Mechanisms for the incorporation of proteins in membranes and organelles

On the amino acid sequence of cytochrome P-450 isozyme 4 from rabbit liver microsomes

Isozyme 4 of rabbit liver microsomal cytochrome P-450 was shown earlier in this laboratory to contain multiple NH2-terminal residues, whereas isozymes 2, 3a, 3b, and 3c have single, unique NH2-terminal sequences. Similar results were obtained with isozyme 4 obtained from animals that were untreated, treated with phenobarbital (which does not induce this isozyme), or induced with beta-naphthoflavone or isosafrole. With the use of selective chemical blocking at seryl or at nonprolyl residues, the complexity of the NH2-terminal sequence has now been shown to be due to the presence of three forms of the cytochrome differing only in the absence of the first or the first two residues: NH2-Ala-Met-Ser-Pro-Ala-Ala-Pro-, NH2-Met-Ser-Pro-Ala-Ala-Pro-, and NH2-Ser-Pro-Ala-Ala-Pro-. These forms may result from variable biological processing. Peptides containing the seven cysteine residues were sequenced and compared with similar peptides reported for other P-450 cytochromes; homology was extensive with respect to two of the cysteine regions in isozyme 4, and a third cysteine region showed partial identity. The sequence of peptides representing about two-thirds of the amino acids in isosafrole-induced cytochrome P-450 isozyme 4 was determined. Comparison with phenobarbital-induced rabbit cytochrome P-450 isozyme 2 indicated about 25% homology. In contrast, comparison of isozyme 4 with rat cytochrome P-450d, which is also induced by isosafrole and for which the sequence has recently been deduced from cDNA [Kawajiri, K., Gotoh, O., Sogawa, K., Tagashira, Y., Muramatsu, M. & Fujii-Kuriyama, Y. (1984) Proc. Natl. Acad. Sci. USA 81, 1649-1653], showed about 70% homology.

Somatostatin discriminates between the intracellular pathways of secretory and membrane proteins

Somatostatin is a14-amino acid peptide hormone that inhibits the secretion of a variety of other polypeptide hormones, including growth hormone. Here we describe an experimental system used to determine whether somatostatin can discriminate in its inhibition between secretory and plasma membrane proteins. Growth hormone-secreting cells (GH3) were infected with vesicular stomatitis virus and pulse-chased with [35S]methionine to follow the simultaneous intracellular transit of growth hormone and the viral membrane glycoprotein, G protein. Secretion of growth hormone was monitored by immunoprecipitation of chase media, while appearance of G protein on the plasma membrane was detected by cell surface labeling and virus purification. In the presence of somatostatin (10 micrograms/ml), the secretion of growth hormone was inhibited by 80%. In contrast, G protein appeared on the plasma membrane with slightly enhanced kinetics. When cells were treated with the ionophore monensin (0.2 microM), there was a dramatic inhibition of both the secretion of growth hormone and the incorporation of G protein into plasma membranes. Our results on the differential effect of somatostatin provide evidence for sorting of secretory and membrane proteins into distinct compartments in the secretory pathway. The data further suggest that this sorting event occurs late in the Golgi complex or after proteins exit from that organelle.

Biosynthesis of microvillar proteins

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Isolation and characterization of a sperm-specific gene family in the nematode Caenorhabditis elegans

The major sperm protein (MSP) of the nematode Caenorhabditis elegans is a low-molecular-weight (15,000) basic protein implicated in the pseudopodial movement of mature spermatozoa. Its synthesis occurs in a specific region of the gonad and is regulated at the level of transcription (M. Klass and D. Hirsh, Dev. Biol. 84:299-312, 1981; S. Ward and M. Klass, Dev. Biol. 92:203-208, 1982; Klass et al., Dev. Biol. 93:152-164, 1982). A developmentally regulated gene family has been identified that codes for this MSP. Whole genomic blots, as well as analysis of genomic clone banks, indicate that there are between 15 and 25 copies of the MSP gene in the nematode genome. Southern blot analysis also indicates that there is no rearrangement or amplification within the MSP gene family during development. No evidence was found of methylation at various restriction sites surrounding the MSP gene family, and similarly, no correlation between methylation and expression was observed. Three distinct members of this MSP gene family have been cloned, and their nucleotide sequences have been determined. Differential screening of a cDNA clone bank made from polyadenylated mRNA from adult males yielded 45 male-specific clones, 32 of which were clones of MSP genes. One of these cDNA clones was found to contain the entire nucleotide sequence for the MSP, including part of the 5' leader and all of the 3' trailing sequence. Genomic clones bearing copies of the MSP genes have been isolated. At least one of the members of this gene family is a pseudogene. Another member of the MSP gene family that has been cloned from genomic DNA contains the entire uninterrupted structural sequence for the MSP in addition to a 5' flanking sequence containing a promoter-like region with the classic TATA box, a sequence resembling the CAAT box, and a putative ribosome-binding sequence. The 3' trailing sequences of the genomic and the cDNA clones contain an AATAAA polyadenylation site.

Differences between translation products of tick-borne encephalitis virus RNA in cell-free systems from Krebs-2 cells and rabbit reticulocytes: involvement of membranes in the processing of nascent precursors of flavivirus structural proteins

Tick-borne encephalitis virus (TBEV) RNA was translated in extracts from Krebs-2 cells and in rabbit reticulocyte lysates. In the former system, two polypeptides, p53 and p13, corresponding to envelope (E) and core (C) proteins of the virion, respectively, were synthesized preferentially. In contrast, the major product in reticulocyte lysates was represented by a heterogeneous set of high-molecular-weight polypeptides which did not appear to include p53 or p13. The reticulocyte lysates, however, acquired the ability to produce structural proteins (p53 at least) after addition of purified membranes isolated from the rough endoplasmic reticulum of Krebs-2 cells. On the other hand, the ability of Krebs-2 extracts to generate identifiable viral structural proteins was lost after degradation of membranes by the nonionic detergent Triton X-100. These findings strongly suggest that membrane-dependent processing of protein precursors is involved in the formation of TBEV structural proteins. Evidence has been obtained that only nascent precursor polypeptides can be processed efficiently into structural proteins in the membrane-dependent reaction.

Microinjection of cultured cells using red-cell-mediated fusion and osmotic lysis of pinosomes: a review of methods and applications

Proteins and other macromolecules can be injected into cultured cells by several different methods. Here we review the strengths and limitations of two of these methods, red-cell-mediated microinjection and osmotic lysis of pinosomes, and indicate how they may be successfully applied to the study of cultured cells.

Signal recognition particle is required for co-translational insertion of cytochrome P-450 into microsomal membranes

Insertion of newly synthesized P-450(1), the major phenobarbital-inducible form of rabbit liver microsomal cytochrome P-450, into microsomal membranes was studied in a wheat germ cell-free translation system programed with total RNA from the liver of a phenobarbital-treated rabbit. P-450(1) synthesized in vitro had the same molecular weight as the mature molecule and was co-translationally inserted into dog pancreas rough microsomal membranes. In the presence of salt-washed microsomes, instead of unwashed ones, the insertion was greatly diminished. It could, however, be restored by supplementation of the system with purified signal recognition particle (SRP), a known component of the membrane translocation machinery for secretory proteins. In the absence of microsomes, SRP inhibited the translation of mRNA encoding P-450(1) and this translation arrest was released by the addition of salt-washed microsomes. On the other hand, SRP did not affect the translation of mRNAs encoding yeast porin and reticulocyte globin, which are mitochondrial and cytosolic proteins, respectively. We conclude that co-translational insertion of P-450(1) into microsomal membranes requires SRP and postulate that P-450(1) possesses an uncleavable signal sequence that can be recognized by SRP.

Isolation and characterisation of a cDNA encoding the precursor for human apolipoprotein AII

cDNA clones encoding human apolipoprotein AII have been isolated from an adult liver cDNA library. Apo AII mRNA was shown to be approximately 600 bases in length by RNA blot hybridisation. The intracellular precursor of apo AII was inferred from the cDNA sequence to be a 100 amino acid polypeptide consisting of the 77 residue mature protein and an additional 23 amino terminal residues. The amino terminal extension, divisible into an 18 residue signal peptide and a 5 residue propeptide, is separated from the first amino acid of mature apo AII by dibasic residues. The 5' untranslated region of the message is 61 bases in length and the 3' untranslated region 113 bases. A polyadenylation signal is situated 14 bases 3' of the poly(A) tail.

Compartmentalization of intracellular membrane glycocomponents is revealed by fracture-label

We used thin-section fracture-label to determine the distribution of wheat-germ agglutinin binding sites in intracellular membranes of secretory and nonsecretory rat tissues as well as in human leukocytes. In all cases, analysis of the distribution of wheat germ agglutinin led to the definition of two endomembrane compartments: one, characterized by absence of the label, includes the membranes of mitochondria and peroxisomes as well as those of the endoplasmic reticulum and nuclear envelope; the other, strongly labeled, comprises the membrane of lysosomes, phagocytic vacuoles, and secretory granules, as well as the plasma membrane. The Golgi apparatus was weakly labeled in all studied tissues.

Characterisation of mRNAs encoding the precursor for human apolipoprotein CI

cDNA clones encoding human apolipoprotein CI have been isolated from an adult liver cDNA library. Apo CI mRNA was shown to have two species of approximately 580 and 560 bases by RNA blot hybridisation. The intracellular precursor of apo CI was inferred from the cDNA sequence to be an 83 amino acid polypeptide consisting of the 57 residue mature protein and an additional 26 residue amino terminal signal peptide. The 5' untranslated regions of the messages are 63 and 40 bases as determined by primer extension and the 3' untranslated region 111 bases. A polyadenylation signal is situated 10 bases 3' of the poly(A) tall. The mRNA level of apo CI in human liver was significantly greater than that of apo All and apo E.

Glucose removal from N-linked oligosaccharides is required for efficient maturation of certain secretory glycoproteins from the rough endoplasmic reticulum to the Golgi complex

1- Deoxynojirimycin is a specific inhibitor of glucosidases I and II, the first enzymes that process N-linked oligosaccharides after their transfer to polypeptides in the rough endoplasmic reticulum. In a pulse-chase experiment, 1- deoxynojirimycin greatly reduced the rate of secretion of alpha 1-antitrypsin and alpha 1-antichymotrypsin by human hepatoma HepG2 cells, but had marginal effects on secretion of the glycoproteins C3 and transferrin, or of albumin. As judged by equilibrium gradient centrifugation, 1- deoxynojirimycin caused alpha 1-antitrypsin and alpha 1-antichymotrypsin to accumulate in the rough endoplasmic reticulum. The oligosaccharides on cell-associated alpha 1-antitrypsin and alpha 1-antichymotrypsin synthesized in the presence of 1- deoxynojirimycin , remained sensitive to Endoglycosidase H and most likely had the structure Glu1- 3Man9GlcNAc2 . Tunicamycin, an antibiotic that inhibits addition of N-linked oligosaccharide units to glycoproteins, had a similar differential effect on secretion of these proteins. Swainsonine , an inhibitor of the Golgi enzyme alpha-mannosidase II, had no effect on the rates of protein secretion, although the proteins were in this case secreted with an abnormal N-linked, partially complex, oligosaccharide. We conclude that the movement of alpha 1-antitrypsin and alpha 1-antichymotrypsin from the rough endoplasmic reticulum to the Golgi requires that the N-linked oligosaccharides be processed to at least the Man9GlcNAc2 form; possibly this oligosaccharide forms part of the recognition site of a transport receptor for certain secretory proteins.

Characterization of translation products of the polyadenylated RNA of free and membrane-bound polyribosomes of rat forebrain

Poly(A)+ RNA (polyadenylated RNA) isolated from membrane-bound and free polyribosomes was translated in reticulocyte lysates, and the products were analysed by two-dimensional gel electrophoresis. Several translation products were specific to membrane-bound polyribosomal mRNA, including polypeptides of 47kDa, 35kDa and 21 kDa, whereas others (e.g. of 37 kDa, 17 kDa and 14 kDa) were specific to free polyribosomal mRNA. Although many products were common to both mRNA species, cross-contamination could be ruled out on the basis of the presence of these and other specific products. The common products included a 68 kDa microtubule-associated protein, tubulin, actin, the brain form of creatine kinase, neuron-specific enolase and protein 14-3-3 and calmodulin, all of which were identified on the basis of two-dimensional gel and peptide analyses. The 35 kDa protein product of membrane-specific mRNA was co-translationally processed in vitro by microsomal membranes, resulting in its cleavage to 33 kDa (and partial glycosylation). The 33 kDa processed protein (but not the 35 kDa precursor) was integrated into both dog pancreas and rat brain microsomal membranes. The occurrence of the enzymes and calmodulin as products of membrane-bound polyribosomal mRNA is discussed in the light of their presence on rat brain synaptic plasma membranes [Lim, Hall, Leung, Mahadevan & Whatley (1983) J. Neurochem. 41, 1177-1182] and their existence in a specific component of axonal flow. It is suggested that some of these translation products of the rough endoplasmic reticulum may represent proteins destined for the plasma membrane. However, the identity and location of the 35 kDa membrane-specific product (or its processed form) still remain unestablished.

Biochemical studies of the maturation of the small Sindbis virus glycoprotein E3

A small glycoprotein (E3) was purified from the culture fluid of Sindbis virus-infected primary chick embryo fibroblasts. Tryptic peptide mapping and pulse-chase studies verified that this protein was produced as a by-product of the cleavage of the precursor protein PE2 to produce the envelope glycoprotein E2. A 2600-fold purification was achieved via a procedure which used differential ethanol precipitation, gel filtration, ion-exchange chromatography, and affinity chromatography on a lentil lectin column. Amino acid composition analysis, N-terminal microsequencing, and labeling studies yielded information about the fine structure of E3 and its relationship to E2 and virion maturation. The N-terminal sequence of E3 is identical to that of PE2, including the result that 90% of the molecules appear to be blocked. The first 19 amino acids are uncharged and presumably serve as the signal sequence for the insertion of PE2 into the membrane of the endoplasmic reticulum, but this sequence is unusual in that it is not immediately cleaved from PE2 and is glycosylated at the asparagine at position 14. The two residues at the C-terminus of E3, Lys-Arg, are removed during or shortly after cleavage from PE2. Labeling studies imply that, although the PE2----E2 + E3 cleavage is necessary for virion budding, these two events are not closely coupled. E3 is cleaved and released into the culture fluid under conditions where virions do not bud, and the kinetics of the appearance of E3 in the culture fluid and E2 in virions are quite dissimilar. The maturation of E3 is discussed as it relates to the processing of cellular membrane and secretory glycoproteins.

The sequences of the N protein gene and intergenic region of the S RNA of pichinde arenavirus

Two overlapping DNA clones representing more than half of the Pichinde arenavirus S RNA segment were cloned into pBR322 and their nucleotide sequences were determined. The analyses predict that the viral nucleocapsid protein (N) is encoded in a reading frame in the viral complementary RNA sequence starting at viral S RNA nucleotide residue 84 from the 3' end and terminating with an opal codon at residues 1767-1769. The position of the termination codon has been confirmed by primer directed dideoxynucleotide sequencing. The N protein has a calculated size of 62,911 Da and a net positive charge of +9. Viral complementary 15 S mRNA that directs the synthesis of N protein and hybridizes to the predicted N gene DNA has been identified in infected cell extracts. A second nonoverlapping reading frame in the viral complementary sequence originates at nucleotide position 1827 and remains open for at least 71 amino acids (i.e., the extent of the second clone). A long stretch of hydrophobic amino acids is near the amino terminus of this predicted gene product. Between the two reading frames is a 60-nucleotide-long noncoding intergenic region. This nucleotide sequence can be arranged in hairpin configuration involving 14 G-C and 4 A-U base pairs. The possible function of this intergenic region in the regulation of transcription and/or translation is discussed.

Coronavirus multiplication: locations of genes for virion proteins on the avian infectious bronchitis virus genome

Six overlapping viral RNAs are synthesized in cells infected with the avian coronavirus infectious bronchitis virus (IBV). These RNAs contain a 3'-coterminal nested sequence set and were assumed to be viral mRNAs. The seven major IBV virion proteins are all produced by processing of three polypeptides of ca. 23, 51, and 115 kilodaltons. These are the core polypeptides of the small membrane proteins, the nucleocapsid protein, and the 155-kilodalton precursor to the large membrane proteins GP90 and GP84, respectively. To determine which mRNAs specify these polypeptides, we isolated RNA from infected cells and translated it in a messenger-dependent rabbit reticulocyte lysate. Proteins of 23, 51, and 110 kilodaltons were produced. Two-dimensional tryptic peptide mapping demonstrated that these proteins were closely related to the major virion proteins. Fractionation of the RNA before cell-free translation permitted the correlation of messenger activities for synthesis of the proteins with the presence of specific mRNAs. We found that the smallest RNA, RNA A, directs the synthesis of P51, the nucleocapsid protein. RNA C, which contains the sequences of RNA A, directs the synthesis of the small membrane protein P23. RNA E directs the synthesis of the large virion glycoproteins. These results supported a model in which only the unique 5'-terminal domain of each IBV mRNA is active in translation and enabled us to localize genes for virion proteins on the IBV genome.

Components of the plasma membrane of growing axons. II. Diffusion of membrane protein complexes

Intramembrane particles (IMPs) of the plasmalemma of mature, synapsing neurons are evenly distributed along the axon shaft. In contrast, IMPs of growing olfactory axons form density gradients: IMP density decreases with increasing distance from the perikarya, with a slope that depends upon IMP size (Small, R., and K. H. Pfenninger, 1984, J. Cell Biol., 98: 1422-1433). These IMP density gradients resemble Gaussian tails, but they are much more accurately described by the equations formulated for diffusion in a system with a moving boundary (a Stefan Problem), using constants that are dependent upon IMP size. The resulting model predicts a shallow, nearly linear IMP density profile at early stages of growth. Later, this profile becomes gradually transformed into a steep nonlinear gradient as axon elongation proceeds. This prediction is borne out by the experimental evidence. The diffusion coefficients calculated from this model range from 0.5 to 1.8 X 10(-7) cm2/s for IMPs between 14.8 and 3.6 nm, respectively. These diffusion coefficients are linearly dependent upon the inverse IMP diameter in accordance with the Stokes-Einstein relationship. The measured viscosity is approximately 7 centipoise. Our findings indicate (a) that most IMPs in growing axons reach distal locations by lateral diffusion in the plasma membrane, (b) that IMPs--or complexes of integral membrane proteins--can diffuse at considerably higher rates than previously reported for iso-concentration systems, and (c) that the laws of diffusion determined for macroscopic systems are applicable to the submicroscopic membrane system.

Viral glycoproteins destined for apical or basolateral plasma membrane domains traverse the same Golgi apparatus during their intracellular transport in doubly infected Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells can sustain double infection with pairs of viruses of opposite budding polarity (simian virus 5 [SV5] and vesicular stomatitis virus [VSV] or influenza and VSV), and we observed that in such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at their characteristic sites. Influenza and SV5 budded exclusively from the apical plasma membrane of the cells, while VSV emerged only from the basolateral surfaces. Immunoelectron microscopic examination of doubly infected MDCK cells showed that the influenza hemagglutinin (HA) and the VSV G glycoproteins traverse the same Golgi apparatus and even the same Golgi cisternae. This indicates that the pathways of the two proteins towards the plasma membrane do not diverge before passage through the Golgi apparatus and therefore that critical sorting steps must take place during or after passage of the glycoproteins through this organelle. After its passage through the Golgi, the HA accumulated primarily at the apical membrane, where influenza virion assembly occurred. A small fraction of HA did, however, appear on the lateral surface and was incorporated into the envelope of budding VSV virions. Although predominantly found on the basolateral surface, significant amounts of G protein were observed on the apical plasma membrane well before disruption of the tight junctions was detectable. Nevertheless, assembly of VSV virions was restricted to the basolateral domain and in doubly infected cells the G protein was only infrequently incorporated into the envelope of budding influenza virions. These observations indicate that the site of VSV budding is not determined exclusively by the presence of G polypeptides. Therefore, it is likely that, at least for VSV, other cellular or viral components are responsible for the selection of the appropriate budding domain.

The complete sequence of the mRNA for the HLA-DR-associated invariant chain reveals a polypeptide with an unusual transmembrane polarity

A non-polymorphic polypeptide is associated intracellularly with the alpha and beta chains of murine Ia antigens and of human HLA-DR antigens. The exact role and the structure of this invariant chain have not been determined so far. A cDNA clone encoding the 33 000 dalton human invariant chain has been isolated. The nucleotide sequence of a near full-length cDNA clone, together with the sequence of the 5' portion of the mRNA determined by primer-extension, are reported here. The protein structure deduced from that sequence shows an unusual feature: the presence of a hydrophobic transmembrane region near the NH2 terminus, and of two glycosylation sites near the middle, indicates that the invariant chain has a polarity of membrane insertion which is inverted relative to histocompatibility antigens and most transmembrane proteins.

Glycosylation and surface expression of the influenza virus neuraminidase requires the N-terminal hydrophobic region

A full-length double-stranded DNA copy of an influenza A virus N2 neuraminidase (NA) gene was cloned into the late region of pSV2330, a hybrid expression vector that includes pBR322 plasmid DNA sequences and the simian virus 40 early region and simian virus 40 late region promoters, splice sequences, and transcription termination sites. The protein encoded by the cloned wild-type NA gene was shown to be present in the cytoplasm of fixed cells and at the surface of "live" or unfixed cells by indirect immunofluorescence with N2 monoclonal antibodies. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of [35S]methionine-labeled proteins from wild-type vector-infected cells with heterospecific N2 antibody showed that the product of the cloned NA DNA comigrated with glycosylated NA from influenza virus-infected cells, remained associated with internal membranes of cells fractionated into membrane and cytoplasmic fractions, and could form an immunoprecipitable dimer. NA enzymatic activity was detectable after simian virus 40 lysis of vector-infected cells. These properties of the product of the cloned wild-type gene were compared with those of the polypeptides produced by three deletion mutant NA DNAs that were also cloned into the late region of the pSV2330 vector. These mutants lacked 7 (dlk), 21 (dlI), or all 23 amino acids (dlZ) of the amino (N)-terminal variable hydrophobic region that anchors the mature wild-type NA tetrameric structure in the infected cell or influenza viral membrane. Comparison of the phenotypes of these mutants showed that this region in the NA molecule also includes sequences that control translocation of the nascent polypeptide into membrane organelles for glycosylation.

Isolation and characterization of a sperm-specific gene family in the nematode Caenorhabditis elegans

The major sperm protein (MSP) of the nematode Caenorhabditis elegans is a low-molecular-weight (15,000) basic protein implicated in the pseudopodial movement of mature spermatozoa. Its synthesis occurs in a specific region of the gonad and is regulated at the level of transcription (M. Klass and D. Hirsh, Dev. Biol. 84:299-312, 1981; S. Ward and M. Klass, Dev. Biol. 92:203-208, 1982; Klass et al., Dev. Biol. 93:152-164, 1982). A developmentally regulated gene family has been identified that codes for this MSP. Whole genomic blots, as well as analysis of genomic clone banks, indicate that there are between 15 and 25 copies of the MSP gene in the nematode genome. Southern blot analysis also indicates that there is no rearrangement or amplification within the MSP gene family during development. No evidence was found of methylation at various restriction sites surrounding the MSP gene family, and similarly, no correlation between methylation and expression was observed. Three distinct members of this MSP gene family have been cloned, and their nucleotide sequences have been determined. Differential screening of a cDNA clone bank made from polyadenylated mRNA from adult males yielded 45 male-specific clones, 32 of which were clones of MSP genes. One of these cDNA clones was found to contain the entire nucleotide sequence for the MSP, including part of the 5' leader and all of the 3' trailing sequence. Genomic clones bearing copies of the MSP genes have been isolated. At least one of the members of this gene family is a pseudogene. Another member of the MSP gene family that has been cloned from genomic DNA contains the entire uninterrupted structural sequence for the MSP in addition to a 5' flanking sequence containing a promoter-like region with the classic TATA box, a sequence resembling the CAAT box, and a putative ribosome-binding sequence. The 3' trailing sequences of the genomic and the cDNA clones contain an AATAAA polyadenylation site.

Identification of distinct messenger RNAs for nuclear lamin C and a putative precursor of nuclear lamin A

The lamins are the major components of the nuclear matrix and are known as lamins A, B, and C with Mr 72,000, 68,000, and 62,000 when analysed by SDS PAGE. These three polypeptides are very similar, as determined by polypeptide mapping and immunological reactivity. Lamins A and C are so homologous that a precursor-product relationship has been proposed. Using an antiserum against nuclear matrix proteins that specifically immunoprecipitates the three lamins, we examined their synthesis in the rabbit reticulocytes lysate. Four bands of Mr 62,000, 68,000, 70,000, and 74,000 were specifically immunoprecipitated when polysomes or polyadenylated RNA were translated in vitro. By two-dimensional gel electrophoresis, the 68,000- and the 62,000-mol-wt proteins were identified as lamins B and C, respectively, and the 74,000-mol-wt polypeptide had properties of a precursor of lamin A. The mRNAs of lamin C and of the putative precursor of lamin A were completely separated by gel electrophoresis under denaturing conditions, and their respective sizes were determined. These results suggest that lamin A is not a precursor of lamin C.

Plasma membrane biogenesis in eukaryotic cells: translocation of newly synthesized lipid

We examined the transfer of sterols and phospholipids from their site of synthesis to the plasma membrane of Acanthamoeba castellanii. Cells were labeled with [3H]acetate, and plasma membrane fractions were isolated under conditions that minimize the nonspecific exchange of lipids between subcellular membrane fractions. Sterols and phospholipids were purified from both whole-cell homogenates and isolated plasma membrane. In whole cells, 3H-labeled lipids were formed, with no apparent time lag, in a linear manner up to 1 hr. Labeled sterol and phospholipids appeared in the plasma membrane, after a 30-min lag, at approximately the same rate. However, the ratio of newly synthesized sterol to phospholipid was significantly enriched in the plasma membrane relative to the whole cell, even at the earlier time points. Pulse-chase experiments indicated that sterols and phospholipids are turned over in the plasma membrane with similar, rather short half-lives. The results of these studies suggest that, although sterols and phospholipids are transported to the cell surface with similar kinetics, some sorting of the lipids must occur at an early stage in membrane biogenesis. The data are consistent with a model of lipid translocation by vesicular transport.

Identification of the proteins exposed on the cytoplasmic surface of the pancreatic zymogen granule

Lactoperoxidase-catalyzed 125I-iodination was used to label pancreatic zymogen granules. Membrane proteins facing the cytoplasmic surface were specifically labeled. Two low molecular weight proteins of 17 000 and 15 000 were intensely labeled at 0 degree C. Another small 13 kDa protein was strongly iodinated at 25 degrees C along with some others, including the 29 kDa subunit of the ATP diphosphohydrolase. The major glycoprotein of the granule membrane was not iodinated but the presence of an iodinated 80 kDa protein suggests that proteolytic fragments of the 92 kDa glycoprotein were accessible to iodination on the intact granule. These proteins localized on the cytoplasmic surface of the granule are believed to play a major role in the exocytotic phenomenon of the exocrine pancreas.

The integral and peripheral proteins of the zymogen granule membrane

This study reports the lipid and protein composition of purified pig zymogen granule membranes. Lipids made up more than 70% of the membrane dry weight and phosphatidylserine constituted 17% of the total phospholipids. The membrane was shown to contain nine major proteins. A protein of Mr 92 000 was the major constituent accounting for more than 25% of the Coomassie blue staining on gels. Six glycoproteins, including the latter, were revealed by periodic acid-Schiff staining and concanavalin A binding. A phase separation technique using partition of intrinsic and extrinsic membrane proteins in Triton X-114 solutions has shown that most of the proteins were integral membrane proteins. The ATP diphosphohydrolase, which is distinctive of the zymogen granule membrane, segregated with integral proteins. These data constitute a detailed description of a purified membrane fraction from pig pancreatic zymogen granules.

18B1: an axon-specific antigen associated with many proteins

We describe an antigen, 18B1, defined by a monoclonal antibody. Immunoperoxidase staining of brain sections shows that 18B1 is selectively associated with neurofilament-rich axons. Antibody staining of sodium dodecyl sulphate-gel blots, on the other hand, shows that 18B1 is associated with a large number of proteins, none of which are structural components of brain neurofilaments. The antigen is sensitive to a variety of proteases but is not degraded by various glycosidases, suggesting that 18B1 is probably an amino acid sequence. Its association with neurofilament-rich axons together with its absence from neurofilaments themselves suggests that it may be involved in mediating interactions between neurofilaments and the proteins that bear it.

Are annulate lamellae in the Drosophila embryo the result of overproduction of nuclear pore components?

Annulate lamellae are cytoplasmic organelles composed of stacked sheets of membrane containing pores that are structurally indistinguishable from nuclear pores. The functions of annulate lamellae are not well understood. Although they may be found in virtually any eucaryotic cell, they occur most commonly in transformed and embryonic tissues. In Drosophila, annulate lamellae are found in the syncytial blastoderm embryo as it is cleaved to form the cellular blastoderm. The cytological events of the cellularization process are well documented, and may be used as temporal landmarks when studying changes in annulate lamellae. By using morphometric techniques to analyze electron micrographs of embryos, we are able to calculate the number of pores per nucleus in nuclear envelopes and annulate lamellae during progressive stages of cellularization. We find that annulate lamellae pores remain at a low level while nuclear envelopes are expanding and acquiring pores in early interphase. Once nuclear envelopes are saturated with pores, however, the number of annulate lamellae pores increases more than 10-fold in 9 min. Over the next 30 min it gradually declines to the initial low level. On the basis of these results, we propose (a) that pore synthesis and assembly continues after nuclear envelopes have been saturated with pores; (b) that these supernumerary pores accumulate transiently in cytoplasmic annulate lamellae; and (c) that because these pores are not needed by the embryo they are subsequently degraded.

Do hydrophobic sequences cleaved from cellular polypeptides induce membrane fusion reactions in vivo?

The concept that a direct interaction between Ca2+ and phospholipids is a major factor in membrane fusion reactions is questioned. Attention is drawn to a number of findings on associations between fusion and the proteolysis of membrane proteins. It is proposed that hydrophobic polypeptides, which are functionally comparable to the fusogenic proteins of certain viruses but which are produced in cells by the endogenous proteolysis of membrane and cellular proteins, may induce membrane fusion reactions in vivo.

Interaction of synthetic signal sequence fragments with model membranes

Peptide fragments corresponding to the signal sequence of chicken lysozyme, labelled with the fluorescent 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) group have been synthesized. The emission characteristics and fluorescence polarization of the dansyl group have been used to study the interaction of signal sequence fragments with liposomes. The peptide fragments bind to liposomes and are associated with the hydrophobic core of the bilayer.

Intracellular transport of influenza virus hemagglutinin to the apical surface of Madin-Darby canine kidney cells

The intracellular pathway followed by the influenza virus hemagglutinin (HA) to the apical surface of Madin-Darby canine kidney cells was studied by radioimmunoassay, immunofluorescence, and immunoelectron microscopy. To synchronize the migration, we used a temperature-sensitive mutant of influenza WSN, ts61, which, at the nonpermissive temperature, 39.5 degrees C, exhibits a defect in the HA that prevents its exit from the endoplasmic reticulum. Upon transfer to permissive temperature, 32 degrees C, the HA appeared in the Golgi apparatus after 10 min, and on the apical surface after 30-40 min. In the presence of cycloheximide, the expression was not inhibited, indicating that the ts defect is reversible; a wave of HA migrated to the cell surface, where it accumulated with a half time of 60 min. After passage through the Golgi apparatus the HA was detected in a population of smooth vesicles, about twice the size of coated vesicles, located in the apical half of the cytoplasm. These HA-containing vesicles did not react with anti-clathrin antibodies. Monensin (10 microM) delayed the surface appearance of HA by 2 h, but not the transport to the Golgi apparatus. Incubation at 20 degrees C retarded the migration to the Golgi apparatus by approximately 30 min and blocked the surface appearance by acting at a late stage in the intracellular pathway, presumably at the level of the post-Golgi vesicles. The initial appearance of HA on the apical surface was in the center; no preference was observed for the tight-junctional regions.

Characterization of plasma membranes and rough endoplasmic reticulum of synovial cells cultured from rheumatoid arthritis patients

[35S]methionine-labelled plasma membrane (PM) and rough endoplasmic reticulum (RER) proteins were analysed from synovial cell cultures derived from rheumatoid arthritis (RA) and reference patients. Pure RER was prepared by a modification of CsCl-containing sucrose-cushion method. The PM proteins were resolved into 30-35 bands using sodium dodecyl sulphate-electrophoresis in polyacrylamide gradient gels. The major polypeptides had apparent molecular masses of 170, 140, 110, 60, 40, 33, 23 and 14 kilodaltons. The major RER polypeptides had the apparent molecular masses of 170, 74, 58 and 35 kD. No reproducible differences were demonstrated in the polypeptide patterns or in the specific activities of the PM and ER (endoplasmic reticulum) marker enzymes between cells from RA and reference patients. The results reflect similarities of membrane structures in RA and reference cells, although differences have been reported in their metabolism.

Glycosylation and surface expression of the influenza virus neuraminidase requires the N-terminal hydrophobic region

A full-length double-stranded DNA copy of an influenza A virus N2 neuraminidase (NA) gene was cloned into the late region of pSV2330, a hybrid expression vector that includes pBR322 plasmid DNA sequences and the simian virus 40 early region and simian virus 40 late region promoters, splice sequences, and transcription termination sites. The protein encoded by the cloned wild-type NA gene was shown to be present in the cytoplasm of fixed cells and at the surface of "live" or unfixed cells by indirect immunofluorescence with N2 monoclonal antibodies. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of [35S]methionine-labeled proteins from wild-type vector-infected cells with heterospecific N2 antibody showed that the product of the cloned NA DNA comigrated with glycosylated NA from influenza virus-infected cells, remained associated with internal membranes of cells fractionated into membrane and cytoplasmic fractions, and could form an immunoprecipitable dimer. NA enzymatic activity was detectable after simian virus 40 lysis of vector-infected cells. These properties of the product of the cloned wild-type gene were compared with those of the polypeptides produced by three deletion mutant NA DNAs that were also cloned into the late region of the pSV2330 vector. These mutants lacked 7 (dlk), 21 (dlI), or all 23 amino acids (dlZ) of the amino (N)-terminal variable hydrophobic region that anchors the mature wild-type NA tetrameric structure in the infected cell or influenza viral membrane. Comparison of the phenotypes of these mutants showed that this region in the NA molecule also includes sequences that control translocation of the nascent polypeptide into membrane organelles for glycosylation.

Effects of exogenous amines on mammalian cells, with particular reference to membrane flow

We have reviewed the evidence that amines accumulate in intracellular vesicles of low pH, such as lysosomes and endosomes. There is consequent elevation of intravesicular pH, and inhibition of receptor-ligand dissociation often results from this pH change. We have argued that the capacity for fusion of such vesicles is also reduced by the high pH. We suggest that the variety of effects of amines on membrane flow and macromolecular transport we describe are at least partly due to such reduced fusion (Figs. 1 and 2). We propose that an internal low pH may facilitate heterologous vesicle-vesicle and vesicle-plasma membrane fusion. There is some evidence that clathrin can accelerate phospholipid vesicle fusion in vitro at low pH (Blumenthal et al., 1983) but no direct evidence on the role of intravesicular pH. This idea is consistent not only with the preceding discussion, but also with the fact that the intracellular membrane-bound compartments least involved in fusion events (e.g. mitochondria) are of neutral or alkaline internal pH. Membrane fusion is certainly required for the formation of vesicles at the periphery of the Golgi apparatus, and possibly earlier in the transport and processing of biosynthetic products in the Golgi (Bergeron et al., 1982). Thus the accumulation of amines in the Golgi may be responsible for several effects on the flow of macromolecules along their translocation pathways. The status of the plasma membrane in this view is complex. It might be argued that the pH dictating the fusion step in endocytosis is that of the extracellular fluid, in which case the inhibitory effects of amines on this process are not explained. However, the rapidity of acidification of the newly formed endocytic vesicles allows the possibility that plasma membrane invaginations might temporarily sequester areas which are of lower pH than that of the bulk extracellular fluid even before fusion, since the proton pumping enzyme(s) are probably present on the plasma membrane. Were this the case, then an acid pH could again be a factor determining membrane fusion at the plasma membrane. The inhibition of endocytosis by weak bases thus may again reflect elevation of pH in a sequestered compartment. From the data on the dependence of response on the concentration of amines, we anticipate that most responses involving membrane flow will be biphasic, with inhibitory effects at low amine concentration, giving way to stimulatory ones at higher concentrations. We suggest that the reported dichotomy between different amines in intracellular membrane fusion systems (D'Arcy Hart, 1982) may result from this concentration dependence.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural localization of rotavirus antigens using colloidal gold

Colloidal gold was used to localize six of the ten known proteins of the simian rotavirus SA11 within infected cells by ultrastructural immunocytochemistry. Monospecific or monoclonal antibodies to selected structural and nonstructural proteins were the primary antisera. The major outer capsid glycoprotein, VP7, was associated with nonenveloped particles, with particles de-enveloped by Triton X-100 and with both nuclear and cytoplasmic inclusions. The protease-sensitive outer capsid protein, VP3, was also found on nonenveloped and de-enveloped particles. The major inner capsid protein, VP6, was accessible to antibodies on some of the nonenveloped particles (presumably single-shelled particles) and on the de-enveloped particles. A monospecific antibody to the gene 11 product, believed to be a precursor to a minor structural protein, VP9, reacted strongly with viroplasmic inclusions. Virus particles were weakly labeled by this antibody. NS35, a nonstructural SA11 protein, was found only in the viroplasms. NS29, a nonstructural glycoprotein, was localized to the cytoplasmic side of the endoplasmic reticulum membrane and to the inside of enveloped virus particles. These data support the hypothesis that NS29 facilitates budding of the virus particles and acquisition of the outer capsid layer.

Are different proteins synthesized in distinct domains in a neuron?

Are different messenger RNA species translated in distinct, limited domains within a cell? For the particular case of the giant R2 neuron from Aplysia californica an answer to this question is possible for the more abundantly synthesized proteins. After brief labeling with 35S-methionine, the R2 neuron soma was frozen and divided into two or three parts. The newly synthesized proteins were analyzed following two-dimensional polyacrylamide gel electrophoresis. No evidence for limited domains of synthesis for 26 abundantly synthesized polypeptides in the R2 neuron was found.

Amphipathic analysis and possible formation of the ion channel in an acetylcholine receptor

Fourier analysis of the hydrophobicities of the acetylcholine receptor subunit sequences reveals regions of amphipathic secondary structure. Prediction of a consensus secondary structure based on this analysis and on an empirical prediction method leads to a testable hypothesis about how the ion channel is formed and might function. Knowledge of the three-dimensional structure of acetylcholine receptors is consistent with features of the model proposed and provides some constraints.

Transport of proteins into mitochondria

There is still much that is obscure concerning the transport of proteins into or through the mitochondrial membrane systems. In addition, as pointed out previously, it is unlikely that the details of the process are the same for proteins destined for different compartments of the organelle. A brief summary of the process for matrix proteins might be as follows: The proteins are synthesized on free polysomes as precursors of higher molecular weight than the native forms. These precursors are liberated into the cell cytosol and subsequently translocated into the mitochondria. This timing might be different in yeast under some circumstances, synthesis being completed in association with the mitochondria. The precursors interact with a receptor in the outer mitochondrial membrane interaction being mediated by the presequences of the precursors. The presequences therefore act as addressing signals as well as possibly playing a role in one or all of (a) solubilization of precursors, (b) prevention of premature assembly into multimeric structures, or (c) maintenance of nonnative configurations required for transport. Interaction occurs with a second receptor, this time in the inner membrane of the mitochondria, interaction being with multiple sites in the polypeptide chain. Transport across the inner membrane then occurs, this transport depending on a transmembrane electrochemical gradient of which the proton component is the essential part. Transport is accompanied or followed by proteolysis of the prepiece, and formation of the native structure. While steps 1 and 2 of this sequence can be considered well established, the remaining steps are still poorly understood or purely hypothetical. Nevertheless, this sequence of events is consistent with known facts about the process and provides a framework for future investigations.