Biogenesis of plasma membrane glycoproteins. Tracer kinetic study of two rat liver plasma membrane glycoproteins in vivo

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice

Background

We recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.

Methods

LPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.

Results

In both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.

Conclusions

The results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

The role of protein synthesis and degradation in the post-transcriptional regulation of rat multidrug resistance-associated protein 2 (Mrp2, Abcc2)

Multidrug resistance-associated protein 2 (Mrp2, Abcc2), an organic anion transporter present in the apical membrane of hepatocytes, renal epithelial cells, and enterocytes, is postulated to undergo post-transcriptional regulation. We hypothesized that Mrp2 protein undergoes altered rates of protein synthesis or degradation consistent with different Mrp2 protein expression. We analyzed Mrp2 synthesis, expression, and degradation in control female, 19- and 20-day pregnant, and pregnenolone-16alpha-carbonitrile (PCN)-treated rats using in vivo metabolic-labeling studies with [35S]cysteine/methionine or [14C]NaHCO3, polysomal distribution analyses and ribonuclease protection assays (RPA). Mrp2 protein was significantly increased in rats treated with PCN for 2 days but significantly decreased in 19-day pregnant rats relative to controls; no significant differences were observed in Mrp2 mRNA expression among these groups. The measured half-lives of 14C-labeled Mrp2 in control, pregnant, and PCN-treated rats were 27, 36, and 22 h, respectively, and were not significantly different. The rate of incorporation of 35S into Mrp2 was highest in PCN-treated rats. Polysomal distribution analysis of Mrp2 mRNA was consistent with increased Mrp2 protein synthesis after PCN treatment. The major transcription-initiation site for rat liver determined by RPA was -98 nucleotides (nt), with other start sites observed at -213, -163, -132, and -71 nt; use of transcription sites did not differ among the groups. Differences in the degradation of Mrp2 protein cannot explain the post-transcriptional regulation of Mrp2 in control, pregnant, and PCN-treated rats. Rather, the observed difference in protein synthesis suggests an intrinsic role for the translational regulation of rat Mrp2 protein.

Transporters on demand: intrahepatic pools of canalicular ATP binding cassette transporters in rat liver

ABC transporter trafficking in rat liver induced by cAMP or taurocholate and [(35)S]methionine metabolic labeling followed by subcellular fractionation were used to identify and characterize intrahepatic pools of ABC transporters. ABC transporter trafficking induced by cAMP or taurocholate is a physiologic response to a temporal demand for increased bile secretion. Administration of cAMP or taurocholate to rats increased amounts of SPGP, MDR1, and MDR2 in the bile canalicular membrane by 3-fold; these effects abated after 6 h and were insensitive to prior treatment of rats with cycloheximide. Half-lives of ABC transporters were 5 days, which suggests cycling of ABC transporters between canalicular membrane and intrahepatic sites before degradation. In vivo [(35)S]methionine labeling of rats followed by immunoprecipitation of (sister of P-glycoprotein) (SPGP) from subcellular liver fractions revealed a steady state distribution after 20 h of SPGP between canalicular membrane and a combined endosomal fraction. After mobilization of transporters from intrahepatic sites with cAMP or taurocholate, a significant increase in the amount of ABC transporters in canalicular membrane vesicles was observed, whereas the decrease in the combined endosomal fraction remained below detection limits in Western blots. This observation is in accordance with relatively large intracellular ABC transporter pools compared with the amount present in the bile canalicular membrane. Furthermore, trafficking of newly synthesized SPGP through intrahepatic sites was accelerated by additional administration of cAMP but not by taurocholate, indicating two distinct intrahepatic pools. Our data indicate that ABC transporters cycle between the bile canaliculus and at least two large intrahepatic ABC transporter pools, one of which is mobilized to the canalicular membrane by cAMP and the other, by taurocholate. In parallel to regulation of other membrane transporters, we propose that the "cAMP-pool" in hepatocytes corresponds to a recycling endosome, whereas recruitment from the "taurocholate-pool" involves a hepatocyte-specific mechanism.

Roles of cysteines in rat dipeptidyl peptidase IV/CD26 in processing and proteolytic activity

The multifunctional type II transmembrane glycoprotein, dipeptidyl peptidase IV (DPPIV, EC 3.4.14.5), is expressed by almost all mammalian cells and is identical to the adenosine deaminase binding protein CD26 on lymphocytes. The extracellular part of rat DPPIV can be divided into three domains the middle part of which harbors 10 of the 12 highly conserved cysteine residues. The cysteine-rich domain is responsible for DPPIV-binding to collagen I and to extracellular ADA. The participation of distinct cysteines in disulfide bridges is not yet known. Titration experiments have shown the presence of six free cysteines and three disulfide bridges in native rat DPPIV. To investigate the role of distinct cysteines in the structure-function relationships of rat DPPIV we constructed 12 different cysteine point mutations (C299, C326, C383, C455, C650 mutated to G; C337, C395, C445, C448, C473, C552, C763 mutated to S). Intracellular translocation to the cell surface of stable transfected Chinese hamster ovary cells was examined with antibodies against different epitopes of DPPIV. Surface expression of mutants C326G, C445S and C448S is inhibited totally; mutants C337S, C455G, C473S and C552S show weak expression only. In parallel, the half-life of these mutants is reduced to < 10% compared with wild-type enzyme. We were able to show that the specific peptidase activity of the mutant protein depends on cell-surface expression, dimerization and the existence of a 150-kDa form demonstrable by nondenaturing SDS/PAGE. We conclude that cysteine residues 326, 337, 445, 448, 455, 473 and 552 in rat DPPIV are essential for the correct folding and intracellular trafficking of this glycoprotein, and therefore for its normal biological properties.

Oligosaccharide reprocessing and recycling of a cell surface glycoprotein in cultured rat hepatocytes

The metabolism of the cell surface glycoprotein dipeptidyl peptidase IV(DPPIV) was studied in cultured rat hepatocytes. In pulse-chase labelling experiments using L-[35S]methionine a 100-kDa high-mannose precursor polypeptide is converted into the mature complex-type 110-kDa glycoprotein. Digestion with exo- and endoglycosidases and metabolic labelling with radioactive sugars demonstrate that the 110-kDa form contains about 6 complex-type oligosaccharides which are fucosylated and sialylated. About 25 min after the beginning of the pulse-labelled glycoprotein appears in the sinusoidal membrane. Physiologically only the 110-kDa form is found in the cell surface. If cell surface DPP IV was desialylated by sialidase at 4 degrees C, it is resialylated during incubation at 37 degrees C. This oligosaccharide reprocessing indicates that the surface glycoprotein has been recycled to the cell compartment containing terminal glycosyltransferases (presumably the trans Golgi system). Two different methods demonstrate internalization of cell surface DPP IV: 1) The complex cell surface DPPIV -anti-DPP IV-antibody -L-[35S]methionine-labelled secondary goat-anti-mouse-antibody formed at 4 degrees C becomes less accessible to trypsin during incubation at 37 degrees C. 2) Part of the complex plasma membrane DPP IV-anti-DPP IV-antibody formed in the cold cannot be recognized by the radioactive secondary antibody after rewarming. Internalization is not blocked by inhibition of protein synthesis with cycloheximide. During internalization of plasma membrane DPP IV its concentration in the membrane remains constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Particulate diadenosine 5',5"'-P1,P3-triphosphate hydrolases in rat brain: two specific dinucleoside triphosphatases and two phosphodiesterase I-like hydrolases

Rat liver and brain differ in the distribution pattern of the total hydrolytic activity on diadenosine 5',5"'-P1,P3-triphosphate (Ap3A) between the soluble and particulate fractions. The Ap3A-hydrolase activity in both the soluble and particulate liver fractions and in the brain soluble fraction had been previously studied in detail. We report now on the brain particulate fraction which, unlike liver, showed a low unspecific phosphodiesterase I-like (PDEaseI, EC 3.1.4.1) activity relative to the specific dinucleoside triphosphatase (Ap3Aase, EC 3.6.1.29). Two PDEaseI-like forms (PDEaseI-A and PDEaseI-B), with different apparent Mrs and kinetic properties, and two Ap3Aases (Ap3Aase-alpha and Ap3Aase-beta) were solubilized with 0.5% Triton X-100 from the particulate fraction. Ap3Aase-alpha resembled the cytosolic Ap3Aase (Ap3Aase-c), a known situation in liver. Comparative to Ap3Aase-alpha, Ap3Aase-beta showed a slightly higher Km (35 vs. 15 micron) and lower isoelectric point (5.25 vs. 5.45); Ap3Aase-beta was absent from the soluble fraction, and its recovery was unaffected by proteinase inhibitors, strongly arguing for distinct soluble and particulate turnover pathways for dinucleoside polyphosphates.

Rapid intramolecular turnover of N-linked glycans in plasma membrane glycoproteins. Extension of intramolecular turnover to the core sugars in plasma membrane glycoproteins of hepatoma

Plasma membrane glycoproteins of rat hepatocytes undergo a rapid terminal deglycosylation in that the terminal sugars of the oligosaccharide side chains are rapidly removed from the otherwise intact glycoproteins [Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl Acad. Sci. USA 80, 4026-4029]. The present paper demonstrates that this rapid intramolecular turnover of plasma membrane glycoproteins is not restricted to peripheral sugars but, in contrast to liver, in hepatoma the core sugars of the oligosaccharide chains are also involved. Intramolecular turnover was measured in Morris hepatoma 7777 in five plasma membrane glycoproteins with Mr of 85,000 (hgp85), 105,000 (hgp105), 115,000 (hgp115), 125,000 (hgp125), 175,000 (hgp175) (hgp = hepatoma glycoprotein) that were isolated and purified to homogeneity by concanavalin-A--Sepharose affinity chromatography and semipreparative SDS gel electrophoresis. Analysis of the carbohydrates of hgp85, hgp105, hgp115 and hgp125 revealed the presence of N-linked oligosaccharides containing L-fucose, D-galactose, D-mannose and N-acetyl-D-glucosamine, but only of trace amounts of N-acetyl-D-galactosamine; hgp175 additionally contained significant amounts of N-acetyl-D-galactosamine, indicating the presence of both N- and O-linked oligosaccharides. As shown by digestion with endoglucosaminidase H, the N-linked oligosaccharides of hgp105, hgp115, hgp125 and hgp175 were of the complex type, whereas hgp85 also contained oligosaccharides of the high-mannose type. Half-lives of the turnover of the oligosacharide chains and of the protein backbone of the five glycoproteins were measured in the plasma membrane in pulse-chase experiments in vivo, using L-[3H]fucose as a marker of terminal sugars, D-[3H]mannose as marker of a core sugar and L-[3H]leucine for labelling the protein backbone. Protein backbones of the five glycoproteins were degraded with individual half-lives ranging over 41-90 h with a mean of 66 h. Compared to the degradation of the polypeptide backbone, both the terminal sugar L-fucose and the core sugar D-mannose turned over with much shorter half-lives averaging about 20 h in the five glycoproteins. The data show that, conversely to liver, within plasma membrane glycoproteins of hepatoma not only peripheral sugars but also core sugars of the oligosaccharides are split off during the life-span of the protein backbone. It may therefore be assumed that this reprocessing of plasma membrane glycoproteins is sensitive to malignant transformation.

Decreased intramolecular turnover of L-fucose in membrane glycoproteins of rat liver during liver regeneration

In plasma membrane glycoproteins of rat liver L-fucose undergoes a rapid intramolecular turnover in that fucose residues are removed from the glycoproteins (Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4026-4029). The present paper demonstrates that the intramolecular turnover of L-fucose is markedly decreased during liver regeneration. Turnover half-lives of L-fucose were measured in regenerating liver by pulse-chase experiments in five plasma membrane glycoproteins (Mr 60,000 (gp60), 80,000 (gp80), 120,000 (gp120), 140,000 (gp140), and 160,000 (gp160). The glycoproteins were isolated from plasma membranes by concanavalin A-Sepharose affinity chromatography and semipreparative NaDodSO4 polyacrylamide gel electrophoresis. L-Fucose turned over in the five glycoproteins with heterogeneous half-lives ranging from 22 h (gp160) to 49 h (gp120). The protein moieties of the glycoproteins were degraded with half-lives ranging from 56 h (gp80) to 107 h (gp140). Relative to the half-life of the protein backbone the half-live of L-fucose was increased in the five membrane glycoproteins by 70% (gp60), 150% (gp80), 182% (gp120), 60% (gp140) and 16% (gp160) during liver regeneration when compared to normal liver. The data show that L-fucose turns over in different membrane glycoproteins with individual rates, and that loss of L-fucose from plasma membrane glycoproteins is reduced in rapidly proliferating liver after partial hepatectomy.

Differential time-course of induction of rat liver gamma-glutamyltransferase and drug-metabolizing enzymes in the endoplasmic reticulum, Golgi and plasma membranes after a single phenobarbital injection. Evaluation of protein variations by two-dimensional electrophoresis

This study was conducted to follow as a function of time the activity of gamma-glutamyltransferase in the various membranes of rat liver cells after a single dose of phenobarbital (PB) (75 mg kg-1 body weight). Gamma-glutamyltransferase induction was maximal 24 h after PB treatment in both the rough endoplasmic reticulum and the plasma membranes. This pattern of induction differed from that of some drug metabolizing enzymes. While total cytochrome P-450 content was enhanced mainly in endoplasmic reticulum until 48 h after PB treatment, UDP-glucuronosyltransferase activity was not greatly altered by PB under the same conditions. The comparison of two-dimensional electrophoretic polypeptide profiles of each subcellular membrane isolated from control and phenobarbital-treated rats revealed important variations induced by PB. In plasma membranes, the heaviest subunit (apparent Mr = 60 x 10(3)) of hepatic gamma-glutamyltransferase was provisionally identified as a collection of polypeptide which differ only by their pI. The concentration of these polypeptides was smaller in the endoplasmic reticulum where they were of lower apparent molecular mass. This suggests that the gamma-glutamyltransferase precursor is already processed at the level of the endoplasmic reticulum but it is still not completely mature or glycosylated. Five days of continuous PB treatment induced by appearance of new gamma-glutamyltransferase isoforms in plasma membranes. We demonstrate that after a single injection of PB, gamma-glutamyltransferase activity increases simultaneously with some drug-metabolizing enzymes, such as total cytochrome P-450 but not with others, such as UDP-glucuronosyltransferases.

Ethanol-induced alterations of plasma membrane assembly in the liver

The effects of acute ethanol administration on the assembly of glycoproteins into the hepatic plasma membrane were studied in the rat. When [14C]fucose and N-acetyl[3H]mannosamine, a sialic acid precursor, were injected following an acute dose of ethanol, the incorporation of these precursors into the total pool of membrane glycoproteins was minimally affected. This finding indicated that ethanol treatment did not appreciably alter the glycosylation of proteins in the Golgi apparatus. However, the assembly of labeled fucoproteins and sialoproteins into the plasma membrane was markedly inhibited in the ethanol-treated animals. This inhibition of plasmalemmal glycoprotein assembly was accompanied by a corresponding accumulation of labeled glycoproteins in the cytosolic fraction of the hepatocyte. The content of labeled glycoproteins in the Golgi complex was not significantly altered by ethanol treatment. These results indicate that ethanol administration impairs the late stages of hepatic plasma membrane assembly and further suggest that ethanol administration interferes with the flow of membrane components from the Golgi apparatus to the surface membrane.

Identification of ecto-nucleoside triphosphate pyrophosphatase in human articular chondrocytes in monolayer culture

In cultured monolayers of human articular chondrocytes we have observed an enzyme activity which catalyzes the extracellular conversion of ATP to AMP and PPi. The enzyme was active at very low concentrations of ATP (microM) and exhibited optimal activity at concentrations of ATP of approx. 100 microM. The enzyme was active in intact cells as judged by measurement of the release of the cytoplasmic marker enzyme lactate dehydrogenase. No increase in production of PPi from ATP was observed on mechanically disrupting the cells and no activity was shed into the medium by intact cells. Activity was stable between days 4 and 8 after subculturing the cells and was not affected by the timing of the final medium change prior to assay. Activity was also observed with other nucleoside triphosphates (GTP, CTP and UTP). We suggest that this activity is attributable to ecto-nucleoside triphosphate pyrophosphatase. This observation may be important in relation to the pathogenesis of the human disease of chondrocalcinosis in which crystals of calcium pyrophosphate dihydrate deposit in articular cartilage.

Occurrence of dinucleosidetriphosphatase in the cytosol and particulate fractions from rat liver

Dinucleosidetriphosphatase (EC 3.6.1.29) is present in both the 37,000 g rat liver supernatant and precipitate (50 mU/g each fraction). These two activities show matching molecular weights, isoelectric points, substrate specificities, Km values, bivalent cation requirements and inhibition by zinc (II). The particulate triphosphatase and a residual dinucleosidetetraphosphatase (EC 3.6.1.17) are solubilized by freeze-thawing or by Triton X-100. Detergent treatment also extracts an unspecific phosphodiesterase I activity (EC 3.1.4.1) which also splits dinucleoside polyphosphates. The above findings suggest the occurrence of cytosolic and particulate degradative pathways for dinucleoside polyphosphates.

Dipeptidyl peptidase IV as a new surface marker for a subpopulation of human T-lymphocytes

Plasma membranes were isolated from normal human lymphocytes as well as from cells of chronic lymphocytic leukemia of the T type. In both cases the bulk of the dipeptidyl peptidase IV activity paralleled the distribution of 5'-nucleosidase and, therefore, was localized in the plasmalemma. Immunofluorescence experiments with normal human lymphocytes and with antibodies against dipeptidyl peptidase IV revealed that this peptidase was accessible on the surface of viable cells. Further, it was demonstrated that the relative number of dipeptidyl peptidase IV-positive cells is much higher in lymphocytes reacting with the OKT4 antibody than in OKT8-positive cells. On the other hand, it has been reported that this peptidase is absent in B lymphocytes and is predominantly found in T cells bearing the Fc receptor for IgM (T mu lymphocytes). Thus, it is concluded that dipeptidyl peptidase IV represents an easily demonstrable surface marker of this lymphocyte subset.

Impaired plasma membrane glycoprotein assembly in the liver following acute ethanol administration

The in vivo effects of acute ethanol administration on hepatic plasma membrane assembly were studied in the rat. When [14C]fucose and [3H]N-acetylmannosamine, a sialic acid precursor, were injected following an acute dose of ethanol, minimal effects on fucose and a slight reduction of sialic acid incorporation into the total pool of hepatic membrane glycoproteins were observed. However, the assembly of labeled fucoproteins and sialoproteins into the plasma membrane was markedly inhibited in the ethanol-treated animals. These results indicate that ethanol administration impairs the late stages of membrane assembly which include the transport of glycoproteins from the Golgi complex to the plasma membrane and/or the insertion of glycoproteins into the membrane.

Modification of the intramolecular turnover of terminal carbohydrates of dipeptidylaminopeptidase IV isolated from rat-liver plasma membrane during liver regeneration

An intramolecular turnover of the terminal carbohydrates L-fucose, N-acetylneuraminic acid and D-galactose is a characteristic property of several liver plasma membrane glycoproteins, first demonstrated for dipeptidylaminopeptidase IV (EC 3.4.14.5., DPP IV). The core carbohydrates D-mannose and N-acetyl-D-glucosamine turn over like the polypeptide chain. The ratio of apparent half-lives of L-fucose and L-methionine of DPP IV is shifted from 0.17 in normal liver to 0.60 in regenerating liver. The ratio of half-lives of N-acetylneuraminic acid and L-methionine is only slightly changed from 0.43 in normal liver to 0.61 in regenerating liver. The ratio of apparent half-lives of D-mannose and L-methionine amounts to 0.80 in normal liver and 0.71 after partial hepatectomy. From this a drastic reduction of the intramolecular turnover of L-fucose on plasma membrane DPP IV in regenerating liver can be derived. The intramolecular N-acetylneuraminic acid turnover is affected to only a minor extent. D-Mannose turns over like the polypeptide in both normal and regenerating liver. The intramolecular L-fucose turnover may be involved in membrane glycoprotein recycling, which presumably is altered in regenerating liver. Additionally, L-fucose could regulate the rate of degradation of DPP IV, since core-fucosylated glycoproteins appear to be resistant to mammalian endo-N-acetylglucosaminidase.

Yeast alpha factor is processed from a larger precursor polypeptide: the essential role of a membrane-bound dipeptidyl aminopeptidase

Alpha factor mating pheromone is a peptide of 13 amino acids secreted by Saccharomyces cerevisiae alpha cells. Nonmating ("sterile," or ste) alpha-cell mutants bearing defects in the STE13 gene do not produce normal alpha factor, but release a collection of incompletely processed forms (alpha factor) that have a markedly reduced specific biological activity. The major alpha-factor peptides have the structures H2N-GluAlaGluAla-alpha factor and H2N-AspAlaGluAla-alpha factor. The ste13 mutants lack a membrane-bound heat-stable dipeptidyl aminopeptidase (DPAPase A) that specifically cleaves on the carboxyl side of repeating -X-Ala- sequences. Absence of DPAPase A and the other phenotypes of a ste13 lesion cosegregate in genetic crosses. The cloned STE13 gene on a plasmid causes yeast cells to overproduce DPAPase A severalfold. A different cloned DNA segment, which weakly suppresses the ste13 defects, causes overproduction of a heat-labile activity (DPAPase B) by about tenfold. Other experiments indicate that DPAPase A action may be rate-limiting for alpha-factor maturation in normal alpha cells.

Membrane bound gamma-glutamyltranspeptidase: its structure, biosynthesis and degradation

gamma-Glutamyltranspeptidase is a glycoprotein composed of heavy and light subunits and associated with the brush border membrane of the kidney and small intestine. gamma GTP solubilized with papain is a hydrophilic enzyme which has lost the membrane binding segments but its catalytic activity is not altered, whereas gamma GTP solubilized with Triton X-100 is a hydrophobic enzyme which contains hydrophobic domain binding to the membrane. Amino acid compositions of these two forms were compared and Triton solubilized enzyme was found to contain 52 amino acid residues more than the papain solubilized form. This difference is due to the heavy subunit not light subunit. Then, end group analysis was carried out and the carboxyl-termini of their light subunits were found to be phenylalanine and those of their heavy subunits were tyrosine, respectively. Although light subunits of two forms contain a common sequence, Thr-Ala (X)-Leu as an amino-terminal portion, that of heavy subunit of Triton X-100 solubilized form contains the sequence Met-Lys-Asn-Arg-Phe-Leu-Val-Leu-Gly-Leu-Val-Ala-Val-Val-Leu-Val-Phe-Val- Ile-Ile-Gly-Leu and the papain-solubilized form contains completely different amino-terminal sequence Gly-Pro-Pro-Leu. It is concluded that an amino-terminal portion of the heavy subunit is a hydrophobic domain consisting of about 20 hydrophobic amino acids and contributes to anchor the enzyme to the membrane. gamma GTP has been known to show great heterogeneity in charge and multiple forms with different isoelectric points are found to be mainly due to differences of their sugar chains. Then the structures of the oligosaccharides attached to gamma GTP were determined. They were found to be all asparagine linked and consisted of neutral and acidic oligosaccharides with remarkable heterogeneity. A correlation between the contents of the acidic oligosaccharides and the isoelectric points of multiple forms of gamma GTP was observed. In addition, multiple forms of gamma GTP were immunologically identical and their protein structures were identical. Next, the mechanisms of biosynthesis of gamma GTP were examined and it was found that two subunits of gamma GTP are synthesized as a precursor protein with a single polypeptide chain of 78,000 daltons. Then processing by limited proteolysis occurs post-translationally, and it is a rather slow process. Since the precursor form is already core glycosylated and fucosylated, proteolytic processing could be carried out after completion of terminal glycosylation at the Golgi membrane or the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)