Post-translational modification of low molecular mass GTP-binding proteins by isoprenoid

Several proteins in mammalian cells are modified post-translationally by the isoprenoid, farnesol. Incubation of cultured cells with [3H]mevalonate, an isoprenoid precursor, results in the labeling of multiple polypeptides, the most prominent of which migrate in the range of 21-26 kDa on sodium dodecyl sulfate-polyacrylamide gels. In Rat-6 fibroblasts transformed by H-ras, one of the farnesylated proteins was identified as p21ras by two-dimensional immunoblotting. However, this protein accounted for only a small proportion of the [3H]mevalonate-derived radioactivity incorporated into 21-26-kDa proteins. Murine erythroleukemia cells, which did not express immunodetectable quantities of p21ras, contained several 21-26-kDa farnesylated proteins distributed in both the cytosolic and particulate fractions. At least eight of these proteins were capable of binding [alpha-32P]GTP on nitrocellulose membranes. Pulse-chase studies showed that the isoprenoid modification did not necessarily result in the translocation of the cytosolic proteins to the cell membrane. A prominent group of carboxyl-methylated proteins in murine erythroleukemia cells overlapped with the 21-26-kDa farnesylated proteins on one-dimensional sodium dodecyl sulfate gels. Methylation of this group of proteins was selectively abolished when cells were treated with lovastatin, an inhibitor of isoprenoid synthesis. Addition of exogenous mevalonate to the lovastatin-treated cells fully restored carboxyl methylation. These studies suggest that the 21-26-kDa farnesylated proteins in mammalian cells are members of a recently discovered family of low molecular mass GTP-binding proteins which, although ras-related, appear to be distinct structurally and possibly functionally from the products of the ras genes. The observed isoprenoid-dependent carboxyl methylation of a group of 21-26-kDa proteins suggests that the low molecular mass GTP-binding proteins may undergo a series of post-translational C-terminal cysteine modifications (i.e. farnesylation, carboxyl methylation) analogous to those recently elucidated for p21ras.     

Detection of low molecular mass GTP-binding proteins in chromaffin granules and other subcellular fractions of chromaffin cells

A homogenate of purified chromaffin cells was fractionated, after removal of the nuclear fraction, by sucrose density gradient ultracentrifugation. The presence and subcellular localization of low molecular mass GTP-binding proteins was explored by incubation of blots of proteins from different subcellular fractions with [alpha-32P]GTP in the presence of Mg2+. The fractions enriched in intact chromaffin granule markers, i.e. catecholamines, chromogranin A, chromogranin B and cytochrome b-561 were also enriched in labelled GTP-binding proteins. Two major labelled components of 23 and 29 kDa were rapidly detected by autoradiography. Traces of 26 and 27 kDa components were also present. These components were detectable in both plasma and granule membranes. In addition to these components, the cytosolic fraction contained another GTP-binding protein of about 20 kDa. Binding of [alpha-32P]GTP was specific and dependent on Mg2+. By analogy to the findings reported in non-mammalian systems, the observations described here suggest the involvement of low molecular mass GTP-binding proteins in the chromaffin cell secretory process.     

Low molecular mass GTP-binding proteins of adrenal chromaffin cells are present on the secretory granule

Adrenal medullary homogenates and chromaffin granule membranes were separated by SDS-polyacrylamide gel electrophoresis and GTP-binding proteins detected using [alpha-32P]GTP binding to nitrocellulose blots. Four GTP-binding polypeptides of 24, 22, 20 and 18 kDa were routinely found in medullary homogenates and all were also found in isolated chromaffin granule membranes. The GTP-binding polypeptides co-sedimented with granule membrane markers following separation on sucrose gradients. On the basis of trypsin sensitivity and resistance to extraction, the GTP-binding proteins appeared to be tightly bound to the cytoplasmic surface of the granules. One or more of the secretory granule GTP-binding proteins could be involved in exocytosis in adrenal chromaffin cells.     

Interaction of mastoparan with the low molecular mass GTP-binding proteins rho/rac

Mastoparan, which has been shown to active G proteins, inhibits the ADP-ribosylation of 20 kDa human platelet membrane proteins catalyzed by Clostridium botulinum exoenzyme C3 half-maximally and maximally (90%) at 20 and 100 microM concentrations, respectively. Inhibition of ADP-ribosylation was enhanced by GTP-gamma S. Mastoparan increased GTP hydrolysis by porcine brain rho protein and stimulated GTP binding in a concentration dependent manner. The data suggest that mastoparan not only interacts with heterotrimeric G proteins but also with low molecular mass GTP-binding proteins of the rho/rac family.     

Low molecular weight GTP-binding proteins in human neutrophil granule membranes

Degranulation of neutrophils involves the differential regulation of the exocytosis of at least two populations of granules. Low molecular weight GTP-binding proteins (LMW-GBPs) have been implicated in the regulation of vesicular traffic in the secretory pathways of several types of cells. In the present study we identify distinct subsets of LMW-GBPs associated with the membranes of neutrophil-specific and azurophilic granules. Ninety-four percent of total [35S]guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) binding activity was equally distributed between the plasma membrane and cytosol with the remaining 6% localized in the granules. In contrast, the cytosol contained only 10% of the total GTPase activity while the specific granules accounted for 13%. [alpha-32P]GTP binding to proteins transferred to nitrocellulose revealed LMW-GBPs in all fractions except the azurophilic granules. The specific granules contained three out of four bands which were found in the plasma membrane; these ranged from 20 to 23 kDa and all were resistant to alkaline extraction. Photoaffinity labeling with [alpha-32P]8-azido-GTP in the presence of micromolar Al3+ identified proteins of 25 and 26 kDa unique to azurophilic granules; these could not be labeled with [alpha-32P]8-azido-ATP and could be extracted by acidic but not alkaline pH. Botulinum C3-mediated [32P]ADP-ribosylation identified proteins of 16, 20, and 24 kDa both in plasma membranes and those of specific granules. An anti-ras monoclonal antibody, 142-24E5, recognized a 20-kDa protein localized to the plasma and specific granule membranes which could not be extracted by alkaline pH, was not a substrate for botulinum C3 ADP-ribosyltransferase, and was translocated from specific granules to plasma membrane after exposure of neutrophils to phorbol myristate acetate. We conclude that neutrophil-specific and azurophilic granules contain distinct subsets of LMW-GBPs which are uniquely situated to regulate the differential exocytosis of these two compartments.     

Compartmentalization of secretory proteins in pancreatic zymogen granules as revealed by immunolabeling on cryo-fixed and molecular distillation processed tissue*

Summary— Immunogold labeling of amylase obtained over zymogen granules of rat pancreatic acinar cells processed through cryofixation, molecular distillation drying and embedding in resins was found to be of high intensity and displayed a particular pattern. Indeed, it was concentrated in certain areas of the granules leaving others devoid of gold particles. This pattern of labeling reflects a strong compartmentalization of the secretory proteins within each granule. In order to assess this phenomenon, we have compared the intensities and the pattern of distribution of the labelings in tissues processed through: chemical fixation with embedding in various resins, cryo-ultramicrotomy and cryo-fixation followed by molecular distillation drying. Serials sections and double labeling experiments were performed for further evaluation of the results and for assessing artefactual displacement of proteins during tissue preparation. The results obtained indicate that the secretory proteins are indeed segregated within the granule which appears thus as a well organized structure. Cryo-fixation combined with molecular distillation appears thus to be superior in terms of preservation of protein antigenicity and retention of cellular components close to their living state.     

Identification of the ral and rac1 gene products, low molecular mass GTP-binding proteins from human platelets

Identification of the GTP-binding proteins from human platelet particulate fractions was attained by their purification via successive column chromatography steps followed by amino acid sequencing. To enhance the likelihood of identifying the GTP-binding proteins, two assays were employed to monitor GTP-binding activities: (i) guanosine 5'-(3-O-[35S]thio)triphosphate (GTP gamma S)-binding followed by rapid filtration and ii) [alpha-32P]GTP-binding following sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting onto nitrocellulose membranes. The latter assay permitted the isolation of a 28-kDa GTP-binding protein that bound [alpha-32P]GTP prominently but was only poorly detected with the GTP gamma S-binding assay. The amino acid sequences of three peptide fragments derived from the 28-kDa protein were identical to regions of the amino acid sequence deduced from a simian ral cDNA with the exception of one conservative substitution (Asp147----Glu). A full length human ral cDNA was isolated from a placental cDNA library, and its deduced amino acid sequence, compared with simian ral, also contained the Asp----Glu substitution along with two other substitutions and an additional three NH2-terminal amino acids. In addition to the 28-kDa protein, two distinct 25-kDa GTP-binding proteins were purified from platelets. One of these proteins has been previously characterized as G25K, an abundant low molecular mass GTP-binding protein. Partial amino acid sequence obtained from the second unidentified 25-kDa protein indicates that it is the product of the rac1 gene; a member of a newly identified gene family which encode for low molecular mass GTP-binding proteins (Didsbury, J., Weber, R.F., Bokoch, G. M., Evans, T., and Snyderman, R. (1989) J. Biol. Chem. 264, 16378-16382). These results identify two new GTP-binding proteins in human platelets, ral, the major protein that binds [alpha-32P]GTP on nitrocellulose transfers, and rac1, a substrate for botulinum C3 ADP-ribosyltransferase.     

Development-dependent expression of low molecular weight GTP-binding proteins in embryonic chicken brain

Expression of low molecular weight GTP-binding proteins in particulate and soluble fractions of embryonic chicken brain was analysed by SDS-PAGE and incubation of blotted proteins with [alpha-32P]GTP. At least seven GTP-binding proteins with apparent molecular weights between 21 and 29 kDa were demonstrated by this technique in membranes and microsomal fractions, whereas only four species were present in the cytosol. Levels of several small GTP-binding proteins were developmentally regulated in membrane and microsomal fractions, but not in the cytosol of embryonic chicken brain. Major GTP-binding proteins G28 and G26 were strongly increased in microsomal but not in membrane fractions between E6 and hatched chicken brain, whereas the minor protein G24 decreased in both membrane and microsomal fractions over this time. The differential expression of low molecular weight GTP-binding proteins in embryonic chicken brain suggests important roles for these proteins in brain development.     

Botulinum ADP-ribosyltransferase C3 but not botulinum neurotoxins C1 and D ADP-ribosylates low molecular mass GTP-binding proteins

Botulinum ADP-ribosyltransferase C3 modified 21-24 kDa proteins in a guanine nucleotide-dependent manner similar to that described for botulinum neurotoxin C1 and D. Whereas GTP and GTP gamma S stimulated C3-catalyzed ADP-ribosylation in the absence of Mg2+, in the presence of added Mg2+ ADP-ribosylation was impaired by GTP gamma S. C3 was about 1000-fold more potent than botulinum C1 neurotoxin in ADP-ribosylation of the 21-24 kDa protein(s) in human platelet membranes. Antibodies raised against C3 blocked ADP-ribosylation of the 21-24 kDa protein by C3 and neurotoxin C1 but neither cross reacted with neurotoxin C1 immunoblots nor neutralized the toxicity of neurotoxin C1 in mice. The data indicate that the ADP-ribosylation of low molecular mass GTP-binding proteins in various eukaryotic cells is not caused by botulinum neurotoxins but is due to the action of botulinum ADP-ribosyltransferase C3. The weak enzymatic activities described for botulinum neurotoxins appear to be due to the contamination of C1 and D preparations with ADP-ribosyltransferase C3.     

Gn-proteins are distinct from ras p21 and other known low molecular mass GTP-binding proteins in the platelet

The 27 kDa platelet membrane protein (Gn27) that binds [alpha-32P]GTP on nitrocellulose blots of SDS-polyacrylamide gels [(1987) Biochem. J. 245, 617-620] was compared with other low molecular mass GTP-binding proteins. Platelet membranes also contained 21 kDa proteins that bound anti-ras p21 antibody and 22-23 kDa proteins that could be ADP-ribosylated by botulinum neurotoxin type D. These groups of proteins were resolved electrophoretically from each other and from Gn27. A low molecular mass GTP-binding protein from bovine brain [(1987) Biochem. J. 246, 431-439] was also resolved from Gn27. At the levels normally present in cell membranes, only Gn-proteins bound significant amounts of [32P]GTP after transfer of protein from SDS-polyacrylamide gels to nitrocellulose.     

Small GTP-binding proteins associated with secretory vesicles of Paramecium

GTP-binding proteins act as molecular switches in a variety of membrane-associated processes, including secretion. One group of GTP-binding proteins, 20-30 kDa, is related to the product of the ras proto-oncogene. In Saccharomyces cerevisiae, ras-like GTP-binding proteins regulate vesicular traffic in secretion. The ciliate protist Paramecium tetraurelia contains secretory vesicles (trichocysts) whose protein contents are released by regulated exocytosis. Using [alpha-32P]GTP and an on-blot assay for GTP-binding, we detected at least seven GTP-binding proteins of low molecular mass (22-31 kDa) in extracts of Paramecium tetraurelia. Subcellular fractions contained characteristic subsets of these seven; cilia were enriched for the smallest (22 kDa). The pattern of GTP-binding proteins was altered in two mutants defective in the formation or discharge of trichocysts. Trichocysts isolated with their surrounding membranes intact contained two minor GTP-binding proteins (23.5 and 29 kDa) and one major GTP-binding protein (23 kDa) that were absent from demembranated trichocysts. This differential localization of GTP-binding proteins suggests functional specialization of specific GTP-binding proteins in ciliary motility and exocytosis.     

Identification of novel GTP-binding proteins in the human neutrophil

We describe here the existence of previously undescribed GTP-binding proteins within the human neutrophil. These proteins specifically bind guanine nucleotides under conditions in which the previously characterized G-proteins are unable to bind. We have partially purified these proteins and present both functional and immunologic data which indicate that they are unrelated to Gn, the major neutrophil pertussis toxin substrate. An additional protein, of apparent molecular mass 22 kDa, may be related to the ras G-protein family. Analysis of the structural and functional characteristics of these novel proteins will promote a better understanding of the process of neutrophil activation.     

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.     

Phospholipase C activity in NCB-20 cells is inhibited by protein kinase A-mediated phosphorylation of low molecular mass GTP-binding proteins.

We have previously shown that bradykinin-induced production of second messengers such as inositol trisphosphate and diacylglycerol in neurotumor cells is inhibited by raising cellular cyclic AMP levels, which in turn inhibit phospholipase C. A monoclonal antibody to phospholipase C-II immunoprecipitated the 140-kDa form of phospholipase C-II from [35S]methionine/[3H]eucine-labeled cells, but not [32P]orthophosphate-labeled phospholipase C-II, following treatment with either forskolin or dibutyryl cyclic AMP. This suggested that phospholipase C is not the target for cyclic AMP-dependent protein kinase-mediated phosphorylation. In vitro studies confirmed that phospholipase C activity was inhibited by raising cellular cAMP levels, and partial sensitivity to Bordetella pertussis toxin suggested the involvement of a GTP-binding protein which could be the target for protein kinase A. The involvement of a GTP-binding protein in coupling the bradykinin receptor to phospholipase C was further suggested by the ability of both guanosine 5'-O-(thio-triphosphate) and fluoride (NaF) to release inositol phosphates from NCB-20 cell membranes previously labeled with [3H]inositol. Both effects were blocked by pretreatment of the cells with protein kinase A activators, further suggesting a GTP-binding protein as the target for protein kinase A-mediated phosphorylation. When whole NCB-20 cell extracts were blotted onto nitrocellulose and incubated with [alpha- 32P]GTP, a major 24-kDa band plus minor bands at 22 and 20 kDa were revealed by autoradiography. A pH 3.0/6.0 soluble (basic protein) NCB-20 cell extract revealed the major 24-kDa band plus the 20-kDa band, and similar basic proteins were shown to be heavily phosphorylated following [32P]orthophosphate labeling and pretreatment with forskolin. The size and ability to bind GTP on Western blots are characteristic of the ras, rho, smg, etc. family of GTP-binding proteins recently suggested to be the much sought after GPLC (Lapetina, E.G., Lacal, J. C., Reep, B. R., and Molina y Vedia, L. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 3131-3134; Wang, P., Nishihata, J., Takabori, E., Yamamoto, K., Toyoshima, S., and Osawa, T. (1989) J. Biochem. (Tokyo) 105, 461-466; Nagata, K.-I., Nagao, S., and Nozawa, Y. (1989) Biochem. Biophys. Res. Commun. 160, 235-242). We propose that GPLC is uniquely sensitive to protein kinase A-mediated phosphorylation and that phosphorylation inhibits stimulus-secretion coupling in these cells.