Mechanisms for the incorporation of proteins in membranes and organelles

Intracellular membrane traffic: pathways, carriers, and sorting devices

Multiple pathways of intracellular membrane traffic have been detected in various cell types. The major established routes are (a) the exocytosis pathway, utilized in secretory cells for the discharge of secretory products, and which is also believed to be used for delivery of intrinsic membrane glycoproteins in all cell types; (b) the plasmalemma to Golgi route, also highly developed in secretory cells, which is believed to be utilized for the recovery and recycling of the membranes of containers used in packaging of secretory products (i.e., secretory granules or vesicles); (c) the lysosomal pathway, which is available in all cells but is the major route utilized in phagocytic cells; (d) the transcellular route, which represents the major type of traffic encountered in nonfenestrated, capillary endothelial cells and also appears to be the preferred route for the transport of immunoglobulins (intact) across cells; and (e) the biosynthetic pathways used for transport of secretory products, lysosomal enzymes, and membrane proteins from the ER to the Golgi complex and for transport of lysosomal enzymes from the Golgi complex to lysosomes in all cell types. It has become clear that cells repeatedly reutilize or recycle the vesicular membranes involved in carrying out these various transport operations. Clathrin-coated vesicles have been found to be involved in transport along all the routes detected so far, suggesting that there are multiple populations of coated vesicles with different transport functions in every cell. It has become clear that considerable sorting of membrane constituents and ligands takes place at the plasmalemma (receptor-mediated uptake), in the Golgi complex, and in endosomes. The Golgi complex is the intracellular site where much of the biosynthetic and recycling membrane traffic converges and where products are sorted and directed to their correct destinations. In summary, we have become aware of the existence of multiple pathways of membrane traffic and of the extensive reutilization or recycling of membranes that occurs in cells. The basic pathways are similar in all cells except that some are emphasized or deemphasized according to the predominant function and organization of a given cell type. What now remains to be done is to determine how these transporting membranes and the membranes of the receiving compartments are constructed, how their specific interactions are controlled, and how individual cell types utilize these pathways to carry out their specific functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Construction of influenza haemagglutinin genes that code for intracellular and secreted forms of the protein

The DNA sequences encoding the amino-terminal signal peptide or the carboxy-terminal hydrophobic anchor have been deleted from a cloned gene coding for the haemagglutinin (HA) of influenza virus. The wild-type gene has previously been shown to be expressed with high efficiency from simian virus 40 (SV40)-HA recombinant vectors into a fully glycosylated protein that is displayed on the infected cell's surface in an antigenically and biologically active form. The anchor-minus HA also is glycosylated but is secreted efficiently into the medium. By contrast, the signal-minus HA is produced only at low levels, is not glycosylated and is located intracellularly.

Synthesis and incorporation of myelin polypeptides into CNS myelin

The distribution of newly synthesized proteolipid protein (PLP, 23 kdaltons) and myelin basic proteins (MBPs, 14-21.5 kdaltons) was determined in microsomal and myelin fractions prepared from the brainstems o1 10-30 d-old rats sacrificed at different times after an intracranial injection of 35S-methionine. Labeled MBPs were found in the myelin fraction 2 min after the injection, whereas PLP appeared first in the rough microsomal fraction and only after a lag of 30 min in the myelin fraction. Cell-free translation experiments using purified mRNAs demonstrated that PLP and MBPs are synthesized in bound and free polysomes, respectively. A mechanism involving the cotranslational insertion into the ER membrane and subsequent passage of the polypeptides through the Golgi apparatus is consistent with the lag observed in the appearance of the in vivo-labeled PLP in the myelin membrane. Newly synthesized PLP and MBPs are not proteolytically processed, because the primary translation products synthesized in vitro had the same electrophoretic mobility and N-terminal amino acid sequence as the mature PLP and MBP polypeptides. It was found that crude myelin fractions are highly enriched in mRNAs coding for the MBPs but not in mRNA coding for PLP. This suggests that whereas the bound polysomes synthesizing PLP are largely confined to the cell body, free polysomes synthesizing MBPs are concentrated in oligodendrocyte processes involved in myelination, which explains the immediate incorporation of MBPs into the developing myelin sheath.

Expression from cloned cDNA of cell-surface secreted forms of the glycoprotein of vesicular stomatitis virus in eucaryotic cells

A cDNA clone of the mRNA encoding the glycoprotein (G) of vesicular stomatitis virus was inserted into plasmid vectors under the control of either the SV40 early promoter (pSV2G) or the SV40 late promoter (pSVGL). Synthesis of G protein was observed in mouse L cells injected with pSV2G DNA or in COS1 cells transfected with pSVGL DNA. Immunofluorescent staining of G protein produced in both cell types showed a pattern of internal and cell-surface staining indistinguishable from that seen in cells infected with vesicular stomatitis virus. The G protein produced in transfected COS1 cells was the size of normal G protein and was glycosylated. Expression of a G protein lacking 79 amino acids from the COOH terminus was also examined. This G protein lacks the transmembrane domain and the hydrophilic COOH terminus, which, we postulated, anchor G protein in the lipid bilayer. This "anchorless" protein is glycosylated and is secreted, albeit slowly.

Synthesis of beta-hexosaminidase in cell-free translation and in intact fibroblasts: an insoluble precursor alpha chain in a rare form of Tay-Sachs disease

RNA was isolated from human term placenta or cultured fibroblasts and translated in a rabbit reticulocyte system in the presence of [35S]methionine; the translation products were immunoprecipitated with antisera made against beta-hexosaminidase or its isolated alpha and beta chains and analyzed by polyacrylamide gel electrophoresis. The largest translated alpha and beta chain polypeptides had Mrs of 65,000 and 59,000, respectively. These are approximately equal to 2,000 greater than the Mrs of precursor chains synthesized by intact fibroblasts and deglycosylated with endo-beta-N-acetylglucosaminidase H suggesting the presence of a signal sequence. RNA of fibroblast cultures from two patients with Sandhoff disease did not direct the translation of immunoprecipitable beta chain; RNA of fibroblast cultures from four patients with Tay-Sachs disease (three of Ashkenazi Jewish descent and one of non-Jewish descent) did not direct the translation of immunoprecipitable alpha chain. In contrast, a normal amount of alpha chain was made in the presence of RNA from the fibroblast culture of another non-Jewish Tay-Sachs patient (GM 1110). Intact fibroblasts from this patient also synthesized the alpha chain as shown by labeling with [3H]leucine; however, strong detergent was required for extraction. The alpha chain could be labeled with [3H]mannose but not with [32P]phosphate; it was neither secreted nor accumulated in the proteolytically processed form, and it disappeared within a day of synthesis. A plausible though not unique explanation is that the insoluble alpha chain is not transported from the endoplasmic reticulum (the site of glycosylation) to the Golgi apparatus (the site of phosphorylation) nor to further points of destination--lysosomes and the exterior of the cell.

Karyophilic proteins: polypeptides synthesized in vitro accumulate in the nucleus on microinjection into the cytoplasm of amphibian oocytes

The specific nucleocytoplasmic compartmentalization of proteins has been examined for some major soluble acidic nuclear proteins in oocytes of different amphibia. Proteins synthesized and radioactively labeled by translation in vitro, by using mRNA from ovaries of the frog Xenopus laevis, were injected into the cytoplasm of living oocytes of Xenopus or of the salamander Pleurodeles waltlii. At various times after injection, nucleus and cytoplasm were manually separated and endogenous and injected proteins were analyzed by two-dimensional gel electrophoresis. We show that several major nucleus-specific proteins of different sizes and electrical charges, including the very acidic proteins N1 and N2 (Mr, 110,000 and 100,000) and N4 (Mr, 34,000), are identical in both forms--i.e., as translation products in vitro and as present in the nucleoplasm. We conclude that significantly different cytoplasmic precursor forms to these nuclear proteins do not exist. The experiments indicate that (i) the translation products contain the signal(s) directing the specific sequestration of these proteins within the nucleus, (ii) post-translational processing is not required for the accumulation of these proteins in the nucleoplasm, and (iii) the signals and the mechanisms involved are not species specific.

Cell-free synthesis of phytochrome apoprotein

A single polypeptide is immunospecifically precipitated by monospecific antiphytochrome from the total translation products of both wheat-germ and rabbit-reticulocyte cell-free protein synthesizing systems programmed with oat (Avena sativa L.) poly(A) RNA. The mobility of this polypeptide is slightly lower on sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis than that of immunoaffinity-purified, 118 kdalton phytochrome and corresponds to an apparent molecular weight of 124 kdalton. Evidence against the possibility that this mobility difference results from intracellular processing of the 124-kdalton protein is provided by extraction of freeze-dried tissue directly into boiling SDS-containing buffer. This procedure yields a phytochrome species with a mobility on SDS polyacrylamide gel electrophoresis indistinguishable from that of the in-vitro translation product. Together the data indicate that the phytochrome polypeptide is synthesized in its mature form in the cell but is subject to modification to a form with lower apparent molecular weight during immunopurification.

Membrane differentiation in the Golgi apparatus of mammalian urinary bladder epithelium

The endomembrane system in superficial and intermediate epithelial cells of mammalian urinary bladder was studied by cytochemistry, thin-section and freeze-fracture electron microscopy to determine the sites where special forms of membrane differentiation first appear. Glutaraldehyde-resistant NADH-ferricyanide reductase, distinctive 11-12 nm intramembrane particles (IMP), and asymmetry of membrane leaflets served as markers of membrane maturation. The three markers were specifically associated with the maturing face of Golgi apparatus and were absent from the remainder of the endomembrane system. Activity of this enzyme was associated with the lateral regions of the maturing face, fusiform vesicles, and the plasmalemma. Asymmetric unit membrane (AUM) plaques were not observed in the Golgi apparatus per se but were present in immature fusiform vesicles that had not detached from the maturing face. When freeze-fracture replicas and thin sections were compared, randomly arranged 11-12 nm IMP first appeared in maturing face membranes that were adjacent to clusters of "free" polyribosomes in the Golgi apparatus region. The proximity of these polyribosomes suggests that they may be related to the coincident appearance of the 11-12 nm IMP in the maturing face membrane. Our observations support the hypothesis that membranes undergo differentiation during "flow" through compartments of the endomembrane system. The lateral regions of the maturing face of the Golgi apparatus appear to be a critical location for the morphogenesis of plasma membranes in urinary bladder.

Biosynthesis of mitochondrial porin and insertion into the outer mitochondrial membrane of Neurospora crassa

Mitochondrial porin, the major protein of the outer mitochondrial membrane is synthesized by free cytoplasmic polysomes. The apparent molecular weight of the porin synthesized in homologous or heterologous cell-free systems is the same as that of the mature porin. Transfer in vitro of mitochondrial porin from the cytosolic fraction into the outer membrane of mitochondria could be demonstrated. Before membrane insertion, mitochondrial porin is highly sensitive to added proteinase; afterwards it is strongly protected. Binding of the precursor form to mitochondria occurs at 4 degrees C and appears to precede insertion into the membrane. Unlike transfer of many precursor proteins into or across the inner mitochondrial membrane, assembly of the porin is not dependent on an electrical potential across the inner membrane.

Carboxy terminus of polyoma middle-sized tumor antigen is required for attachment to membranes, associated protein kinase activities, and cell transformation

We have constructed a transformation-defective polyoma virus mutant (Py 1387-T) that directs the synthesis of a normal small tumor antigen, a functional large tumor antigen, and a truncated (51,000-dalton) middle-sized tumor (mT) antigen that lacks 37 amino acids at its COOH terminus. The shortened mT polypeptide is missing the hydrophobic "tail" thought to be responsible for the anchorage of this protein into the plasma membrane and is in fact in cytosol fractions. This truncated mT polypeptide is inactive in an in vitro protein kinase assay and is altered in its phosphorylation in vivo. Mutant 1387-T differs from wild-type virus in having a T.A base pair instead of a C.G base at nucleotide position 1387. This change was introduced into viral DNA by using a synthetic undecanucleotide as a specific mutagen. Wild-type polyoma DNA was rendered single stranded by molecular cloning into coliphage M13. The oligonucleotide, which hybridizes with a mismatch at the site to be altered, was used to prime the synthesis of double-stranded closed circular DNA. Progeny recombinant phage were screened by DNA sequence analysis for the desired base change. The polyoma mutant was reconstructed from recombinant phage replicative form DNA molecules containing the mutation.

gamma Heavy chain disease in man: cDNA sequence supports partial gene deletion model

Human gamma heavy chain disease (HCD) is characterized by the presence in serum of a short monoclonal Ig gamma chain unattached to light chains. Although most HCD proteins have internal deletions, in some the defect is NH2-terminal. The OMM gamma 3 HCD serum protein is of the latter type, having undergone an extensive NH2-terminal deletion with a sequence starting within the hinge. A cell line synthesizing the OMM protein has enabled us to study the biogenesis of the abnormal molecule. In vitro translation of isolated mRNA yields a protein containing a hydrophobic NH2-terminal leader sequence. In the intact cell, the precursor molecule is processed normally to yield a protein with an NH2-terminal sequence homologous to the beginning of the variable (V) region. The nucleotide sequence of cDNA prepared from the OMM mRNA encodes a 19-amino acid leader followed by the first 15 residues of the V region. An extensive internal deletion encompasses the remainder of the V and the entire CH1 domain. Immediately following the short V region, there is information in the cDNA for the entire normal hinge. The primary synthetic product is thus an internally deleted molecule that undergoes postsynthetic degradation to yield the NH2-terminally deleted serum protein. The structure of the OMM mRNA suggests that the protein abnormality results from a partial gene deletion rather than defective splicing.

Recovery of ribophorins and ribosomes in "inverted rough" vesicles derived from rat liver rough microsomes

Treatment of rat liver rough microsomes (3.5 mg of protein/ml) with sublytical concentrations (0.08%) of the neutral detergent Triton X-100 caused a lateral displacement of bound ribosomes and the formation of ribosomal aggregates on the microsomal surface. At slightly higher detergent concentrations (0.12-0.16%) membrane areas bearing ribosomal aggregates invaginated into the microsomal lumen and separated from the rest of the membrane. Two distinct classes of vesicles could be isolated by density gradient centrifugation from microsomes treated with 0.16% Triton X-100: one with ribosomes bound to the inner membrane surfaces ("inverted rough" vesicles) and another with no ribosomes attached to the membranes. Analysis of the fractions showed that approximately 30% of the phospholipids and 20-30% of the total membrane protein were released from the membranes by this treatment. Labeling with avidin-ferritin conjugates demonstrated that concanavalin A binding sites, which in native rough microsomes are found in the luminal face of the membranes, were present on the outer surface of the inverted rough vesicles. Freeze-fracture electron microscopy showed that both fracture faces had similar concentrations of intramembrane particles. SDS PAGE analysis of the two vesicle subfractions demonstrated that, of all the integral microsomal membrane proteins, only ribophorins I and II were found exclusively in the inverted rough vesicles bearing ribosomes. These observations are consistent with the proposal that ribophorins are associated with the ribosomal binding sites characteristic of rough microsomal membranes.

A family of genes that codes for ELH, a neuropeptide eliciting a stereotyped pattern of behavior in Aplysia

We describe a particularly advantageous experimental system for studying gene structure, expression and modulation in the nervous system. In the marine mollusc Aplysia, the bag cells, two discrete clusters of neurons, secrete a peptide of known behavioral function. This neuroactive peptide, egg-laying hormone (ELH), produces a characteristic and stereotypic behavioral repertoire, consisting first of a cessation of walking and inhibition of feeding, followed by head waving and egg laying. We have cloned the genes encoding ELH and characterized their organization and expression. At least five distinct genes for ELH exist within the chromosome. Sequence analysis of one recombinant clone unambiguously identifies a contiguous stretch of nucleotides that encodes the 36 amino acids of ELH. Transcription of this small multigene family results in the expression of at least five distinct RNA transcripts encoding ELH. The pattern of transcripts differs strikingly in different tissues: bag cells express three distinct mRNA species, whereas the atrial gland, a secretory reproductive gland, expresses two distinct mRNAs. Several other neuronal and nonneuronal tissues do not express ELH RNA. In vitro these mRNAs produce a series of long polypeptide precursors that must be processed to generate the active ELH peptide. This processing event is likely to generate several additional neuroactive peptides. Thus the same peptide, ELH, may be released in association with different combinations of other neuroactive peptides. The concept of combinatorial sets of neuropeptides, each bearing one overlapping peptide ELH, and each directing a differing pattern of behavior, greatly expands the information potential of a small set of genes.

Retinal ligatin recognizes glycoproteins bearing oligosaccharides terminating in phosphodiester-linked glucose

Ligatin is a filamentous, baseplate protein that binds and localizes peripheral glycoproteins to the external cell surface. Glycoproteins coisolated with ligatin from embryonic chicken neural retina and radiolabeled with 32P are retained by an affinity column containing covalently bound retinal ligatin. Elution is achieved preferentially by alpha-glucose 1-phosphate and, to a limited extent, by mannose 6-phosphate. Treatment with endo-beta-N-acetylglucosaminidase H prevents the proteins from binding to the column and results in the release of high-mannose-type oligosaccharides containing 32P. The simplest of these oligosaccharides is unaffected by alkaline phosphatase unless the treatment is preceded by mild acid hydrolysis. Enzymatic and chemical analyses suggest that the phosphate is present in phosphodiester bonds linking penultimate mannose residues to terminal glucose residues.

The purification of M13 procoat, a membrane protein precursor

Many membrane proteins and most secreted proteins are initially made as precursors with an N-terminal leader sequence. We now report the isolation of M13 procoat, the precursor of the membrane-bound form of M13 coat protein. There are 40 000 copies of M13 procoat protein/cell during M13 amber 7 virus infection. Purified procoat is quantitatively cleaved by isolated leader peptidase to yield mature-length coat protein. Rabbit antibodies to M13 procoat will precipitate procoat but not coat, suggesting that the antibody molecules are specifically recognizing the leader sequence or the conformation which it induces in the whole procoat molecule.