Mechanisms for the incorporation of proteins in membranes and organelles

Four secretory proteins synthesized by hepatocytes are transported from endoplasmic reticulum to Golgi complex at different rates

Pulse-chase experiments in conjunction with subcellular fractionation and quantitative immunoprecipitation have been used to study the intracellular transport of four secretory proteins, albumin, transferrin, prealbumin and retinol-binding protein, in isolated rat hepatocytes. The proteins were found to be transported from the endoplasmic reticulum (ER) to the Golgi complex (GC) at greatly different rates (t1/2 = 14-137 min), indicating that transport of secretory proteins between these organelles is effected by a selective, possibly receptor-mediated process and not through bulk phase transfers. The transport from the Golgi complex to the medium was rapid for all proteins (t1/2 approximately 15 min) and possibly occurred at the same rate. Consistent with these kinetic data, the amount of a rapidly transported protein (albumin) in the GC fraction was found to be high (relative to its amount in the ER fraction) whereas the amount of a slowly transported protein (transferrin) in the GC fraction was found to be low, as determined by radioimmunoassays.

Dynamic nature of the association of large tumor antigen and p53 cellular protein with the surfaces of simian virus 40-transformed cells

A molecular complex of simian virus 40 large tumor antigen (T-Ag) and p53 cellular protein is present on the surface of simian virus 40-transformed mouse cells. The stability of the association of the two proteins with the cell surface was characterized. Cells were either surface iodinated by the lactoperoxidase technique or metabolically labeled with [35S]methionine, and surface antigens were detected by differential immunoprecipitation with specific antibodies immediately after labeling or after incubation at 37 degrees C. A rapid, concomitant disappearance of T-Ag and p53 from the cell surface was observed. The half-life of iodinated surface T-Ag was less than 30 min, whereas that of [35S]methionine-labeled surface T-Ag was 1 to 2 h. Although T-Ag and p53 were rapidly lost, both were also rapidly replaced on the cell surface, since newly exposed molecules could be detected when cells were reiodinated after a 2-h chase period. Control experiments established that the loss of the surface molecules was not induced by the iodination reaction. The appearance of surface T-Ag was prevented when cellular protein synthesis was inhibited with cycloheximide. The disappearance and replacement of T-Ag and p53 appeared to be energy-independent processes, as neither was inhibited by sodium azide or 2,4-dinitrophenol. Incubation of iodinated cells at 4 degrees C did block the loss of T-Ag and p53. These observations suggest that T-Ag and p53 are coordinately turned over in the plasma membrane. The nature of the association of the T-Ag-p53 complex with the cell surface can best be described as highly dynamic.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Effect of ethanol on hepatic secretory proteins

Both acute and chronic ethanol administration inhibit the secretion of albumin and glycoproteins from the liver. Impairment of posttranslational steps of the secretory process are mainly involved in this secretory defect, although in some instances altered synthesis of the protein moiety may be a factor. Decreased secretion following ethanol administration results in the intrahepatic retention of export proteins. The secretory defect is a consequence of the metabolism of ethanol and is likely mediated via acetaldehyde, although more conclusive proof is still required. The manner by which acetaldehyde impairs the secretory process is unknown, but may be related to its high reactivity with hepatocellular proteins. The specific posttranslational steps or processes involved in the secretory defect are still unclear; however, it appears that the final steps of secretion (post-Golgi events) may be the primary site of impairment. Impaired secretion of proteins from the liver could contribute to altered levels of plasma proteins and hepatomegaly as well as to the liver injury observed in the alcoholic.

An overview of pancreatic exocrine secretion

Genes for all proteins have encoded in their DNA sequences, information that specifies where these proteins will localize within the cell. Nascent translation products of transcripts of these genes, that possess a specific NH2-terminal signal sequence, are able to translocate into a specialized membranous conducting system called the endoplasmic reticulum (ER), or can be incorporated directly into a target organelle (i.e. mitochondrion). Nascent polypeptides lacking this signal sequence remain in the cytosol. Once segregation into the ER has occurred each protein appears to migrate at a characteristic rate to a connecting organelle; the Golgi Complex. Here, enzymatic modifications of these proteins determines the organelle with which each will eventually become associated. In secretory tissues such as the exocrine pancreas, many different proteins are directed into specialized secretory structures called zymogen granules. These granules are maintained in a "ready-to-release" state by steroid hormones. In the absence of estrogens and glucocorticoids the zymogen granules disappear. Physiologically, secretion from the exocrine pancreas is brought about by parasympathetic nerve stimulation, or by the gut hormone cholecystokinin (CCK). Interaction of acetylcholine or CCK with specific receptors on pancreatic acinar cells initiates the process of exocytosis; that is, fusion of the zymogen granule membrane with the plasma membrane resulting in extracellular release of the contents of the secretory granule.

Functional histone antibody fragments traverse the nuclear envelope

Factors important in the translocation process of proteins across the nuclear membrane were studied by microinjecting either fluoresceinated nonimmune IgG and F(ab)2 or the corresponding molecules, prepared from antisera to histones, into the nucleus and cytoplasm of human fibroblasts. Intact IgG from both preparations remained at the site of injection regardless of whether it was injected into the nucleus or the cytoplasm. In contrast, nonimmune F(ab)2 distributed uniformly throughout the cell. The F(ab)2 derived from affinity-pure antihistone moves into the nucleus after cytoplasmic injection and remains in the nucleus after nuclear microinjection. The migration of the antihistone F(ab)2 into the nucleus results in inhibition of uridine incorporation in the nuclei of the microinjected cells. We conclude that non-nuclear proteins, devoid of specific signal sequences, traverse the nuclear membrane and accumulate in the nucleus provided their radius of gyration is less than 55A and the nucleus contains binding sites for these molecules. These findings support the model of "quasibifunctional binding sites" as a driving force for nuclear accumulation of proteins. The results also indicate that active F(ab)2 fragments, microinjected into somatic cells, can bind to their antigenic sites suggesting that microinjection of active antibody fragments can be used to study the location and function of nuclear components in living cells.

Intracellular processing of the vesicular stomatitis virus glycoprotein and the Newcastle disease virus hemagglutinin-neuraminidase glycoprotein

The kinetics of intracellular transport of the vesicular stomatitis virus (VSV) glycoprotein (G) and the Newcastle disease virus (NDV) hemagglutinin-neuraminidase (HN) glycoprotein in chicken embryo cells were compared. To assay for the appearance of pulse-labelled glycoprotein at the cell surface, an antibody-binding assay was developed which allowed the precipitation of only those molecules on the outside surfaces of infected cells. Using this assay, it was found that pulse-labelled VSV G protein appeared at the cell surface with a half-time of approximately 27 min, while pulse-labelled NDV HN glycoprotein reached the cell surface with a half-time of approximately 78 min. To determine the transit time of these glycoproteins to trans-Golgi membranes, the kinetics of the acquisition of endoglycosidase H resistance was analyzed. The half-time of the transit of the G protein to the trans-Golgi membranes was found to be approximately 13 min while that of the HN glycoprotein was found to be approximately 60 min. Since the G protein migrates to the trans-Golgi membranes with a half-time of 13 min, and the cell surface with a half-time of 27 min, the half-time for the transit between the trans-Golgi membrane and the plasma membrane must be approximately 14 min. In a similar analysis, the half-time for the transit of the HN glycoprotein from the trans-Golgi membrane to the plasma membrane must be approximately 18 min, a time not significantly different from that of the G protein. Thus the difference in the kinetics of the intracellular transport of these two glycoproteins resides primarily in the transit from the rough endoplasmic reticulum to the trans-Golgi membranes. These results argue against a non-selective mechanism for the transport of plasma membrane glycoproteins to the cell surface.

Influenza virus hemagglutinin containing an altered hydrophobic carboxy terminus accumulates intracellularly

The influenza virus hemagglutinin (HA) glycoprotein synthesized from cloned DNA in a simian virus 40 vector is expressed on the surface of infected primate cells. Previously, it has been demonstrated that mutant HAs lacking the hydrophobic carboxy terminus fail to anchor on the cell surface and therefore are secreted extracellularly. During analysis of additional HA deletion mutants derived from an HA-simian virus 40 recombinant, we found a mutant with an altered hydrophobic carboxy terminus that exhibited another phenotype. This deletion mutant, dl-12, produced HA that was neither secreted nor expressed on the infected cell surface. The mutant HA was similar to the wild-type HA in apparent molecular weight and extent of glycosylation as assayed by endoglycosidase H sensitivity. The mutant HA localized near the perinuclear region of infected cells as indicated by an indirect immunofluorescence assay. Sequence analysis showed that a 5-base-pair deletion had occurred before the region encoding the hydrophobic carboxy terminus. Nevertheless, the physicochemical properties of the wild-type HA carboxy terminus were maintained in that the truncated HA carboxy terminus consisted of predominantly hydrophobic amino acids followed by several charged amino acids residues. This similarity in the carboxy terminus between the wild-type and mutant HAs may be responsible for the functional similarities observed. In spite of these similarities, the mutant HA failed to mature at the surface. These results suggest that the maturation of the mutant HA is blocked during a late stage in the transit to the cell surface.

Synthetic leader peptide modulates secretion of proteins from microinjected Xenopus oocytes

To investigate the role of the leader peptide in modulating secretion from living cells, we injected a synthetic peptide into Xenopus oocytes. The peptide consisted of the NH2-terminal leader sequence of mouse immunoglobulin light chain precursor. We found that the leader peptide has two different roles in regulating secretion from the oocytes. First, it competitively inhibits the synthesis of secretory and membrane proteins but not of cytoplasmic proteins. The inhibition occurs both with oocyte proteins and with proteins directed by coinjected myeloma mRNA. The inhibition reaches a maximum 2 hr after injection and decays within 3 hr. It appears to be mediated through the cell membrane, because 125I-labeled leader peptide segregates into the membrane fraction of microinjected oocytes simultaneously with the interference with methionine incorporation. A second role of the microinjected leader peptide is to induce a rapid acceleration in the rate of export of secretory proteins from the oocyte. The maximal enhancement effect is obtained upon injection of 50 ng of leader peptide per oocyte. It is not merely due to the small size, negative charge, or hydrophobicity of the peptide, because enhanced secretion does not occur when glucagon, poly-L-glutamic acid, or Triton X-100 is injected. Furthermore, immunoreaction of the peptide with specific antibodies prior to microinjection prevents the accelerated export. Our observations indicate that in Xenopus oocytes, the leader peptide is involved in both translocation and later step(s) in the secretory pathway.

beta-Adrenergic receptor regulation of N-linked protein glycosylation in rat parotid acinar cells

We have investigated the relationship between beta-adrenergic receptor stimulation and protein glycosylation and secretion in rat parotid gland cells in vitro. The potent beta-adrenergic agonist (-)-isoproterenol increases [3H]mannose incorporation into newly synthesized glycoproteins. This effect is enhanced if cells are first preincubated with dolichyl phosphate and is not observed after muscarinic-cholinergic or alpha-adrenergic stimulation of cells. The increase in [3H]mannose incorporation is abolished by incubation of cells with tunicamycin, suggesting that the glycosylation events being studied involved asparagine-linked oligosaccharides. The extent of increase in glycosylation is dependent on the concentration of (-)-isoproterenol to which cells are exposed. (+/-)-Propanolol totally abolishes the (-)-isoproterenol-induced increase in [3H]mannose incorporation, in a manner similar to its effects on exocrine secretion. Our findings suggest that beta-adrenergic receptor activation has a profound influence on N-linked protein glycosylation in rat parotid cells in addition to eliciting exocrine protein release.

Vesicular stomatitis virus glycoprotein, albumin, and transferrin are transported to the cell surface via the same Golgi vesicles

Human hepatoma cells, infected by vesicular stomatitis virus, offer a good system to study simultaneously the intracellular localization of a well defined transmembrane glycoprotein (VSV-G), a secretory glycoprotein (transferrin), and a nonglycosylated secretory protein (albumin). We used monospecific antibodies in combination with 5- and 8-nm colloidal gold particles complexed with protein A to immunolabel these proteins simultaneously in thin frozen sections of hepatoma cells. VSV-G, transferrin, and albumin are present in the same rough endoplasmic reticulum cisternae, the same Golgi compartments, and the same secretory vesicles. In the presence of the ionophore monensin intracellular transport is blocked at the trans cisternae of the Golgi complex, and VSV-G, transferrin, and albumin accumulate in dilated cisternae, which are apparently derived from the trans-Golgi elements. Glycoproteins, synthesized and secreted in the presence of monensin, are less acidic than those in control cultures. This is probably caused by a less efficient contact between the soluble secretory proteins and the membrane-bound glycosyltransferases that are present in the most monensin-affected (trans) Golgi cisternae.

Cloning and sequencing of a full length cDNA coding for human retinol-binding protein

We have isolated and sequenced a cDNA clone coding for human Retinol Binding Protein. The sequence indicates that Retinol Binding Protein is synthesized as a single polypeptide chain precursor which is then matured to the secreted protein by removal of a leader peptide. Southern and Northern blot analysis suggest that the gene is present in one or few copies per haploid genome and is transcribed in a single mRNA species.

Direct enzyme transfer from lymphocytes is specific

Lymphocytes are known to interact directly with other cells in vivo and in vitro, and have recently been shown to transfer the lysosomal enzyme, beta-glucuronidase, to fibroblasts from patients with an inherited deficiency of the enzyme. This process requires cell-cell contact, is unaffected by inhibitors of 'classical' receptor-mediated endocytosis and is abolished by inhibitors of protein synthesis. Although it is not yet known to what extent the transfer of enzymes by direct cellular interaction is a general phenomenon, a similar mechanism could possibly be involved in the transfer of other lysosomal enzymes in vivo and in the exchange of protein in vitro. We show here that the direct transfer of enzymes from lymphocytes to fibroblasts is restricted to only certain lysosomal enzymes.

Synthesis and processing of glycoprotein D of herpes simplex virus types 1 and 2 in an in vitro system

We carried out studies of in vitro translation and processing of glycoprotein D (gD) of herpes simplex virus types 1 and 2 by using mRNA from cells infected for 6 h and a reticulocyte lysate translation system. Polypeptides of 49,000 daltons were immunoprecipitated with anti-gD-1 sera. Each in vitro-synthesized molecule had the same methionine tryptic peptide profile as the respective in vivo precursors, pgD-1 and pgD-2. In addition, the polypeptides synthesized in vitro were larger than the corresponding molecules synthesized in the presence of tunicamycin. This suggested that each of the gD polypeptides synthesized in vitro contained a transient N-terminal signal sequence. When the translation mixture was supplemented with pancreatic microsomes, each of the gD polypeptides was converted cotranslationally to a larger-molecular-weight form. Processing involved addition of three N-asparagine-linked oligosaccharides and removal of the signal peptide. When trypsin was added after in vitro processing, a polypeptide which was 3,000 daltons smaller than the in vitro-modified form of gD was immunoprecipitated. Experiments with endo-beta-N-acetylglucosaminidase H showed that this polypeptide still contained the three N-asparagine-linked oligosaccharides. Two monoclonal antibodies, 57S (group V) and 17O (group VII), were used to further orient gD in microsomes. The group V determinant was located in the trypsin-sensitive 3,000-dalton fragment, and the group VII determinant was located in the portion of gD which was protected from trypsin. We concluded that gD is oriented with the three glycosylation sites inside the vesicles and that 3,000 daltons containing the group V determinant are located outside. Immunofluorescence studies indicated that the group V determinant of gD is inside the plasma membrane of herpes simplex virus-infected cells and that the group VII determinant is outside. This cellular orientation is consistent with predictions based on the in vitro experiments.

Complete amino acid sequence and predicted membrane topology of phenobarbital-induced cytochrome P-450 (isozyme 2) from rabbit liver microsomes

The complete amino acid sequence of phenobarbital-induced isozyme 2 of rabbit liver microsomal cytochrome P-450 (P-450LM2) is presented. The polypeptide consists of 491 residues with a calculated Mr of 55,755. The rabbit isozyme is 77% identical to the corresponding rat cytochrome, P-450b, as deduced from cDNA, with 96% of the hydrophobic, 88% of the anionic, and 83% of the cationic positions conserved. The secondary structure of isozyme 2 was predicted and a model was developed for the membrane topology of this cytochrome. Of the two highly conserved cysteinyl peptides in P-450LM2, P-450b, and bacterial P-450cam, we favor, on the basis of our model, the one nearer the NH2 terminus (Cys-152 in P-450LM2) as the source of the thiolate ligand to the heme iron atom. The recently reported sequence of the apparently identical protein [Heinemann, F. S. & Ozols, J. (1983) J. Biol. Chem. 258, 4195-4201] has two fewer residues and differs in 14 other amino acid assignments.

Processing of herpes simplex virus-1 glycans in cells defective in glycosyl transferases of the Golgi system: relationship to cell fusion and virion egress

We studied herpes simplex virus-1 (HSV-1) glycan structure and the expression of HSV-1 functions regulated by viral glycoproteins in Ric21 cells (P. VISCHER and R. C. HUGHES, Eur. J. Bioch. 117, 275-284, 1981). This is a line of ricin-resistant mutant BHK cells defective in the enzymes of the Golgi system which add terminal sugars to N-linked glycans. Two kinds of alterations were observed in the glycosylation of HSV glycoproteins in Ric21 cells. First, there was a defective processing of complex glycans leading to a reduction of biantennary and triantennary species and an increase of incompletely processed monosialylated oligosaccharides. Second, there was an overall reduction in the accumulation of HSV-1 glycoproteins. We found that (i) the release of herpesvirions from Ric21 cells was markedly lower than that from BHK cells, possibly reflecting reduced terminal sugar addition which, in turn, might affect the intracellular transport of glycoproteins. (ii) HSV-1 (MP)-infected Ric21 cells fused with a low efficiency. Furthermore, polycaryocytosis was reduced or abolished in BHK and in Ric21 cells exposed to neuraminidase, indicating that the presence of sialic acid residues in the cell surface glycans is essential for cells to interact in a fashion that brings cell fusion. (iii) Although capsid assembly was comparable, the rate of accumulation of infectious virus decreased in Ric21 cells. Infectivity of released virions from Ric21 and BHK cells was similar, in agreement with previous studies showing that complex-type glycans do not appear to be required for herpesvirion infectivity. The decrease in infectious HSV-1 yield seems to correlate with overall reduced ability to synthesize glycoproteins.

Synthesis and maturation of the yeast vacuolar enzymes carboxypeptidase Y and aminopeptidase I

We have studied the two vacuolar enzymes carboxypeptidase Y and aminopeptidase I from Saccharomyces cerevisiae with respect to biosynthesis, maturation and transfer from their site of synthesis into the organelle. The levels of translatable mRNA for these two proteins increase more than 10-fold at the end of the exponential growth period on glucose as carbon source and decrease again in the stationary phase. Two precursors of carboxypeptidase Y have been identified by in vivo pulse-labelling with [35S]methionine. These differ in their amount of carbohydrate as shown by inhibition of N-linked glycosylation with tunicamycin. The first is a protein with an apparent molecular weight of 67 kDa, which can be converted into the mature 60-kDa protein via an intermediate of 69 kDa. In the pep4-3 mutant, which is disturbed in the maturation of several vacuolar enzymes (Hemmings, B.A., Zubenko, G.S., Hasilik, A. and Jones, E.W. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 435-439), the 69-kDa precursor accumulates in the vacuole. This suggests that the final proteolytic cleavage of carboxypeptidase Y can occur in the vacuole.

Membrane biogenesis in the sprouting neuron. I. Selective transfer of newly synthesized phospholipid into the growing neurite

Our goal was to elucidate the pathway of newly synthesized phospholipid into the growing neurite. This was accomplished in pulse-chase studies with the phospholipid precursor [3H]glycerol, using sprouting explant cultures of rat superior cervical ganglion as an experimental system. After the pulse with the precursor and various chase periods, we separated perikarya and neurites microsurgically and extracted their phospholipids. The phospholipid extract from the perikarya exhibited a steep rise followed by a rapid decline in specific radioactivity. In contrast, an increase in neuritic specific radioactivity of phospholipid was observed only after a lag period of approximately 60 min. Nearly quantitative transfer of newly synthesized phospholipid from the perikarya into the neurites could be demonstrated. Both the decline in perikaryal specific radioactivity and the increase in its neuritic counterpart, i.e., the proximodistal transfer, could be blocked with the microtubule drug colchicine and the metabolic uncoupler, 2,4-dinitrophenol. These observations indicate preferential export of newly synthesized phospholipid from the perikaryon (the major or exclusive site of synthesis) into the growing neurites, most likely by rapid axoplasmic transport of formed elements.

Studies on subcellular fractions which are involved in myelin membrane assembly: isolation from developing mouse brain and characterization by enzyme markers, electron microscopy, and electrophoresis

An extensive scheme for the subcellular fractionation of myelinating mouse brain is presented. Several centrifugation procedures for the separation of membranes involved in myelinogenesis are critically appraised, and guidelines for selection of centrifugation conditions are given. Characteristics of subcellular fractions are presented in the form of electron micrographs; also presented are distribution of RNA and protein; electrophoretic profiles of membrane proteins, and verification of the myelin-specific basic proteins, proteolipid protein, and glycoprotein by the immuno-electroblot technique; and the distribution of eight marker enzyme activities. Myelin-related membranes were found to differ both qualitatively and quantitatively in their complement of myelin-specific proteins. These myelin-containing fractions appear to represent different stages of myelination that coexist in developing mouse brain. These results provide the fundamental methodologies and background information for kinetic radioisotope analysis of intracellular events in the assembly of myelin presented in a companion article.

Protein production and processing in young adult and aged rat submandibular gland cells in vitro

Protein production and processing were evaluated in vitro in dispersed submandibular gland cells from young adult (4-6 months) and aged (24 month) rats. A modest decrease (approx. 25%) in protein production (incorporation of radiolabeled amino acids into 10% CCl3COOH-insoluble material) was found with old cells in both continuous-pulse and pulse-chase studies. Also new protein processing, followed by gel electrophoresis and autofluorography of radiolabeled samples, showed specific, marked alterations in old cells. In particular a significant difference in the processing of a 225 000 molecular weight glycoprotein (likely the major rat submandibular mucin) was detected.

Transport and topology of galactosyltransferase in endomembranes of HeLa cells

HeLa cell membranes were studied for the distribution and orientation of the Golgi marker enzyme uridine diphosphate-galactose:beta-D-N-acetylglucosamine beta, 1-4 transferase (GT). Short pulse labeling in the presence of [35S]methionine resulted in two precursor species (Mr = 44,000 and 47,000), present in a microsomal fraction with a density of 1.18 g/ml in sucrose, presumably derived from the rough endoplasmic reticulum. Processing of the N-linked oligosaccharide(s) occurred only after the precursor molecules migrated to lighter density fractions, presumably derived from the Golgi complex. The mature GT molecules (Mr = 54,000) contain O-linked oligosaccharides as shown by beta-elimination of metabolically incorporated [3H]galactose. The O-glycosylation occurred mainly in the light density fractions. The topology of GT was studied on membrane fractions after labeling with [35S]methionine as well as immunocytochemically on ultrathin cryosections at the electron microscope level. Our results indicate that both the antigenic determinants of GT as well as polypeptide chain are present intramembraneously and at the luminal side of the membranes of the Golgi complex and rough endoplasmic reticulum.

Altered cytoplasmic domains affect intracellular transport of the vesicular stomatitis virus glycoprotein

We have altered the structure of the COOH-terminus of the vesicular stomatitis virus (VSV) glycoprotein (G) by introducing deletions into a cDNA clone encoding G protein. We examined the effects of these deletions on intracellular transport of G protein after expression of the deleted genes in eucaryotic cells under control of the SV40 late promoter. To prevent readthrough of translation into vector sequences, we introduced synthetic DNA linkers containing translation stop codons at the site of the deletion. G proteins that lacked the cytoplasmic domain and most of the transmembrane domain were secreted slowly from the cells. Deletion mutants affecting the structure of the cytoplasmic domain fell into two classes. The first class completely arrested transport of the protein to the cell surface at a stage prior to acquisition of complex oligosaccharides. The second class showed severely reduced rates of complex sugar addition although the proteins were eventually transported to the cell surface. Indirect immunofluorescence microscopy suggested that mutant proteins in both classes may accumulate in the rough endoplasmic reticulum.

Expression of Semliki Forest virus proteins from cloned complementary DNA. II. The membrane-spanning glycoprotein E2 is transported to the cell surface without its normal cytoplasmic domain

The E2 protein (422 amino acid residues long) of Semliki Forest virus is a spanning membrane protein which is made in the rough endoplasmic reticulum of the infected cell and transported to the cell surface. The cytoplasmic domain of this protein comprises 31 amino acid residues. We introduced deletions of various sizes into the gene region encoding this part of the protein molecule and analyzed the transport behavior of the mutant proteins. The deletions were made using exonuclease digestions of cloned cDNA encoding the E2 protein. When the mutated DNA molecules, engineered into an expression vector, were introduced into nuclei of baby hamster kidney 21 cells, membrane proteins with cytoplasmic deletions were expressed and routed to the cell surface in the same way as the wild-type protein. This suggests that the cytoplasmic domain of the E2 protein does not carry information that is needed for its transport from the rough endoplasmic reticulum to the cell surface.

Vinblastine inhibits the maturation of the precursor of mitochondrial aspartate aminotransferase. Vincristine and six other cytoskeleton inhibitors do not show this effect

Cytoskeleton inhibitors were tested in chicken embryo fibroblast cultures for possible effects on the import of the precursor of mitochondrial aspartate aminotransferase into mitochondria. Vinblastine (50 microM) increased the steady-state pool of the precursor 2.5-fold in pulse experiments with [35S]methionine. If the precursor was accumulated during a pulse in the presence of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and then chased under diluting CCCP, vinblastine (50 microM) prolonged the half-life of the precursor from 0.5 min in the control to 3 min. Other cytoskeleton inhibitors, i.e. vincristine (25 to 150 microM), colchicine (50 microM), nocodazole (50 microM), podophyllotoxin (50 microM), taxol (45 microM), cytochalasin D (20 microM) and phalloidin (25 microM) did not show this effect. The observed inhibition by vinblastine does not seem to relate to its action on microtubuli.

The 60S ribosomal subunit is altered in the skeletal muscle of dystrophic hamsters

Polysomes from the skeletal muscle of normal and dystrophic hamsters were dissociated into ribosomal subunits by treatment with puromycin and the subunits from both strains were reassociated in all possible combinations. When their protein synthesis activity was assayed in a poly(U)-directed cell-free system at a low magnesium concentration, the reassociated ribosomes from dystrophic hamsters were less active than the ribosomes from control animals. The ribosomal defect is a property of the 60S subunit and is due to a ribosomal component rather than to abnormal binding of a non-ribosomal protein.

Role of steroids in secretion--modulating effect of triamcinolone and estradiol on protein synthesis and secretion from the rat exocrine pancreas

Following adrenalectomy of male rats, or adrenalectomy and ovariectomy of females, there was marked depletion of zymogen granules in acinar cells of the pancreas. Within 9 h after treatment with either triamcinolone or 17 beta-estradiol, complete restoration of these secretory vesicles was observed. This repletion was not inhibited by actinomycin-D. Supernatant fractions (100,000 g, 60 min) of rat pancreas, from both normal and surgically altered animals, contained proteins that bound [3H]-triamcinolone and [3H]-estradiol, suggesting that the action of these hormones is exerted directly on the pancreas. Binding of both steroid hormones required the presence of an additional coligand referred to as accessory factor. In addition, the binding proteins for [3H]-triamcinolone and [3H]-estradiol eluted in similar positions after Sephadex G-200 and CM Affi-gel Blue chromatography. It is uncertain, however, whether a single protein binds both steroid hormones since they had different binding isotherms. Scatchard analysis of binding of [3H]-estradiol yielded a single straight line of negative slope from which it was calculated that there were about 4.4 pmol of binding sites per mg protein, having an average apparent Kd of about 5 X 10(-8) M. Similar analysis of the data for [3H]-triamcinolone yielded a straight line of zero slope indicating nonsaturable binding of hormone at concentrations as high as 10 microM. Since both [14C]-L-leucine incorporation into protein and amylase secretion were affected markedly by the steroid-hormonal status of the animal, it is presumed that steroid-bound complexes in acinar cells of the pancreas modulate synthesis and secretion of protein.

Insulin as a surface marker on isolated cells from rat pancreatic islets

Immunoreactive insulin was shown to exist as a surface molecule in the plasma membrane of dispersed rat pancreatic islet cells. The intact cells were stained by immunofluorescence with a guinea pig antisera specific for insulin. The hormone on the cell surface could not be accounted for by insulin bound to specific receptors or nonspecifically absorbed to cells. Thus, surface insulin was demonstrated to be a specific membrane antigen for islet cells. Furthermore, the proportion of islet cells with insulin on the cell surface was directly correlated with insulin secretion in several different settings. This correspondence was demonstrated by varying the glucose concentration in the medium, by withholding Ca2+, which inhibits secretion, and by adding theophylline, which potentiates secretion. Consequently, these results suggested that insulin as a membrane protein was a marker for cells that actively secreted the hormone and may have been derived in the fusion process of secretory granules with the plasma membrane.

Alteration of membrane barrier in stripped rough microsomes from rat liver on incubation with GTP: its relevance to the stimulation by this nucleotide of the dolichol pathway for protein glycosylation

The membrane barrier of stripped rough microsomes from rat liver is markedly altered on incubation with GTP at 37 degrees C: after 30 min the structure-linked latency of mannose-6-phosphatase was considerably reduced, and esterase and nucleoside diphosphatase were partly released into the suspension medium. This phenomenon was already maximal with 30 microM GTP and was specific for this nucleotide. Similar conditions enhance the dolichol-mediated glycosylation of protein in microsomes incubated with uridine diphosphate N-acetylglucosamine and guanosine diphosphate mannose (Godelaine, D., H. Beaufay, M. Wibo, and A. Amar-Costesec, 1979, Eur. J. Biochem., 96:17-26; Godelaine, D., H. Beaufay, and M. Wibo, 1979, Eur. J. Biochem., 96:27-34). The GTP-induced permeability and glycosylation activities evolved in parallel in rough microsomes subjected to various treatments to detach the ribosomes and were maximal after removal of congruent to 60% of the RNA. In addition, GTP had no effect of this type in smooth microsome subfractions. Triton X-100, in spite of complex inhibitory effects on glycosylation reactions, mimicked the action of GTP by increasing the amount of microsomal dolichylphosphate that reacts with uridine diphosphate N-acetylglucosamine and by enhancing synthesis of dolichylpyrophosphoryl-chitobiose at concentrations greater than 2 mg/ml. Thus, GTP may activate dolichol-mediated glycosylation reactions in stripped microsomes by lowering the permeability barrier that prevents access of sugar nucleotides to the inner aspect of the membrane. The genuine role of GTP in the functioning of the endoplasmic reticulum membrane in situ remains unknown. Because GTP seems to act only on rough microsomes, we hypothesize that this role is somehow related to biosynthesis of protein by the rough endoplasmic reticulum.

Receptor sites involved in posttranslational transport of apocytochrome c into mitochondria: specificity, affinity, and number of sites

Assembly of cytochrome c involves a series of steps: synthesis of apocytochrome c on free ribosomes, specific binding of apocytochrome c to the mitochondrial surface, transfer across the outer membrane, covalent addition of protoheme, refolding of the polypeptide chain, and association of holocytochrome c with its functional sites at the inner membrane. The binding step of apocytochrome c to Neurospora crassa mitochondria was studied by inhibiting the subsequent transfer steps with the heme analogue deuterohemin. The binding sites are highly specific for mitochondrial apocytochromes c. Bound labeled Neurospora apocytochrome c was competitively displaced by unlabeled apocytochrome c from various species. These exhibited different abilities for displacement. Apocytochrome c from Paracoccus denitrificans, the amino-terminal (heme-binding) fragment of Neurospora apocytochrome c, and Neurospora holocytochrome c did not recognize the binding sites. Polylysine did not interfere with apocytochrome c binding. Apocytochrome c is reversibly bound. The binding sites are present in limited number. High-affinity binding sites were present at about 90 pmol/mg of mitochondrial protein. They displayed an association constant of 2.2 X 10(7) M-1. Apocytochrome c was imported into mitochondria and converted to holocytochrome c directly from the binding sites when inhibition by deuterohemin was relieved. We conclude that the apocytochrome c binding sites on mitochondria represent receptors that function in the recognition and import of this precursor by mitochondria.

Structure of a mouse submaxillary messenger RNA encoding epidermal growth factor and seven related proteins

The structure of the messenger RNA (mRNA) encoding the precursor to mouse submaxillary epidermal growth factor (EGF) was determined from the sequence of a set of overlapping complementary DNA's (cDNA). The mRNA is unexpectedly large, about 4750 nucleotide bases, and predicts the sequence of preproEGF, a protein of 1217 amino acids (133,000 molecular weight). The EGF moiety (53 amino acids) is flanked by polypeptide segments of 976 and 188 amino acids at its amino and carboyxl termini, respectively. The amino terminal segment of the precursor contains seven peptides with sequences that are similar but not identical to EGF.

Synthesis of rat-liver lactate dehydrogenase and characterization of its mRNA

Rat liver lactate dehydrogenase (LDH) has been synthesized in hepatocytes and in a cell-free translation system. The subunit synthesized in both systems displayed the same electrophoretic mobility upon sodium dodecyl sulfate/polyacrylamide slab gel electrophoresis. Sequence analysis of the subunits translated in vitro and synthesized in vivo indicated N-acetyl-alanyl-alanine for both N termini. Thus, the newly synthesized subunit does not exhibit an amino-terminal extension. The mRNA for the lactate dehydrogenase subunit was exclusively found in free polysomes. A size of 2120 +/- 240 nucleotides was estimated for the mRNA. Only about 50% of these nucleotides are needed to code for the polypeptide chain of the enzyme.

Carp preproinsulin cDNA sequence and evolution of insulin genes

The nucleic acid sequence of the preproinsulin cDNA of carp (Cyprinus carpio), cloned in the PstI-site of pBR322 (1), has been determined. The sequenced insert of 439 bp includes the complete coding information for carp preproinsulin (108 amino acids), 10 nucleotides of the 5'-and 105 nucleotides of the 3'-nontranslated regions. The nucleotide sequence confirms the previously established amino acid sequence of carp insulin (2) and determines those of the signal (21 aa 1) and C-peptide (35 aa 1). The observed shortness of the signal peptide of carp preproinsulin and the N-terminal addition of 2 amino acids to the carp insulin B-chain suggest that the cleavage site of the signal peptidase has moved. Calculations based on the comparison of known preproinsulin cDNA sequences showed that the evolutionary distance between fresh water and salt water teleostians is not smaller than between man and chicken.

Hepatoma secretory proteins migrate from rough endoplasmic reticulum to Golgi at characteristic rates

In eukaryotic cells, secretory proteins and glycoproteins migrate from the rough endoplasmic reticulum, their site of synthesis, through Golgi vesicles before being released from the cell. Cellular and viral integral plasma membrane glycoproteins are co-translationally inserted into the rough endoplasmic reticulum membrane and follow a similar pathway to the cell surface. Previous studies using endoglycosidase H (Endo H) suggested that in rat hepatoma cells the vesicular stomatitis virus (VSV) G protein, albumin and transferrin migrate from the rough endoplasmic reticulum to the Golgi apparatus at different rates. Here we show directly that in human hepatoma HepG2 cells, five secreted proteins mature from the rough endoplasmic reticulum to Golgi vesicles at characteristic rates which differ at least threefold. The results are incompatible with bulk-phase movement of the luminal contents of the endoplasmic reticulum, and suggest that there is a membrane-bound receptor that selectively mediates the transport of secretory proteins from the rough endoplasmic reticulum to the Golgi.

Active influenza virus neuraminidase is expressed in monkey cells from cDNA cloned in simian virus 40 vectors

We have replaced the late genes of simian virus 40 (SV40) with a cloned cDNA copy of the neuraminidase (NA; EC 3.2.1.18) gene of the WSN (H1N1) strain of human influenza virus. When the SV40-NA recombinant virus was complemented in a lytic infection of monkey cells with a helper virus containing an early region deletion mutant, influenza NA was expressed and readily detected by immunofluorescence as well as by immunoprecipitation of in vivo labeled proteins with monoclonal antibodies against NA. In addition, the expressed NA exhibited enzymatic activity by cleaving the sialic acid residue from alpha-2,3-sialyllactitol. The expressed protein was glycosylated and transported to the cell surface, and it possessed the same molecular weight as the NA of WSN virus grown in monkey cells. Because the structure of NA is quite different from that of other integral membrane proteins and includes an anchoring region at the NH2 terminus consisting of hydrophobic amino acids, we also constructed deletion mutants of NA in this region. Replacement of DNA coding for the first 10 NH2-terminal amino acids with SV40 and linker sequences had no apparent effect on NA expression, glycosylation, transport to the cell surface, or enzymatic activity. However, further deletion of NA DNA encoding the first 26 amino acids abolished NA expression. These data suggest that the hydrophobic NH2-terminal region is multifunctional and is important in biosynthesis and translocation of NA across the membrane as well as in anchoring the protein.

Immunocytochemical localization of alpha-D-mannosidase II in the Golgi apparatus of rat liver

Mannosidase II is involved in the trimming of alpha-1,6-mannosyl residues during the biosynthesis of glycoproteins containing N-linked oligosaccharides of the complex type. A highly specific polyclonal antibody (IgG) was isolated from rabbits immunized with a homogeneous preparation of mannosidase II prepared from rat liver. With this antibody, light and electron microscopic immunocytochemical studies on rat liver reveal that essentially all mannosidase II in hepatocytes is localized in the Golgi apparatus, the only other site with reaction product being the endoplasmic reticulum. The indirect immunocytochemical method used in this study involved three major steps: exposure of aldehyde-fixed tissue to immune and nonimmune IgG, treatment with staphylococcal protein A labeled with horseradish peroxidase, and incubation in diaminobenzidine to reveal sites of peroxidase activity. The procedures described overcome major problems in immunocytochemistry, allowing preservation of antigenic sites and maintaining adequate ultrastructural integrity. The in situ localization of other carbohydrate-processing enzymes, involved in either trimming or attachment of sugar residues, should be possible with this procedure. Because biosynthetic precursors of the processing enzymes may be revealed by an immunocytochemical approach, it is potentially significant that mannosidase II reaction product is present in areas of the endoplasmic reticulum as well as in the Golgi apparatus.

Secretion of plant storage globulin polypeptides by Xenopus laevis oocytes

Xenopus oocytes injected with poly(A)-containing RNA from developing cotyledons of field beans (Vicia faba L. var. minor) synthesize precursor polypeptides to the major storage globulins legumin and vicilin. These polypeptides are secreted into the medium without proteolytic cleavage of the legumin propolypeptides into the mature disulfide-linked alpha and beta chains. Similarly, storage globulin polypeptides from pea (Pisum sativum L.) and french bean (Phaseolus vulgaris L.) were secreted from oocytes. Inhibition of glycosylation by tunicamycin does not prevent secretion. This first report on the secretion of plant polypeptides by Xenopus oocytes shows that (a) intracellular deposition of storage proteins in membrane-bounded organelles (protein bodies) of plants and extracellular secretion have step(s) in common, and (b) the cell, in addition to the mRNA, determines the final destination of these polypeptides.

Nuclear localization of herpesvirus proteins: potential role for the cellular framework

Two herpes simplex virus proteins, the major capsid protein and the major DNA binding protein, are specifically localized to the nucleus of infected cells. We have found that the major proportion of these proteins is associated with the detergent-insoluble matrix or cytoskeletal framework of the infected cell from the time of their synthesis until they have matured to their final binding site in the cell nucleus. These results suggest that these two proteins may interact with or bind to the cellular cytoskeleton during or soon after their synthesis and throughout transport into the cell nucleus. In addition, the DNA binding protein remains associated with the nuclear skeleton at times when it is bound to viral DNA. Thus, viral DNA may also be attached to the nuclear framework. We have demonstrated that the DNA binding protein and the capsid protein exchange from the cytoplasmic framework to the nuclear framework, suggesting the direct movement of the proteins from one structure to the other. Inhibition of viral DNA replication enhanced the binding of the DNA binding protein to the cytoskeleton and increased the rate of exchange from the cytoplasmic framework to the nuclear framework, suggesting a functional relationship between these events. Inhibition of viral DNA replication resulted in decreased synthesis and transport of the capsid protein. We have been unable to detect any artificial binding of these proteins to the cytoskeleton when solubilized viral proteins were mixed with a cytoskeletal fraction or a cell monolayer. This suggested that the attachment of these proteins to the cytoskeleton represents the actual state of these proteins within the cell.

A 64-kilodalton membrane protein of Bacillus subtilis covered by secreting ribosomes

The complexed (ribosome-bearing) membrane fraction of Bacillus subtilis contains several proteins (CM-proteins) that are virtually absent from the ribosome-free fraction and hence might be components of the apparatus of protein secretion. We have determined, by trypsin digestion and by labeling with a nonpenetrating reagent (diazoiodosulfanilic acid), the accessibility of four of these proteins on the two surfaces of the membrane, as exposed either in protoplasts or in inverted membrane vesicles. The 68-kilodalton protein is a transmembrane protein and the 45-kilodalton protein faces only the external surface, whereas the 31-kilodalton protein is inaccessible from either side. Of particular interest is the 64-kilodalton protein: it can be digested by trypsin, and can bind antibody, on the cytoplasmic surface, but only after the ribosomes have been released. This protein is thus evidently a component of the apparatus of protein secretion, closely covered by secreting ribosomes. Whether the other CM-proteins are also involved in protein secretion is uncertain.