcellular fractionation of synaptosomes. They both copurify with synaptic vesicles prepared from squid optic lobe. Synaptobrevin (18.1 kDa) is present at high levels in synaptic terminals but at very low levels in axon. Rab3A (27.5 kDa) is a member of the rab family of low-molecular weight GTP-binding proteins which regulates vesicle traffic in secretory and endocytic processes. As resolved with 2-dimensional gels, squid neurons contain at least 16 GTP-binding species (19-29 kDa), and most of these are present in both soluble and particulate fractions. The 24 kDa class of GTP-binding proteins is highly enriched in axonal transport organelles. The characterization of synaptobrevin and rab3A in squid synaptic vesicles extends their known distributions to invertebrates and points to a fundamental importance of these proteins in neurotransmitter release.

Expression and localization of smg p25A (= rab3A) in cultured rat hippocampal cells

We have studied the expression of smg p25A and synaptophysin in cultured hippocampal neurons isolated from 5-day-old rat brain by an immunocytochemical technique. In a dispersed cell culture seeded on astrocyte monolayers, well-branching neurite proliferation was observed along with age in culture. The synaptophysin immunoreactivity was present in the neuronal cell bodies and neurites at 1 and 5 days in vitro (DIV) and was eventually localized to discrete areas along neurites at 15 DIV while the immunoreactivity in cell bodies became less prominent. On the other hand, the smg p25A immunoreactivity was observed in the neuronal cell bodies and neurites at 1 through 15 DIV. The immunoreactivity for smg p25A or synaptophysin was not observed in astrocytes and this finding was confirmed by an immunoblot analysis. These results indicate that smg p25A as well as synaptophysin is present exclusively in neurons and suggest that these two synapse-associated proteins have different sites of function and different kinetics of synthesis, transport, and/or turnover in cultured hippocampal neurons.

Brain cysteine proteinase inhibitors II: evidence that a 21-kDa papain-binding component resembles ras p21

A 21-kDa protein extracted from rat or bovine brain at high pH was purified on alkylated-papain and shown to have dual ras-like and cysteine proteinase inhibitory (CPI) properties. This was demonstrated by its GTP-binding activity, cross-reactivity toward pan-reactive ras p21 monoclonal antibody, and inhibition of papain. The material eluted earlier than cystatins or kininogens on the alkylated papain-affinity column and was devoid of other CPIs based on immunoblot analysis. In a second procedure, ras p21s isolated from rat or bovine brain membranes by cholate extraction and purified by gel-permeation and hydrophobic interaction were shown to act also as potent CPIs, inhibiting rat brain cathepsin L, papain, or rat brain cathepsin B with Ki values of 3, 11, and 167 nM, respectively. This component cross-reacted with the monospecific anti-ras, but not with other anti-CPIs, and represented 3-4% of total GTP binding present in homogenates. The specific activity of the purified 21 kDa component was 4.7 nmol GTP-gamma-S bound per mg protein. The data support the notion that brain ras p21s constitute a separate group of CPIs and are available for regulating some aspects of brain protein turnover.

Small GTP-binding proteins in human neuroblastoma cell lines

The presence of small G proteins was investigated by [gamma-35S]GTP-binding in 3 human neuroblastoma cell lines. IMR-32, SK-N-BE and SH-SY5Y, before and after treatment with differentiating agents (dibutyryl-cAMP, 5-bromodeoxyuridine or retinoic acid) which induce the appearance of secretory organelles. One major component of about 24 kDa and 3 minor components of smaller Mr were found to bind specifically [gamma-35S]GTP in all 3 cell lines already before differentiation. Differentiation did not affect the expression of small G proteins in IMR-32 cells and only modestly affected it in the other two cell lines. The possibility that the expression of small G proteins in neuroblastoma cells is not coupled with the assembly of secretory organelles is discussed.

Molecular cloning of a GTPase activating protein specific for the Krev-1 protein p21rap1

The rap1/Krev-1 gene encodes a ras-related protein that suppresses transformation by ras oncogenes. We have purified an 88 kd GTPase activating protein (GAP), specific for the rap1/Krev-1 gene product, from bovine brain. Based on partial amino acid sequences obtained from this protein, a 3.3 kb cDNA was isolated from a human brain library. Expression of the cDNA in insect Sf9 cells resulted in high level production of an 85-95 kd rap1GAP that specifically stimulated the GTPase activity of p21rap1. The complete deduced amino acid sequence is not homologous to any known protein sequences, including GAPs specific for p21ras. Northern and Western blotting analysis indicate that rap1GAP is not ubiquitously expressed and appears most abundant in fetal tissues and certain tumor cell lines, particularly the Wilms' kidney tumor, SK-NEP-1, and the melanoma, SK-MEL-3, cell lines.

GTP-binding proteins and potassium channels involved in synaptic plasticity and learning

Inhibition of potassium channels is possibly the first step in the sequence of biochemical events leading to memory formation. These channels appear to be regulated directly or indirectly by GTP-binding proteins (G proteins), which may themselves be affected by phosphorylation and dephosphorylation in response to elevated calcium levels or other phenomena resulting from the blockage of the potassium channels. A wide variety of cellular phenomena, from transcriptional changes to axonal transport, are thus capable of being initiated by these events.

Isolation and characterization of rapid transport vesicle subtypes from rabbit optic nerve

Subcellular fractionation of rabbit optic nerve resolves three populations of membranes that are rapidly labelled in the axon. The lightest membranes are greater than 200 nm and are relatively immobile. The intermediate density membranes consist of 84 nm vesicles which disappear from the nerve with kinetics identical to those of the rapid component. A third population of membranes, displaying a distinct protein profile, is present in the most dense region of the gradient. Immunological characterization of these membranes suggests the following. (1) The lightest peak contains rapidly transported glucose transporter and most of the total glucose transporters present in the nerve; this peak is therefore enriched in axolemma. (2) The intermediate peak contains rapidly transported glucose transporters and synaptophysin, an integral synaptic vesicle protein, and about half of the total synaptophysin; this peak therefore contains transport vesicles bound for both the axolemma and the nerve terminal, and these subpopulations can be separated by immunoadsorption with specific antibodies against the aforementioned proteins. (3) The heaviest peak contains rapidly transported synaptophysin and tachykinin neuromodulators and about half of the total synaptophysin, and 80% of the total tachykinins present in the nerve; this peak appears to represent a class of synaptic vesicle precursor bound for the nerve terminal exclusively. (4) Synaptophysin is present in the membranes of vesicles carrying tachykinins. (5) Both the intermediate and the heaviest peaks are enriched in kinesin heavy chain, suggesting that both vesicle classes may be transported by the same mechanism.

Low molecular weight GTP-binding proteins associated with zymogen granule membranes from rat pancreas

We report here that at least seven low Mr GTP-binding proteins (range 21.5 to 29 kDa) are associated with the membranes of zymogen granules from rat pancreas. GTP binding proteins of similar Mr but in different relative proportions were found in the cytosolic fraction. Treatment of intact granules with either trypsin or proteinase K caused the complete digestion of all the GTP-binding proteins, indicating that the proteins are located on the cytoplasmic face of the granule membrane. All the GTP-binding proteins were relatively resistant to extraction by 1.0M NaCl, 6.0M urea and 0.2M Na2CO3 (pH 11.0) but partitioned into the detergent phase of Triton X 114 extracts indicating that the proteins are tightly associated with the granule membrane. By analogy with the function of other small Mr GTP-binding proteins in regulation of membrane fusion events in eukaryotic cells, we suggest that these low Mr GTP-binding proteins in the pancreatic acinar cell may be involved in regulated secretion.

Clathrin-coated vesicles from human placenta contain GTP-binding proteins

Biochemical and morphological techniques were used to investigate the GTP-binding proteins of clathrin-coated vesicles. Binding of [3H]GTP to clathrin coats was demonstrated by electron microscopic autoradiography. Purified coated vesicles bound 5.2 pmol [35S]GTP gamma S/mg protein. Addition of GTP or GTP gamma S, but not ATP nor GMP, inhibited binding of [35S]GTP gamma S to intact coated vesicles and Triton X-100-extracted coats. A series of 23-24 kDa GTP-binding proteins with isoelectric points between pH 5-8 were detected in coated vesicles. We suggest that the low molecular weight ras-like GTP-binding protein(s) play a role in regulating vesicle-mediated protein transport or signal transduction within intracellular organelles.

Expression of smg p25A, a ras p21-like small GTP-binding protein, during postnatal development of rat cerebellum

Changes in expression and localization of smg p25A, a ras p21-like small GTP-binding protein, in developing rat brain were analyzed in comparison with those of synaptophysin, a well-known synaptic vesicle-specific protein. The smg 25A mRNA was detected in whole brain of rat fetus at 14 days of gestation and its level was increased along with the age and reached the maximum level at postnatal day (P) 20. In postnatal cerebellum, the smg25A mRNA level was also increased age-dependently and the maximum level was observed at P30. Immunoblot analysis with an anti-smg p25A monoclonal antibody (MAb SG-11-7) and an anti-synaptophysin monoclonal antibody (SY 38) showed that expression of both smg p25A and synaptophysin was increased age-dependently in postnatal rat cerebellum. By immunofluorescent cytochemical study with the anti-smg p25A antibody, bright fluorescence was observed in the molecular layer of cerebellum and it was increased in accordance with the cerebellar development. In early postnatal cerebellum, the perikarya of Purkinje cells and the white matter were brightly stained with the antibody, but the fluorescence of these portions was faint in adult cerebellum. The anti-synaptophysin monoclonal antibody also stained the molecular layer of cerebellum but the perikarya of Purkinje cells and the white matter had only a weak immunoreactivity with the antibody irrespective of the age. These results indicate that smg p25A is predominantly present in the nerve terminals and that its amount is increased along with the development of postnatal rat cerebellum. Our results also suggest that smg p25A and synaptophysin have the different kinetics of synthesis, transport, and/or turnover in developing rat cerebellum.

Isoprenylation of the low molecular mass GTP-binding proteins rac 1 and rac 2: possible role in membrane localization

Ras proteins can be modified at their COOH-terminal cysteine in the motif Cys-Ali-Ali-Xaa by a farnesyl isoprenoid. This modification is essential for membrane association and biological activity of ras proteins. A similar COOH-terminal amino acid sequence, Cys-Xaa-Ali-Xaa, exists in the ras-related GTP-binding proteins rac 1 and rac 2. To determine whether these proteins were similarly modified, COS cells were transfected with rac 1 and rac 2 cDNA and expressed proteins were labeled with [3H]mevalonic acid. We report here that both rac 1 and rac 2 are post-translationally modified by addition of an isoprenoid group, the likely site of which is the COOH-terminal cysteine. Isoprenylation was found only in racs associated with particulate cell fractions, suggesting that this modification may be associated with membrane localization of the proteins. These data specifically identify mammalian low molecular mass GTP-binding proteins other than ras that undergo post-translational modification and further define the COOH-terminal consensus sequence, Cys-Ali-Ali-Xaa, as an isoprenylation signal. This sequence may identify a larger family of low molecular mass GTP-binding proteins which are isoprenylated.

The cellular functions of small GTP-binding proteins

A substantial number of novel guanine nucleotide binding regulatory proteins have been identified over the last few years but the function of many of them is largely unknown. This article will discuss a particular family of these proteins, structurally related to the Ras oncoprotein. Approximately 30 Ras-related small guanosine triphosphate (GTP)-binding proteins are known, and from yeast to man they appear to be involved in controlling a diverse set of essential cellular functions including growth, differentiation, cytoskeletal organization, and intracellular vesicle transport and secretion.