Specific binding of [alpha-32P]GTP to cytosolic and membrane-bound proteins of human platelets correlates with the activation of phospholipase C

A mutation in Rab38 small GTPase causes abnormal lung surfactant homeostasis and aberrant alveolar structure in mice

The chocolate mutation, which is associated with oculocutaneous albinism in mice, has been attributed to a G146T transversion in the conserved GTP/GDP-interacting domain of Rab38, a small GTPase that regulates intracellular vesicular trafficking. Rab38 displays a unique tissue-specific expression pattern with highest levels present in the lung. The purpose of this study was to characterize the effects of Rab38-G146T on lung phenotype and to investigate the molecular basis of the mutant gene product (Rab38(cht) protein). Chocolate lungs exhibited a uniform enlargement of the distal airspaces with mild alveolar destruction as well as a slight increase in lung compliance. Alveolar type II cells were engorged with lamellar bodies of increased size and number. Hydrophobic surfactant constituents (ie, phosphatidylcholine and surfactant protein B) were increased in lung tissues but decreased in alveolar spaces, consistent with a malfunction in lamellar body secretion and the subsequent cellular accumulation of these organelles. In contrast to wild-type Rab38, native Rab38(cht) proteins were found to be hydrophilic and not bound to intracellular membranes. Unexpectedly, recombinant Rab38(cht) proteins retained GTP-binding activity but failed to undergo prenyl modification that is required for membrane-binding activity. These results suggest that the genetic abnormality of Rab38 affects multiple lysosome-related organelles, resulting in lung disease in addition to oculocutaneous albinism.

Analysis and expression of Rab38 in oculocutaneous lung disease

Rab38 is a low-molecular-weight G-protein highly expressed in melanocytes of the skin and alveolar type II cells in the lung. A point mutation in the postulated GTP/GDP-interacting domain of Rab38 has been identified as the genetic lesion responsible for oculocutaneous albinism (OCA) in chocolate (cht) mice. Another point mutation that prevents translation of Rab38 mRNA is the molecular basis of the Ruby gene mutation causing the phenotype of OCA and prolonged bleeding time in Fawn-Hooded and Tester-Moriyama rats. Cht mice show conspicuously enlarged lamellar bodies in alveolar type II cells and abnormal lung structure. Triton X-114 phase partitioning of cht mouse lung showed that Rab38cht-protein was recovered in the aqueous phase. We produced recombinant Rab38cht-protein using a baculovirus/insect cell-protein expression system. The results demonstrate that Rab38cht-protein is inactive due to reduced membrane binding and enhanced intracellular degradation. Rab38 is a new strong candidate gene for human Hermansky-Pudlak syndrome (HPS) that is characterized by OCA, bleeding diathesis, and lung disease.

Small GTP-binding proteins are associated with chitosomes and vesicles carrying glucose oxidase from Mucor circinelloides

Fractions enriched with chitosomes and vesicles carrying glucose oxidase (GOX) activity from the dimorphic zygomycete Mucor circinelloides were obtained using two successive sucrose gradients, the first a linear-log and the second an isopycnic gradient. Using an [alpha-(32)P]GTP-binding assay, we detected the association of small GTP-binding proteins (21 and 17 kDa) with both types of vesicles. In addition, by ADP-ribosylation with C3 exotoxin, and Western blot analysis with specific antibodies, we identified the small GTPases RhoA (Rho1p) and Rab8, and a 17 kDa protein, with pI values of 6.0, 6.1, and 6.2 and molecular masses of 21, 21 and 17 kDa, respectively, associated with those vesicles carrying GOX activity. Rab and Cdc42 proteins with pI values of 6.1 and 6.2 and molecular masses of 21 and 17 kDa, respectively, were found associated with chitosomes. These data indicate the presence in M. circinelloides of low molecular mass G-proteins in chitosomes and in vesicles carrying GOX activity. The difference in association of Rho1 and Cdc42, with vesicles carrying GOX activity and chitosomes, respectively, indicates that each of these proteins probably controls formation, transport and specific plasma membrane site docking of the respective vesicles.

Expression and characterization of Rab38, a new member of the Rab small G protein family

Rab38 is a new member of the Rab small G protein family that regulates intracellular vesicle trafficking. Rab38 is expressed in melanocytes and it has been clarified that a point mutation in the postulated GTP-binding domain of Rab38 is the gene responsible for oculocutaneous albinism in chocolate mice. However, basic information regarding recombinant protein production, intracellular location, and tissue-specific expression pattern has not yet been reported. We produced recombinant Rab38 using a baculovirus/insect cell-protein expression system. A combination of Triton X-114 phase separation and nickel-affinity chromatography yielded exclusively prenylated Rab38 that bound [alpha-32P]-GTP. The mRNA and the native protein were expressed in a tissue-specific manner, e.g., in the lung, skin, stomach, liver, and kidney. Freshly isolated rat alveolar type II cells were highly positive for the mRNA signal, but the signal was rapidly lost over time. Immunofluorescence staining demonstrated that expressed GST-tagged Rab38 was mainly co-localized with endoplasmic reticulum-resident protein and also partly with intermittent vesicles between the endoplasmic reticulum and the Golgi complex. These results indicate that Rab38 is expressed non-ubiquitously in specific tissues and regulates early vesicle transport relating to the endoplasmic reticulum, and hence suggest that Rab38 abnormality may cause multiple organ diseases as well as oculocutaneous albinism.

Toxoplasma gondii Rab6 mediates a retrograde pathway for sorting of constitutively secreted proteins to the Golgi complex

Toxoplasma gondii relies on protein secretion from specialized organelles for invasion of host cells and establishment of a parasitophorous vacuole. We identify T. gondii Rab6 as a regulator of protein transport between post-Golgi dense granule organelles and the Golgi. Toxoplasma Rab6 was localized to cisternal rims of the late Golgi and trans-Golgi network, associated transport vesicles, and microdomains of dense granule and endosomal membranes. Overexpression of wild-type Rab6 or GTP-activated Rab6(Q70L) rerouted soluble dense granule secretory proteins to the Golgi and endoplasmic reticulum and augmented the effect of brefeldin A on Golgi resorption to the endoplasmic reticulum. Parasites expressing a nucleotide-free (Rab6(N124I)) or a GDP-bound (Rab6(T25N)) mutant accumulated dense granule proteins in the Golgi and associated transport vesicles and displayed reduced secretion of GRA4 and a delay in glycosylation of GRA2. Activated Rab6 on Golgi membranes colocalized with centrin during mitosis, and parasite clones expressing Rab6 mutants displayed a partial shift in cytokinesis from endodyogeny (formation of two daughter cells) to endopolygeny (multiple daughter cells). We propose that Toxoplasma Rab6 regulates retrograde transport from post-Golgi secretory granules to the parasite Golgi.

Rab proteins mediate Golgi transport of caveola-internalized glycosphingolipids and correct lipid trafficking in Niemann-Pick C cells

We recently showed that human skin fibroblasts internalize fluorescent analogues of the glycosphingolipids lactosylceramide and globoside almost exclusively by a clathrin-independent mechanism involving caveolae. In contrast, a sphingomyelin analogue is internalized approximately equally via clathrin-dependent and caveolar routes. Here, we further characterized the caveolar pathway for glycosphingolipids, showing that Golgi targeting of sphingolipids internalized via caveolae required microtubules and phosphoinositol 3-kinases and was inhibited in cells expressing dominant-negative Rab7 and Rab9 constructs. In addition, overexpression of wild-type Rab7 or Rab9 (but not Rab11) in Niemann-Pick type C (NP-C) lipid storage disease fibroblasts resulted in correction of lipid trafficking defects, including restoration of Golgi targeting of fluorescent lactosylceramide and endogenous GM(1) ganglioside, and a dramatic reduction in intracellular cholesterol stores. Our results demonstrate a role for Rab7 and Rab9 in the Golgi targeting of glycosphingolipids and suggest a new therapeutic approach for restoring normal lipid trafficking in NP-C cells.

A magnetosome-specific GTPase from the magnetic bacterium Magnetospirillum magneticum AMB-1

Magnetic bacteria produce intracellular vesicles that envelope single domain magnetite crystals. Although many proteins are present in this intracellular vesicle membrane, five are specific to this membrane. A 16-kDa protein, designated Mms16, is the most abundant of the magnetosome-specific proteins, and to establish its function we cloned and sequenced its gene from Magnetospirillum magneticum AMB-1. This was achieved by determination of the N-terminal amino acid sequence of the protein following two dimensional polyacrylamide gel electrophoresis, and sequencing of the gene was performed by gene walking using anchored polymerase chain reaction. Mms16 contains a putative ATP/GTP binding motif (P-loop). Recombinant Mms16 with a hemagglutinin tag, was expressed in Escherichia coli and purified. Recombinant Mms16 protein could bind GTP and showed GTPase activity. GTP was the preferred substrate for Mms16-catalyzed nucleotide triphosphate hydrolysis. These results suggest that a novel protein specifically localized on the magnetic particle membrane, Mms16, is a GTPase. Mms16 protein showed similar characteristics to small GTPases involved in the formation of intracellular vesicles. Furthermore, addition of the GTPase inhibitor AlF(4)- also inhibited magnetic particle synthesis, suggesting that GTPase is required for magnetic particles synthesis.

Analysis of low-molecular-weight GTP-binding proteins in two functionally different intestinal epithelial cell lines

Low molecular weight GTP-binding proteins are molecular switches that are believed to play pivotal roles in cell growth, differentiation, cytoskeletal organization, and vesicular trafficking. In this study, for the first time, members of this family of proteins in two functionally different intestinal epithelial cell lines are identified and characterized. [alpha-32P]GTP blot overlay assays of cytosolic and membranous fractions revealed the presence of specific GTP-binding proteins in the range of 20-30 kDa (small GTPases) in both fractions, with considerably higher amounts in the membranous insoluble fraction. Analysis by two-dimensional electrophoresis, immunoprecipitation using monoclonal and sequence-specific polyclonal antibodies, and C3 exoenzyme-mediated ADP ribosylation demonstrated the presence of Ras, Rap, Rho, Rac, Rab, and several other small GTPases. The pattern of small GTP-binding proteins corresponded to the characteristics of the cell lines. Caco-2 cells showed a Rab5 protein that is known to be involved in endocytosis but was not found in T84 cells. On the contrary Rab3 has been shown to participate in secretory processes. It is highly expressed in T84 cells (sixfold compared to Caco-2 cells).

Cloning and direct G-protein regulation of phospholipase D from tobacco

Phospholipase D (PLD) and heterotrimeric G-proteins are involved in plant signal transduction pathways at the plasma membrane. There is evidence suggesting that PLD acts downstream from G-proteins, but a direct interaction of specific members has not been shown. In the present paper, a PLD cDNA clone was isolated from tobacco, expressed as a GST fusion in bacteria, and the recombinant protein was purified by glutathione affinity. Its enzymatic properties identified it as an alpha-type PLD. The alpha-subunit of a G-protein from tobacco was isolated in a similar way. Both proteins were functional in biochemical assays. When the G-protein was included in the PLD assay, a strong dosage-dependent inhibition of the PLD activity was observed. Different control proteins did not exhibit this inhibitory effect. When GST-NtGPalpha1 was activated by incubation with GTPgammaS the inhibitory activity was greatly reduced. These results provide a first indication for a direct regulation of PLDalpha by a heterotrimeric G-protein alpha-subunit in plants.

Accumulation of RhoA, RhoB, RhoG, and Rac1 in fibroblasts from Tangier disease subjects suggests a regulatory role of Rho family proteins in cholesterol efflux

Tangier disease (TD) is an inherited disorder of lipid metabolism characterized by very low high density lipoprotein (HDL) plasma levels, cellular cholesteryl ester accumulation and reduced cholesterol excretion in response to HDL apolipoproteins. Molecular defects in the ATP binding cassette transporter 1 (ABCA1) have recently been identified as the cause of TD. ABCA1 plays a key role in the translocation of cholesterol across the plasma membrane, and defective ABCA1 causes cholesterol storage in TD cells. However, the exact relationship of many of the biochemical and morphological abnormalities in TD to ABCA1 is unknown. Since small GTP-binding proteins are important regulators of many cellular functions, we characterized these proteins in normal and TD fibroblasts using the [alpha-32P]GTP overlay technique and Western blotting of SDS and isoelectric focusing gels. Our results indicate that GTP-binding proteins of the Rho family (RhoA, RhoB, RhoG, Rac-1) are enriched in fibroblasts from TD patients. The accumulation of small G proteins may have potential implications for the TD phenotype and the regulation of cholesterol excretion in TD cells.

Proteomics of rat liver Golgi complex: minor proteins are identified through sequential fractionation

The discovery of novel proteins resident to the Golgi complex will fuel our future studies of Golgi structure/function and provide justification for proteomic analysis of this organelle. Our approach to Golgi proteomics was to first isolate and characterize the intact organelle free of proteins in transit by use of tissue pretreated with cycloheximide. Then the stacked Golgi fraction was fractionated into biochemically defined subfractions: Triton X-114 insoluble, aqueous, and detergent phases. The aqueous and detergent phases were further fractionated by anion-exchange column chromatography. In addition, radiolabeled cytosol was incubated with stacked Golgi fractions containing proteins in transit, and the proteins bound to the Golgi stacks in an energy-dependent manner were characterized. All fractions were analyzed by two-dimensional (2-D) gel electrophoresis and identification numbers were given to 588 unique 2-D spots. Tandem mass spectrometry was used to analyze 93 of the most abundant 2-D spots taken from preparative Triton X-114 insoluble, aqueous and detergent phase 2-D gels. Fifty-one known and 22 unknown proteins were identified. This study represents the first installment in the mammalian Golgi proteome database. Our data suggest that cell fractionation followed by biochemical dissection of specific classes of molecules provides a significant advantage for the identification of low abundance proteins in organelles.

Rab3D, a small GTP-binding protein implicated in regulated secretion, is associated with the transcytotic pathway in rat hepatocytes

Rab3 isotypes are expressed in regulated secretory cells. Here, we report that rab3D is also expressed in rat hepatocytes, classic models for constitutive secretion. Using reverse transcriptase polymerase chain reaction (RT-PCR) with primers specific for rat rab3D, we amplified a 151 base pair rab3D fragment from total RNA extracted from primary cultures of rat hepatocytes. Immunoblot analysis using polyclonal antibodies to peptides representing the N- and C-terminal hypervariable regions of murine rab3D recognized a protein of approximately 25 kd in hepatocyte lysates, hepatic subcellular fractions, and tissue extracts. The distribution of rab3D was primarily cytosolic; however, only membrane-associated rab3D significantly bound guanosine triphosphate (GTP) in overlay assays. Several lines of investigation indicate that rab3D is associated with the transcytotic pathway. First, rab3D was enriched in a crude vesicle carrier fraction (CVCF), which includes transcytotic carriers. Vesicular compartments immunoisolated from the CVCF on magnetic beads coated with anti-rab3D antibody were enriched in the transcytosed form of the polymeric IgA receptor (pIgA-R), but lacked not only the pIgA-R precursor form associated with the secretory pathway, but also a Golgi marker protein. Second, indirect immunofluorescence on frozen liver sections and in polarized cultured hepatocytes localized rab3D-positive sites at or near the apical plasma membrane and to the pericanalicular cytoplasm. Finally, cholestasis induced by bile duct ligation (BDL), a manipulation known to slow transcytosis, caused rab3D to accumulate in the pericanalicular cytoplasm of cholestatic hepatocytes. Our results indicate that rab3D plays a role in the regulation of apically directed transcytosis in rat hepatocytes.

Analysis of recombinant human ADP-ribosylation factors by reversed-phase high-performance liquid chromatography and electrospray mass spectrometry

Two complementary approaches utilizing reverse-phase high-performance liquid chromatography and liquid chromatography/mass spectrometry were developed to analyze recombinantly produced Group I and Group II human ADP-ribosylation factors (ARFs). We observe that the NH2 termini of Group II ARFs (ARF4 and ARF5) are efficiently processed by removal of the initiating methionine. In contrast, the NH2 termini of Group I ARFs (ARF1 and ARF3), although fully deformylated, undergo only partial methionine cleavage. This result is unexpected as ARFs are canonical substrates for methionine processing in both bacterial and eukaryotic systems, but it may explain the difficulties encountered by many researchers attempting to produce myristoylated ARFs in Escherichia coli. Additionally, we observe that a significant fraction of purified ARF4 contains a modification which we demonstrate to be consistent with mono-glutathionation. Both methionine retention and glutathione modification may impact ARF function and the methods presented here should be employed to determine the quality of recombinant ARFs.

Translocation of Arf1 to the secretory granules in rat parotid acinar cells

We investigated the interaction of ADP-ribosylation factor (Arf) with the secretory granules in rat parotid acinar cells. The 20. 5-kDa small-molecular-mass GTP-binding protein in the cytosolic fraction of rat parotid acinar cells was identified as ADP-ribosylation factor1 by using a pan-Arf monoclonal antibody and isotype-specific polyclonal antibodies for Arf proteins 1, 3, 5, and 6. Incubation of the cytosolic fraction with isolated secretory granule membranes in the presence of GTPgammaS resulted in the translocation of Arf1 from the cytosolic fraction to the secretory granule membranes. The translocation was not observed in the presence of GDPbetaS in place of GTPgammaS, indicating that the process is GTP-dependent. The immunoelectron microscopy experiment confirmed Arf1 is translocated to the secretory granules. A prior treatment of the granule membranes with trypsin inhibited the translocation of Arf1 at 2 mM Mg2+, but had no effect in the absence of Mg2+ (condition of spontaneous conversion of Arf-GDP to Arf-GTP). Thus, the trypsin-sensitive nucleotide exchange activity for Arf1 is probably associated with the secretory granule membranes. These results demonstrate Arf1 translocates to the secretory granules in rat parotid acinar cells.

Stimulation of rat liver mitochondrial fusion by an outer membrane-derived aluminum fluoride-sensitive protein fraction

In normal livers, hepatocytes contain a large number of spheroidal mitochondria. Mitochondrial morphology changes drastically in liver disease, but the underlying fusion-fission mechanisms are not known. We detected GTP- and aluminum fluoride-dependent membrane fusion events between rat liver mitochondria. Separation of outer mitochondrial membrane-derived proteins led to a subfraction containing a 60-kDa protein band that is detected by specific antibodies directed to common amino acid sequences of the GTP-binding site or carboxyl-terminus of eukaryotic heterotrimeric G-protein alpha subunits. Addition of this subfraction and aluminum fluoride to permeabilized rat hepatocytes triggered a substantial morphological change in hepatic mitochondria, giving them the three-dimensional appearance of a tubulovesicular network. Comparative stereology using electron and confocal microscopy showed that these morphological changes represent true mitochondrial fusion. Addition of aluminum fluoride alone induces a more limited change in mitochondrial morphology, from spheroidal organelles to short rods. Mitochondria maintained their normal membrane potential and overall membrane ultrastructure after all these morphological changes. Our results reveal that mammalian liver mitochondria contain proteins that stimulate mitochondrial fusion and suggest that members of the GTPase superfamily control the normalcy of mitochondrial morphology, which is closely linked to physiological cellular energetics.

Identification of caveolin and caveolin-related proteins in the brain

Caveolae are 50-100 nm, nonclathrin-coated, flask-shaped plasma membrane microdomains that have been identified in most mammalian cell types, except lymphocytes and neurons. To date, multiple functions have been ascribed to caveolae, including the compartmentalization of lipid and protein components that function in transmembrane signaling events, biosynthetic transport functions, endocytosis, potocytosis, and transcytosis. Caveolin, a 21-24 kDa integral membrane protein, is the principal structural component of caveolae. We have initiated studies to examine the relationship of detergent-insoluble complexes identified in astrocytes to the caveolin-caveolae compartment detected in cells of peripheral tissues. Immunolocalization studies performed in astrocytes reveal caveolin immunoreactivity in regions that correlate well to the distribution of caveolae and caveolin determined in other cell types, and electron microscopic studies reveal multiple clusters of flask-shaped invaginations aligned along the plasma membrane. Immunoblot analyses demonstrate that detergent-insoluble complexes isolated from astrocytes are composed of caveolin-1alpha, an identification verified by Northern blot analyses and by the cloning of a cDNA using reverse transcriptase-PCR amplification from total astrocyte RNA. Using a full-length caveolin-1 probe, Northern blot analyses suggest that the expression of caveolin-1 may be regulated during brain development. Immunoblot analyses of detergent-insoluble complexes isolated from cerebral cortex and cerebellum identify two immunoreactive polypeptides with apparent molecular weight and isoelectric points appropriate for caveolin. The identification of caveolae microdomains and caveolin-1 in astrocytes and brain, as well as the apparent regulation of caveolin-1 expression during brain development, identifies a cell compartment not detected previously in brain.

An expression cloning method to identify monomeric GTP-binding proteins by GTP overlay

We have developed a method for identifying monomeric GTP-binding proteins that is based on probing plasmid expression libraries with [alpha-32P]GTP. The method involves the production of nitrocellulose replica filter lifts from a plasmid cDNA expression library and treatment of the filters with chloroform vapor to lyse the Escherichia coli and to denature and inactivate endogenous E. coli GTP-binding proteins, thus allowing the direct identification of cDNA clones which encode Ras-like small GTP-binding proteins by ligand blotting. Using this procedure we have cloned a series of small Ras-like GTP-binding proteins from human retina. The method relies on a functional test, ligand specificity of the expressed proteins, to identify candidate molecules. This results in the isolation of predominantly full-length cDNA clones without relying on DNA sequence similarity. Thus, this method may be particularly useful for the cloning of novel Ras-related GTP-binding proteins which share limited sequence similarity with previously identified members of the Ras superfamily.

Identification of caveolin and caveolin-related proteins in the brain

Caveolae are 50-100 nm, nonclathrin-coated, flask-shaped plasma membrane microdomains that have been identified in most mammalian cell types, except lymphocytes and neurons. To date, multiple functions have been ascribed to caveolae, including the compartmentalization of lipid and protein components that function in transmembrane signaling events, biosynthetic transport functions, endocytosis, potocytosis, and transcytosis. Caveolin, a 21-24 kDa integral membrane protein, is the principal structural component of caveolae. We have initiated studies to examine the relationship of detergent-insoluble complexes identified in astrocytes to the caveolin-caveolae compartment detected in cells of peripheral tissues. Immunolocalization studies performed in astrocytes reveal caveolin immunoreactivity in regions that correlate well to the distribution of caveolae and caveolin determined in other cell types, and electron microscopic studies reveal multiple clusters of flask-shaped invaginations aligned along the plasma membrane. Immunoblot analyses demonstrate that detergent-insoluble complexes isolated from astrocytes are composed of caveolin-1alpha, an identification verified by Northern blot analyses and by the cloning of a cDNA using reverse transcriptase-PCR amplification from total astrocyte RNA. Using a full-length caveolin-1 probe, Northern blot analyses suggest that the expression of caveolin-1 may be regulated during brain development. Immunoblot analyses of detergent-insoluble complexes isolated from cerebral cortex and cerebellum identify two immunoreactive polypeptides with apparent molecular weight and isoelectric points appropriate for caveolin. The identification of caveolae microdomains and caveolin-1 in astrocytes and brain, as well as the apparent regulation of caveolin-1 expression during brain development, identifies a cell compartment not detected previously in brain.

Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptors is mediated by post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. To dissect the sorting machinery that regulates budding, targeting, and fusion of rhodopsin carrier vesicles, their GTP-binding protein composition has been studied using multiple means including high-resolution two-dimensional gel electrophoresis and [32P]GTP overlays of renatured proteins. These studies indicate a succession on rhodopsin-bearing vesicles of rab6, rab11, rab3 and rab8, all members of the small GTP-binding protein family of the known regulators of membrane trafficking. In this review the role of rab proteins in post-Golgi trafficking of rhodopsin is discussed.

Small GTP-binding protein, Rab6, is associated with secretory granules in atrial myocytes

Rab proteins, a subfamily of small GTP-binding proteins, have been shown to play key roles in regulation of vesicular traffic in eukaryotic cells. In this study, we have intended to identify, the atrial granule-associated Rab proteins that seem to be required for formation or intracellular transport of the granules. Atrial granules contained at least four small GTP-binding proteins, and we have demonstrated by biochemical analysis that one of the small GTP-binding proteins associated with the atrial granules is a Rab6 protein (Rab6p). Rab6p was also detected in highly purified zymogen granules of pancreatic exocrine gland. Immunogold electron microscopy performed on ultrathin cryosections of rat auricle revealed that Rab6p was associated with the atrial granule membranes. Association of Rab6p with the atrial granule membranes was also confirmed by immunodiffusion electron microscopy in agarose-embedded atrial granules. These data indicate that Rab6p is associated with the atrial granules and that it might function in the intracellular traffic of the secretory granules in the atrial myocytes.

Characteristic epitope recognition pattern of autoantibodies against eukaryotic ribosomal protein L7 in systemic autoimmune diseases

OBJECTIVE

To define the epitope-recognition pattern and the fine specificity of the autoantibody response to protein L7 in patients with rheumatic diseases.

METHODS

The epitope-recognition pattern was studied by enzyme-linked immunosorbent assay utilizing overlapping fragments of L7. The fine specificity was examined by binding inhibition and isoelectric focusing.

RESULTS

We observed a disease-specific epitope-recognition pattern of anti-L7 autoantibodies. There was one immunodominant epitope that was recognized by all anti-L7-positive sera from patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and systemic sclerosis (SSc). Additional recognition of minor epitopes was observed; it arises by intramolecular epitope spreading and was correlated with disease activity in SLE patients. SSc patients differed from SLE and RA patients in that their sera did not recognize certain minor epitopes. The major epitope was recognized by high-affinity autoantibodies of limited heterogeneity. Minor epitopes were recognized by heterogeneous low-affinity autoantibodies.

CONCLUSION

The anti-L7 autoantibody response is oligoclonal. Additional B cell clones are activated by antigen during active phases of disease.

A low-molecular-mass GTP-binding protein in the cytosol of germinated wheat embryos

A low-molecular-mass protein able to bind GTP in both native and SDS-denaturating conditions was detected in the cytosol of embryos from wheat (Triticum aestivum L.) seeds germinated for 40 h. The protein fulfilled most of the distinguishing criteria common to eukaryotic small GTP-binding proteins. It retained the ability to bind GTP after SDS/PAGE and nitrocellulose blotting. The protein eluted from Sephadex G-200 gel filtration with a Ve/Vo value corresponding to a molecular mass of 18 kDa, whereas on SDS/PAGE the molecular mass was 20 kDa. The native protein, which showed an intrinsic GTPase activity highly sensitive to NaF, bound the guanine nucleotide with high specificity and with a relatively high affinity (Kd approximately 85 nM). The GTP-binding protein was not detectable in other subcellular fractions; in the microsomal fraction, two other peptides of low molecular mass (23.5 and 21.5 kDa) with GTP-binding activity were detected. These results indicate that in the cytosolic fraction of germinating wheat embryos there is a 20-kDa protein which is biochemically similar to the known small GTP-binding proteins that currently have been detected almost exclusively in the membrane fraction of plant material.

Synergistic induction of anchorage-independent growth of NIH3T3 mouse fibroblasts by cysteine proteinase inhibitors and a tumor promoter

We have previously reported that Ras protein is a potent cysteine proteinase inhibitor. In order to examine whether the cysteine proteinase-inhibitory activity of Ras is involved in carcinogenesis, the effects of the following probes were investigated. Cystatin alpha is a cysteine proteinase-specific inhibitor and has some amino acid sequence homology with Ras. Ras has a CAAX motif (C, cysteine; A, aliphatic amino acid; X, any amino acid) at the carboxyl terminus, which is indispensable for the biological activity. Thus, cystatin alpha carrying a CAAX motif (cystatin alpha-CVLS) was examined. A v-Ha-Ras deletion mutant, Ras delta 42-49, has undetectable GTP binding activity, yet it retains a similar protease inhibitory activity to that of wild-type v-Ras. These genes were inserted into a eukaryotic inducible expression vector and transfected into NIH3T3 cells. The expression was effectively induced by treatment with a glucocorticoid hormone, dexamethasone. The expression of cystatin alpha-CVLS or Ras delta 42-49 alone induced neither transformation nor morphological changes. However, when their expression was induced in the presence of a tumor-promoting phorbol ester, a remarkable increase in the anchorage-independent growth was observed in cystatin alpha-CVLS- and Ras delta 42-49-transfected clones. These results suggest that cysteine proteinase inhibitors and a tumor promoter synergistically transformed NIH3T3 cells. It is thus possible that the cysteine proteinase-inhibitory activity of Ras might play a key role in the early stage of carcinogenesis.

Identification of rab12 as a secretory granule-associated small GTP-binding protein in atrial myocytes

A subfamily of small GTP-binding proteins, rab, has been shown to be involved in regulation of vesicular traffic in eukaryotic cells. The goal of this study was to identify the rab proteins associated with atrial secretory granules. A [32P]GTP-overlay assay showed the presence of multiple small GTP-binding proteins on the atrial granules. By biochemical analysis, we have demonstrated that one of the small GTP-binding proteins associated with the atrial granules is a rab12 protein (rab12p), one of the rab proteins that are most closely related to a Sec4 protein of yeast. Association of rab12p with the atrial granules was confirmed by immunogold electron microscopy. Immunoprecipitation followed by immunoblot analysis with anti-rab12 antibody showed that in addition to atria, rab12p was expressed in multiple other organs and cell lines. These results suggest that rab12p may function in vesicular traffic in multiple diverse types of cells.

Sec6, Sec8, and Sec15 are components of a multisubunit complex which localizes to small bud tips in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the products of at least 14 genes are involved specifically in vesicular transport from the Golgi apparatus to the plasma membrane. Two of these genes, SEC8 and SEC15, encode components of a 1-2-million D multi-subunit complex that is found in the cytoplasm and associated with the plasma membrane. In this study, oligonucleotide-directed mutagenesis is used to alter the COOH-terminal portion of Sec8 with a 6-histidine tag, a 9E10 c-myc epitope, or both, to allow the isolation of the Sec8/15 complex from yeast lysates either by immobilized metal affinity chromatography or by immunoprecipitation. Sec6 cofractionates with Sec8/15 by immobilized metal affinity chromatography, gel filtration chromatography, and by sucrose velocity centrifugation. Sec6 and Sec15 coimmunoprecipitate from lysates with c-myc-tagged Sec8. These data indicate that the Sec8/15 complex contains Sec6 as a stable component. Additional proteins associated with Sec6/8/15 were identified by immunoprecipitations from radiolabeled lysates. The entire Sec6/8/15 complex contains at least eight polypeptides which range in molecular mass from 70 to 144 kD. Yeast strains containing temperature sensitive mutations in the SEC genes were also transformed with the SEC8-c-myc-6-histidine construct and analyzed by immunoprecipitation. The composition of the Sec6/8/15 complex is disrupted specifically in the sec3-2, sec5-24, and sec10-2 strain backgrounds. The c-myc-Sec8 protein is localized by immunofluorescence to small bud tips indicating that the Sec6/8/15 complex may function at sites of exocytosis.

Phenotypic instability of transgenic tobacco plants and their progenies expressing Arabidopsis thaliana small GTP-binding protein genes

Chimeric genes consisting of the cauliflower mosaic virus 35S promoter, a cDNA encoding a small GTP-binding protein from Arabidopsis thaliana (ara-2 or ara-4) and the terminator of the nopaline synthase gene were cloned into a binary vector. Tobacco leaf tissues were transformed with this plasmid via Agrobacterium-mediated transformation. Transgenic plants possessing either ara-2 or ara-4 occasionally showed morphological abnormalities in leaves and other organs. However, such alterations were not always associated with co-transferred characters, such as kanamycin tolerance, and they arose in no more than 10% of the transgenic plants. Such phenomena were also observed in the progenies of the primary transgenic plants. Despite such unusual inheritance of the phenotypic abnormalities, GTP-binding activity of the inserted ara gene products was detected in all plants tested.

rab8 in retinal photoreceptors may participate in rhodopsin transport and in rod outer segment disk morphogenesis

Small GTP-binding protein rab8 regulates transport from the TGN to the basolateral plasma membrane in epithelial cells and to the dendritic plasma membrane in cultured hippocampal neurons. In our approach to identify proteins involved in rhodopsin transport and sorting in retinal photoreceptors, we have found, using [32P]GTP overlays of 2D gel blots, that six small GTP-binding proteins are tightly bound to the post-Golgi membranes immunoisolated with a mAb to the cytoplasmic domain of frog rhodopsin. We report here that one of these proteins is rab8. About 50% of photoreceptor rab8 is membrane associated and approximately 13% is tightly bound to the post-Golgi vesicles. By confocal microscopy, antibody to rab8 specifically labels calycal processes and the actin bundles of the photoreceptor inner segment that extend inward to the junctional complexes that comprise the outer limiting membrane. Anti-rab8 shows a striking periodicity of high density labeling at 1 +/- 0.12 microns intervals along the actin bundles. Rhodopsin-bearing post-Golgi membranes cluster around the base of the cilium where rab8 and actin are also co-localized, as revealed by confocal microscopy of retinal sections double labeled with anti-rab8 and phalloidin. Microfilaments have been implicated in rod outer segment (ROS) disk morphogenesis. Our data suggest that rab6, which we have previously localized to the post-Golgi compartment, and rab8 associate with the post-Golgi membranes sequentially at different stages of transport. rab8 may mediate later steps that involve interaction of transport membranes with actin filaments and may participate in microfilament-dependent ROS disk morphogenesis.

Mapping of Ras-related GTP-binding proteins by GTP overlay following two-dimensional gel electrophoresis

For identification of Rab, Rac, Rho, Ral, Rap, and Arf proteins on two-dimensional polyacrylamide gels, we have expressed full-length cDNAs of members of these protein families with the T7 RNA polymerase-recombinant vaccinia virus expression system. Membrane preparations from cells expressing the cDNAs were subjected to high-resolution two-dimensional polyacrylamide gel electrophoresis followed by [alpha-32P]GTP ligand blotting. We have mapped 28 small GTP-binding proteins relative to their isoelectric points and according to their molecular weights and by immunoblotting with specific antibodies. Rab and Rho proteins could be specifically identified by extraction of streptolysin O-permeabilized Madin-Darby canine kidney (MDCK) cells with Rab- and Rho-GDP dissociation inhibitor. We applied the reference mapping to analyze the GTP-binding patterns of synaptosome fractions from rat brain. The purified synaptosomes exhibited specific enrichment of Rab3a, Rab5a, Ral, and several other GTPases. This approach and the map we have produced should provide a useful aid for the analysis of the expression and localization of members of all families of small GTP-binding proteins in various cell types and subcellular fractions.

Purification and characterization of smooth muscle cell caveolae

Plasmalemmal caveolae are a membrane specialization that mediates transcytosis across endothelial cells and the uptake of small molecules and ions by both epithelial and connective tissue cells. Recent findings suggest that caveolae may, in addition, be involved in signal transduction. To better understand the molecular composition of this membrane specialization, we have developed a biochemical method for purifying caveolae from chicken smooth muscle cells. Biochemical and morphological markers indicate that we can obtain approximately 1.5 mg of protein in the caveolae fraction from approximately 100 g of chicken gizzard. Gel electrophoresis shows that there are more than 30 proteins enriched in caveolae relative to the plasma membrane. Among these proteins are: caveolin, a structural molecule of the caveolae coat; multiple, glycosylphosphatidylinositol-anchored membrane proteins; both G alpha and G beta subunits of heterotrimeric GTP-binding protein; and the Ras-related GTP-binding protein, Rap1A/B. The method we have developed will facilitate future studies on the structure and function of caveolae.

In vitro mutation analysis of Arabidopsis thaliana small GTP-binding proteins and detection of GAP-like activities in plant cells

Previously, we have reported the molecular cloning of ara genes encoding a small GTP-binding protein from Arabidopsis thaliana. The criterion based on amino acid sequences suggest that such an ara gene family can be classified to be of the YPT/rab type. To examine the biochemical properties of ARA proteins, several deletions and point mutations were introduced into ara cDNAs. Mutant proteins were expressed in E. coli as GST-chimeric molecules and analyzed in terms of their GTP-binding or GTP-hydrolysing ability in vitro. The results indicate that four conserved amino acid sequence regions of ARA proteins are necessary for GTP-binding. A point mutation of Asn at position 72 for ARA-2, or 71 for ARA-4, to Ile decreased GTP-binding and a point mutation of Gln at position 126 for ARA-2, or 125 for ARA-4, to Leu suppressed GTP-hydrolysis activity. Furthermore, certain factors associated with the membrane fraction accelerated GTPase activities of ARA proteins, suggesting the presence of GTPase activating protein(s) (GAP(s)) in the vesicular transport system of higher plant cells.

Two-dimensional gel mapping of small GTPases reveals transformation-specific changes during oncogenesis

Epithelial cells transformed by oncogenes in vitro change their gene expression program, thereby losing features of cell polarity and cell adhesion. Using ras-transformed mammary epithelial cells, we have investigated the expression of other small GTP-binding proteins by high-resolution two-dimensional gel electrophoresis and direct GTP ligand binding after renaturing transfer onto nitrocellulose. Ras-transformed cells lost the expression of one epithelial-specific GTP-binding protein (21-22 kDa, pI 4.5-4.8) and instead expressed a fibroblast GTP-binding protein (21-22 kDa, pI 4.8-5.0).

Several small GTP-binding proteins are strongly down-regulated in simian virus 40 (SV40) transformed human keratinocytes and may be required for the maintenance of the normal phenotype

High resolution two-dimensional (2-D) gel electrophoresis in combination with the blot overlay nucleotide binding assay was used to reveal low molecular weight GTP-binding proteins expressed by primary cultured, normal human keratinocytes. Forty one small GTP-binding proteins (30 isoelectric focusing, IEF; and 11 nonequilibrium pH gradient electrophoresis, NEPHGE) ranging in molecular weights from 18,000 to 30,000 and isoelectric points from 4.4 to 8.0 were detected and mapped in the master human keratinocyte database. Four GTP-binding proteins were identified by 2-D gel immunoblotting and these correspond to rap1 and 2 and two forms of rab6. ras-Proteins are most likely present in the [alpha 32P]GTP 2-D gel blots but their levels may be too low to be detected by immunoblotting. Quantitative changes in the relative expression levels of [alpha 32P]GTP-binding proteins in normal proliferating and simian virus 40 (SV40) transformed human keratinocytes (K 14) were determined by scintillation counting of the radioactive spots excised from the nitrocellulose blots. The results showed that thirteen of these proteins were not expressed in transformed K14 keratinocytes, implying that they may play a role in the maintenance of the normal cell phenotype.

Cytosolic ARFs are required for vesicle formation but not for cell-free intra-Golgi transport: evidence for coated vesicle-independent transport

We investigated the role of ADP-ribosylation factors (ARFs) in Golgi function using biochemical and morphological cell-free assays. An ARF-free cytosol produced from soluble Chinese hamster ovary (CHO) extracts supports intra-Golgi transport by a mechanism that is biochemically indistinguishable from control transport reactions: ARF-free transport reactions are NSF-dependent, remain sensitive to the donor Golgi-specific inhibitor ilimaquinone, and exhibit kinetics that are identical to that of control reactions containing ARFs. In contrast, ARF-free cytosol does not support the formation of coated vesicles on Golgi cisternae. However, vesicle formation is reconstituted upon the addition of ARF1. These data suggest that neither soluble ARFs nor coated vesicle formation are essential for transport. We conclude that cell-free intra-Golgi transport proceeds via a coated vesicle-independent mechanism regardless of vesicle formation on Golgi cisternae.

Mapping small GTP-binding proteins on high-resolution two-dimensional gels by a combination of GTP binding and labeling with in situ periodate-oxidized GTP

We compared two approaches to identify and map small GTP-binding proteins in combination with high-resolution two-dimensional (2-D) gel electrophoresis. The first approach involved direct GTP ligand binding after a renaturing transfer onto nitrocellulose. In the second, affinity labeling with in situ periodate-oxidized GTP was used in permeabilized cells (Peter, M. E., She, J., Huber, L. A. and Terhorst, C. Anal. Biochem. 1993, 210, 77-85). Analysis by 2-D gel electrophoresis revealed a number of distinct intracellular small GTP-binding proteins in Madine-darby canine kidney strain II cells (MDCKII). Using specific antibodies the electrophoretic coordinates of rab4, rap1a/b, and rap2 were identified for native as well as for crosslinked GTPases. These methods allow the identification of small GTP-binding proteins in total cell lysates and purified subcellular fractions, providing excellent markers throughout the course of differentiation and development.

Correlated expression of the 97 kDa sarcoendoplasmic reticulum Ca(2+)-ATPase and Rap1B in platelets and various cell lines

Evidence has accumulated that cyclic AMP (cAMP)-induced phosphorylation of a Ras-related protein (Rap1) regulates platelet Ca2+ transport. As this transport was recently found to be controlled by two isoforms of sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA), the 100 kDa SERCA2b and the newly identified 97 kDa SERCA, we attempted to establish which isoform is involved in this regulation. For this purpose, we studied the expression and regulation of both the SERCA and Rap1 isoforms in platelets, haemopoietic cells and various cancer cell lines. SERCA2b was shown to be equally expressed in all the cell lines tested, as determined by detection of its phosphoenzyme formation and by Western blotting using an isoform-specific antibody. In contrast, the expression of the 97 kDa SERCA, studied by the same methods, varied from total absence in the cancer cells to high levels in the megakaryocytic cell lines. With regard to the potential regulatory Rap1 proteins, Western blotting showed different expression of total Rap1 isoforms among the cell lineages, thus ruling out any possible relationship between Rap1 and SERCA2b. However, the expression of Rap1 proteins correlated with that of the 97 kDa SERCA isoform. More refined analysis of the rap1A and rap1B isoforms by reverse transcription PCR and by determining cAMP-induced phosphorylation of Rap1B, i.e. its functional mechanism, confirmed the correlation between Rap1B and the 97 kDa SERCA expression. This relationship was also established by the concerted up-regulation of these two proteins demonstrated in the pathological model of platelets from hypertensive rats. It is concluded that the expressions of 97 KDa SERCA and Rap1B are related, suggesting that regulation of the platelet Ca(2+)-ATPase system by cAMP-induced phosphorylation of Rap1B specifically involves the 97 kDa SERCA.

Redistribution of a rab3-like GTP-binding protein from secretory granules to the Golgi complex in pancreatic acinar cells during regulated exocytosis

Regulated secretion from pancreatic acinar cells occurs by exocytosis of zymogen granules (ZG) at the apical plasmalemma. ZGs originate from the TGN and undergo prolonged maturation and condensation. After exocytosis, the zymogen granule membrane (ZGM) is retrieved from the plasma membrane and ultimately reaches the TGN. In this study, we analyzed the fate of a low M(r) GTP-binding protein during induced exocytosis and membrane retrieval using immunoblots as well as light and electron microscopic immunocytochemistry. This 27-kD protein, identified by a monoclonal antibody that recognizes rab3A and B, may be a novel rab3 isoform. In resting acinar cells, the rab3-like protein was detected primarily on the cytoplasmic face of ZGs, with little labeling of the Golgi complex and no significant labeling of the apical plasmalemma or any other intracellular membranes. Stimulation of pancreatic lobules in vitro by carbamylcholine for 15 min, resulted in massive exocytosis that led to a near doubling of the area of the apical plasma membrane. However, no relocation of the rab3-like protein to the apical plasmalemma was seen. After 3 h of induced exocytosis, during which time approximately 90% of the ZGs is released, the rab3-like protein appeared to translocate to small vesicles and newly forming secretory granules in the TGN. No significant increase of the rab3-like protein was found in the cytosolic fraction at any time during stimulation. Since the protein is not detected on the apical plasmalemma after stimulation, we conclude that recycling may involve a membrane dissociation-association cycle that accompanies regulated exocytosis.

Identification of a dominant epitope of glutamic acid decarboxylase (GAD-65) recognized by autoantibodies in stiff-man syndrome

Glutamic acid decarboxylase (GAD) is the enzyme that synthesizes the neurotransmitter gamma-aminobutyric acid (GABA) in neurons and in pancreatic beta cells. It is a major target of autoimmunity in Stiff-Man syndrome (SMS), a rare neurological disease, and in insulin-dependent diabetes mellitus. The two GAD isoforms, GAD-65 and GAD-67, are the products of two different genes. GAD-67 and GAD-65 are very similar to each other in amino acid sequence and differ substantially only at their NH2-terminal region. We have investigated the reactivity of autoantibodies of 30 Stiff-Man syndrome patients to GAD. All patient sera contained antibodies that recognize strongly GAD-65, but also GAD-67, when tested by immunoprecipitation on brain extracts and by immunoprecipitation or immunocytochemistry on cells transfected with either the GAD-65 or the GAD-67 gene. When tested by Western blotting, all patient sera selectively recognized GAD-65. Western blot analysis of deletion mutants of GAD-65 demonstrated that autoantibodies are directed predominantly against two regions of the GAD-65 molecule. All SMS sera strongly recognized a fragment contained between amino acid 475 and the COOH terminus (amino acid 585). Within this region, amino acids 475-484 and 571-585 were required for reactivity. The requirement of these two discontinuous segments implies that the epitope is influenced by conformation. This reactivity is similar to that displayed by the monoclonal antibody GAD 6, suggesting the presence of a single immunodominant epitope (SMS-E1) in this region of GAD-65. In addition, most SMS sera recognized at least one epitope (SMS-E2) in the NH2-terminal domain of GAD-65 (amino acids 1-95). The demonstration in SMS patients of a strikingly homogeneous humoral autoimmune response against GAD and the identification of dominant autoreactive target regions may help to elucidate the molecular mechanisms of GAD processing and presentation involved in GAD autoimmunity. Moreover, the reactivity reported here of GAD autoantibodies in SMS partially differs from the reactivity of GAD autoantibodies in insulin-dependent diabetes mellitus, suggesting a link between the pattern of humoral autoimmunity and the clinical condition.

Rab6 is associated with a compartment that transports rhodopsin from the trans-Golgi to the site of rod outer segment disk formation in frog retinal photoreceptors

The biogenesis of light sensitive membranes in retinal rod photoreceptors involves polarized sorting and targeting of newly synthesized rhodopsin to a specialized domain, the rod outer segment (ROS). We have isolated and characterized the population of post-Golgi membranes that mediate intracellular transport of rhodopsin. In the present study we have examined the association of small (20-25 kDa) GTP-binding (G) proteins with these membranes. We found that one of the small G proteins, rab6, behaves like an integral membrane protein of the post-Golgi vesicles, although approximately 30% of rab6 is soluble. The distribution of the membrane-associated and the soluble forms is highly polarized. By confocal and EM immunocytochemistry it can be seen that most of rab6 is associated with the photoreceptor trans-Golgi cisternae, trans-Golgi network (TGN) and post-Golgi vesicles. The photoreceptor axon and synaptic terminal are unlabeled, but dendrites of deeper retinal layers are labeled. The distribution of rab6 across sucrose density gradient fractions parallels the distribution of sialyltransferase (a TGN marker) activity. About 9% of membrane-bound rab6 is associated, however, with the rhodopsin-bearing sialyltransferase-free post-Golgi vesicles, which represent a very small fraction (< 1%) of the total retinal membranes. Rab6 is absent from the mature ROS disk membranes but it is present at the sites of new ROS disk formation and in the ROS cytoplasm. This suggests that rab6 becomes soluble upon disk membrane formation. Therefore, rab6 may function not only as a component of the sorting machinery of photoreceptors that delivers rhodopsin to its appropriate subcellular domain but may also participate in some aspects of ROS disk morphogenesis.

Rab8, a small GTPase involved in vesicular traffic between the TGN and the basolateral plasma membrane

Small GTP-binding proteins of the rab family have been implicated as regulators of membrane traffic along the biosynthetic and endocytic pathways in eukaryotic cells. We have investigated the localization and function of rab8, closely related to the yeast YPT1/SEC4 gene products. Confocal immunofluorescence microscopy and immunoelectron microscopy on filter-grown MDCK cells demonstrated that, rab8 was localized to the Golgi region, vesicular structures, and to the basolateral plasma membrane. Two-dimensional gel electrophoresis showed that rab8p was highly enriched in immuno-isolated basolateral vesicles carrying vesicular stomatitis virus-glycoprotein (VSV-G) but was absent from vesicles transporting the hemagglutinin protein (HA) of influenza virus to the apical cell surface. Using a cytosol dependent in vitro transport assay in permeabilized MDCK cells we studied the functional role of rab8 in biosynthetic membrane traffic. Transport of VSV-G from the TGN to the basolateral plasma membrane was found to be significantly inhibited by a peptide derived from the hypervariable COOH-terminal region of rab8, while transport of the influenza HA from the TGN to the apical surface and ER to Golgi transport were unaffected. We conclude that rab8 plays a role in membrane traffic from the TGN to the basolateral plasma membrane in MDCK cells.