Messenger RNA of opsin from bovine retina: isolation and partial sequence of the in vitro translation product

Co-translational targeting and translocation of the amino terminus of opsin across the endoplasmic membrane requires GTP but not ATP

The tight coupling between ongoing translation and translocation across the mammalian endoplasmic reticulum has made it difficult to determine the requirements that are specific for translocation. We have developed an in vitro assay that faithfully mimics the co-translational targeting and translocation of the amino terminus of opsin without ongoing translation. Using this system we demonstrate that this post-translational targeting and translocation requires nucleotide triphosphates but not cytosolic proteins. The addition of GTP alone was sufficient to fully restore targeting. The addition of ATP was not specifically required, and non-hydrolyzable analogs of ATP that blocked 90% of the ATPase activity also had no inhibitory effect on translocation.

The opsin family of proteins

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Synthesis and maturation of recombinant human tumor necrosis factor in eukaryotic systems

The biosynthesis of human tumor necrosis factor (hTNF) was studied. The amino-terminal extension of the hTNF precursor (26 kDa polypeptide) was not cleaved off in a cell-free system supplemented with dog pancreas microsomes. Correct maturation of pre-hTNF was nevertheless not restricted to the macrophage system: in the medium of a TNF-producing, transformed CHO cell line, a (weak) approximately 20 kDa, an approximately 18.5 kDa (doublet) and a 17 kDa TNF polypeptide, the latter corresponding to mature hTNF, were revealed by specific immunoprecipitation. Similar results were obtained with Xenopus laevis oocytes, injected with hTNF mRNA, except that the 20 kDa band was lacking. The results are discussed in relation to the secretion mechanism of hTNF.

Proteolytic processing and compartmentalization of the primary translation products of mammalian apolipoprotein mRNAs

The steps involved in the initial assembly of apolipoproteins and lipids into supramolecular arrays (nascent lipoprotein particles) are largely unknown. Examination of the proteolytic processing and compartmentalization of the primary translation products of apolipoprotein mRNAs represents one approach to deciphering the molecular details of lipoprotein assembly. The structures of the primary translation products of seven mammalian apolipoprotein mRNAs has been determined in the past several years. The organization of apolipoprotein signal peptides is typical of eukaryotic prepeptides, although an unusual degree of sequence conservation is present among the signal segments of apo AI, AIV, and E. For those apolipoprotein sequences studied in detail, SRP-dependent cotranslational translocation and proteolytic processing appears to be highly efficient and results in sequestration of the processed protein within the lumen of the endoplasmic reticulum (ER). However the mechanism by which these lipid-binding proteins avoid arrest during their translocation through the lipid bilayer of the ER membrane remains obscure. The two principal human HDL apolipoproteins undergo novel extracellular post-translational proteolytic processing, which results in removal of nonhomologous propeptides. The proteases responsible for proapo AI and AII processing appear to be different. The processing of these proapolipoproteins provides a potential series of steps for regulating the ordered assembly of HDL constituents.

Bovine opsin has more than one signal sequence

By deletion of selected segments from a bovine opsin complementary DNA clone and subsequent analysis of transcripts in a cell-free translation-translocation system, we have localized two out of four theoretically conceivable signal sequences required for the integration of opsin into microsomal membranes.

Synthesis and localization of a development-specific protein in sclerotia of Sclerotinia sclerotiorum

A development-specific protein (SSP) makes up about 35 to 40% of the total protein in sclerotia of the fungus Sclerotinia sclerotiorum. The protein consists of three charge isomers, with one isomer making up 80 to 90% of the total. In vitro translation of poly(A)+ RNA isolated from cells in early stages of sclerotia formation revealed that 44% of the amino acids incorporated was into SSP. In vivo- and in vitro-synthesized forms of SSP migrated at identical rates on both isoelectric focusing and denaturing polyacrylamide gels, indicating that SSP was not synthesized as a larger precursor. This was significant because SSP accumulated in membrane-bound, organellelike structures which resemble protein bodies found in seeds of many higher plants.

Biosynthesis and processing of ribophorins in the endoplasmic reticulum

Ribophorins are two transmembrane glycoproteins characteristic of the rough endoplasmic reticulum, which are thought to be involved in the binding of ribosomes. Their biosynthesis was studied in vivo using lines of cultured rat hepatocytes (clone 9) and pituitary cells (GH 3.1) and in cell-free synthesis experiments. In vitro translation of mRNA extracted from free and bound polysomes of clone 9 cells demonstrated that ribophorins are made exclusively on bound polysomes. The primary translation products of ribophorin messengers obtained from cultured hepatocytes or from regenerating livers co-migrated with the respective mature proteins, but had slightly higher apparent molecular weights (2,000) than the unglycosylated forms immunoprecipitated from cells treated with tunicamycin. This indicates that ribophorins, in contrast to all other endoplasmic reticulum membrane proteins previously studied, contain transient amino-terminal insertion signals which are removed co-translationally. Kinetic and pulse-chase experiments with [35S]methionine and [3H]mannose demonstrated that ribophorins are not subjected to electrophoretically detectable posttranslational modifications, such as proteolytic cleavage or trimming and terminal glycosylation of oligosaccharide side chain(s). Direct analysis of the oligosaccharides of ribophorin l showed that they do not contain the terminal sugars characteristic of complex oligosaccharides and that they range in composition from Man8GlcNAc to Man5GlcNAc. These findings, as well as the observation that the mature proteins are sensitive to endoglycosidase H and insensitive to endoglycosidase D, are consistent with the notion that the biosynthetic pathway of the ribophorins does not require a stage of passage through the Golgi apparatus.

Isolation and nucleotide sequence of the gene encoding human rhodopsin

We have isolated and completely sequenced the gene encoding human rhodopsin. The coding region of the human rhodopsin gene is interrupted by four introns, which are located at positions analogous to those found in the previously characterized bovine rhodopsin gene. The amino acid sequence of human rhodopsin, deduced from the nucleotide sequence of its gene, is 348 residues long and is 93.4% homologous to that of bovine rhodopsin. Interestingly, those portions of the polypeptide chain predicted to form loops on the cytoplasmic face of rhodopsin are perfectly conserved between the human and bovine proteins.

Assembly in vitro of a spanning membrane protein of the endoplasmic reticulum: the E1 glycoprotein of coronavirus mouse hepatitis virus A59

The E1 glycoprotein of coronavirus mouse hepatitis virus A59 was synthesized in vitro by translation of viral mRNA in the presence of dog pancreatic microsomes. Its disposition in the membrane was investigated by digestion with proteases and by selective NH2-terminal labeling. The protein spans the membrane, but only small portions from the NH2 and COOH terminus are exposed respectively in the lumenal and cytoplasmic domains; the bulk of the molecule is apparently buried in the membrane. The protein lacks a cleavable leader sequence and does not acquire its characteristic O-linked oligosaccharides in rough microsomes. It may enter the membrane at any stage during synthesis of the first 150 amino acid residues. These unusual features of the protein might help to explain why it is not transported to the cell surface in vivo but remains in intracellular membranes, causing the virus to bud there.

Coding assignment and nucleotide sequence of simian rotavirus SA11 gene segment 10: location of glycosylation sites suggests that the signal peptide is not cleaved

A cloned DNA copy of simian rotavirus SA11 genomic segment 10 was used to confirm the assignment of the nonstructural glycoprotein NCVP5 to this gene. Determination of the nucleotide sequence for gene 10 indicated that NCVP5 is 175 amino acids in length and has an N-terminal hydrophobic region with the characteristics of a signal sequence for membrane translocation. Unexpectedly, this region was also the location for the only two potential glycosylation sites within the molecule, asparagine residues 8 and 18. The carbohydrates carried by NCVP5 were of the high-mannose type, Man9GlcNAc and Man8GlcNAc, with the mannose 9 species predominating; no complex oligosaccharides were present. If these asparagine residues are the sites for carbohydrate attachment, this implies that cleavage of the putative signal peptide does not occur during the maturation of this nonstructural glycoprotein.

In vitro biosynthesis of the human cell surface receptor for transferrin

The human cell surface receptor for transferrin is a transmembrane phosphoglycoprotein composed of two disulphide linked and apparently identical subunits of Mr 90 000. Using an affinity purified, polyclonal rabbit antibody, we have studied the in vitro biosynthesis of this receptor. The primary translation product, synthesised in a rabbit reticulocyte lysate programmed with human placental RNA, appears to have the same Mr (78 000) as the unglycosylated molecule immunoprecipitated from tunicamycin-treated cells. In the presence of a dog pancreatic microsomal system the cell free system accurately reproduces the glycosylation and the asymmetric transmembrane integration.

Isolation and characterization of the CNBr peptides from the proteolytically derived N-terminal fragment of ovine opsin

Ovine rhodopsin may be cleaved in situ by Staphylococcus aureus V8 proteinase into two membrane-bound fragments designated V8-L (27 000 mol.wt.) and V8-S (12 000 mol.wt.). After purification of the proteolysed complex by affinity chromatography in detergent using concanavalin A immobilized on Sepharose 4B, the two polypeptide fragments may be separated by gel-permeation chromatography on Sephadex LH-60. Digestion of the N-terminal-derived V8-L fragment with CNBr in 70% (v/v) trifluoroacetic acid resulted in a peptide mixture that could be fractionated by procedures involving gel-permeation chromatography in organic and aqueous solvents and the use of differential solubility. The complete or partial sequences of all ten peptides are reported.

Fine structure of a periciliary ridge complex of frog retinal rod cells revealed by ultrahigh resolution scanning electron microscopy

We studied the junctional region between rod inner segments (RIS) and outer segments (ROS) in frog retinas by high resolution scanning electron microscopy (SEM). Retinas of dark adapted or light exposed Rana pipiens were critical-point-dried and RIS and ROS were split and coated with ultrathin metal films of niobium and chromium--or decorated with gold--and imaged in a new SE-I imaging mode. The connecting cilium (CC) usually broke at the base of the RIS and remained attached to the ROS. The outer part of the CC plasmalemma expanded to form liplike protrusions beyond which disks evaginated with successively larger diameter until they reached the full width of the ROS. The CC rose out from an invagination of the RIS apical plasma membrane (PM). On the lateral walls of this invagination, a highly ordered complex of nine symmetrically arrayed ridges and grooves rose steeply and extended laterally approximately 0.4-1 micron on the adjacent RIS PM. On the apical plasmalemma, the ridges and grooves formed groups of three to four parallel rows that surrounded the invagination. The grooves were bridged by filaments anchored at the top edges of the ridges. This highly ordered structure we term the periciliary ridge complex (PRC). Its ninefold symmetry apparently reflects the 9 + 0 microtubule organization of the CC axoneme. The three-dimensional structure revealed by SEM was confirmed by transmission electron microscopy (TEM) of sections of Epon-embedded retinas. TEM-immunocytochemistry on thin sections of retinas embedded in glutaraldehyde cross-linked albumin suggested that the PRC and the CC may participate in opsin transport and disk morphogenesis.

Ultrastructural localization of glycerolipid synthesis in rod cells of the isolated frog retina

The incorporation of two glycerolipid precursors, 3H-glycerol and 3H-choline, into rod cells of the isolated frog retina has been studied using quantitative electron microscope autoradiography. The results indicate that the endoplasmic reticulum (ER) is the major site of early incorporation of these precursors suggesting that the ER is the primary site of lipid synthesis. Of the different types of ER present in rod cells, the rough ER (RER) and nuclear envelope predominate in this activity. The organized region of smooth ER (SER) in the subellipsoid region does not appear to be of major quantitative importance, although SER closely intermingled with RER in the myoid region may be involved to some extent. We also compared the pattern of labelling observed at various incubation times in 3H-glycerol and 3H-choline with that observed with 3H-leucine. Differences were observed between the pattern of lipid and protein labelling, particularly in the labelling of the Golgi apparatus, mitochondria, plasma membrane, presynaptic terminals and outer segments. This suggests that lipids and proteins may differ in some aspects of the routes and mechanisms by which they are transported from their sites of synthesis to the membrane delimited compartments for which they are destined.

In vitro biosynthesis, core glycosylation, and membrane integration of opsin

A membrane-integrated , core-glycosylated form of bovine opsin was synthesized in vitro when bovine retina mRNA was translated in a wheat germ cell-free system supplemented with dog pancreas microsomal vesicles; glycosylation and integration of opsin into membranes were coupled to translation. Proteolysis with themolysin was used to probe the orientation of opsin within the dog pancreas microsomal membrane, and to compare it with that of opsin in rod cell disk membranes isolated from bovine retina. Intact microsomal or disk vesicles were required for production of discrete, membrane-associated thermolysin fragments of opsin; no discrete opsin fragments were detected when membranes were incubated with thermolysin in the presence of the nonionic detergent, Triton X-100. The major opsin fragments produced by themosylin treatment of intact microsomal vesicles resembled those from disk vesicles in their size, oligosaccharide content, and order of appearance. In each case, the first cleavage of opsin took place at the COOH-terminus, generating a glycosylated fragment, O', which was only slightly smaller than intact opsin. Both the microsomal and disk membrane forms of O' were next cleaved internally; glycosylated fragments of similar sizes in both cases were detected which were derived from the NH(2)-terminal portion of O'. Several smaller NH(2)-terminal fragments of opsin were detected only in thermolysin-treated microsomal membranes, and not in disk membranes. The data suggest that the topology of opsin integrated into dog pancreas microsomal vesicles is similar to that in rod cell disk vesicles, although not identical. In each case, the glycosylated NH(2)-terminal region of opsin is located within the lumen of the vesicle, while discrete COOH-terminal and internal segments of opsin apparently emerge at the outer, cytoplasmic face of the membrane. Thus, opsin in the heterologous microsomal membrane, like its counterpart in the native disk membrane, may cross the bilayer at least three times. The internal domain of the polypeptide that emerges at the outer membrane surface is apparently more highly exposed in the case of opsin in microsomal membranes, evidenced by the additional internal thermolysin cleavage sites detected.

Assembly of proteins into membranes

Two pathways for protein assembly into biological membranes have been proposed. The "signal hypothesis" emphasizes the role of specific membrane proteins in binding the growing polypeptide and conducting it into the bilayer during its synthesis. The "membrane-triggered folding" hypothesis emphasizes self-assembly and the role of changing protein conformation during transfer from an aqueous compartment into a membrane. These ideas provide a framework for reviewing recent data on the biogenesis of membrane proteins.

The four cytoplasmically made subunits of yeast mitochondrial cytochrome c oxidase are synthesized individually and not as a polyprotein

Subunit-specific antisera prepared against each of the four cytoplasmically made subunits (IV, V, VI, and VII) of yeast mitochondrial cytochrome c oxidase (EC 1.9.3.1) were used to precipitate immunoreactive polypeptides that were synthesized either in vitro, in a cell-free protein-synthesizing system programmed with total yeast mRNA, or in vivo, in intact cells and in spheroplasts, under conditions of pulse labeling, pulse-chase labeling, and continuous labeling. Using N-formyl-[35S]Met-rTNA as the only radioactively labeled component in the cell-free system, we demonstrated (i) that each of the four cytoplasmically made subunits is synthesized as a separate entity and not as part of a polyprotein as was claimed by others; (ii) that subunits IV, V, and VI are synthesized as precursors, larger by 1500-3000 daltons than their mature counterparts; in contrast, subunit VII is not synthesized as a larger precursor. Precursor forms of subunits IV, V, and VI identical to those synthesized in vitro were also detected in vivo by pulse-labeling of spheroplasts. The observed disappearance of these larger forms after a chase is compatible with the notion that they represent short-lived precursors that are rapidly converted to their mature counterparts during or shortly after import into mitochondria. Furthermore, using N-formyl-[35S]Met-tRNA, we provide definitive evidence that two of the cytoplasmically made subunits (beta and gamma) of another oligomeric inner mitochondrial membrane protein (F1-ATPase, EC 3.6.1.3) are not synthesized as part of a polyprotein but as individual precursors.

Synthesis and insertion of cytochrome P-450 into endoplasmic reticulum membranes

Treatment of rats with phenobarbital leads to a substantial increase in levels of translatable liver cytochrome P-450 mRNA. This mRNA is primarily associated with ribosomes bound to endoplasmic reticulum membranes which in an in vitro system synthesized approximately 10 times more cytochrome P-450 than did free polysomes from the same animals. Cytochrome P-450 synthesized by rough microsomes in vitro appears to be directly inserted into the membranes because it was not released by a treatment with low detergent concentrations that released albumin and other microsomal content proteins. The amino-terminal amino acid sequence of cytochrome P-450 synthesized in an mRNA-dependent system resembles in hydrophobicity the signal segment of presecretory proteins and therefore may serve to insert the polypeptide into the membrane during synthesis. In contrast to the situation with secretory proteins and several other membrane proteins, however, the putative insertion signal of cytochrome P-450 is not removed by a membrane-associated peptidase and remains in the mature polypeptide.