The affinity of rat liver ribosome subunits for degranulated rough microsomal membranes

Potassium and magnesium ion concentrations affected the extent but not the specificity of binding in vitro of 60-S and 40-S ribosome subunits to degranulated rough microsomal membranes from rat liver. Scatchard plots revealed that under ionic conditions most likely to resemble those in vivo, the affinity constants for binding 60-S subunits were approximately four-times greater than those characterizing 40-S subunit binding. Further, the extent to which subunits bound at saturation was close to the level of ribosomes present in intact membranes.     

Lipid composition and turnover of rough and smooth microsomal membranes in rat liver

Subfractions of rat liver microsomes (rough, smooth I, and smooth II), isolated in a cation-containing sucrose gradient system, were analyzed. After removal of adsorbed and luminal protein, these subfractions had the same phospholipid/protein ratio, about 0.40. Both the classes and the relative amounts of phospholipids were similar in the three subfractions, but the relative amounts of neutral lipids (predominantly free cholesterol and triglycerides) were higher in smooth I and especially in smooth II than in rough microsomes. Various pieces of evidence indicate that the neutral lipids are tightly bound to the membranes. Glycerol-(3)H was incorporated into the phospholipids of the rough and smooth I microsomes significantly faster than into those of the smooth II membranes; (32)P incorporation followed a similar but less pronounced pattern. Acetate-(3)H was incorporated into the free cholesterol of smooth I microsomes only half as fast as into the other two subfractions. Injection of phenobarbital increased the cellular phospholipid and neutral lipid content in the rough and smooth I, but not in the smooth II microsomes. Consequently, the neutral lipid/phospholipid ratio of all three subfractions remained unchanged after phenobarbital treatment. It is concluded that the membranes of the rough and the two smooth microsomal subfractions from rat liver have a similar phospholipid composition, but are dissimilar in their neutral lipid content and in the incorporation rate of precursors into membrane lipids.     

Facilitated transfer of cholesteryl ester between rough and smooth microsomal membranes by plasma lipid transfer protein

The accessibility of intracellular membrane cholesteryl esters to removal was tested with plasma lipid transfer protein as a tool. Incubation of a mixture of non-radioactive smooth microsomes + rough microsomes prelabeled with cholesteryl ester resulted in slight movement (2-4%) of radioactive cholesteryl ester into smooth microsomes. With the addition of increasing amounts of plasma lipid transfer protein to the mixture, the % transfer of cholesteryl ester into smooth microsomes progressively increased until a plateau was reached at 14%. Movement of cholesteryl ester in the reverse direction was examined with non-radioactive rough microsomes as an acceptor and smooth microsomes prelabeled with cholesteryl ester as a donor. The pattern of the % cholesteryl ester transferred in the reverse and forward direction was almost identical in the presence of plasma lipid transfer protein, showing bidirectional movement of cholesteryl ester between membranes.     

Studies on the mechanism of 5-fluoroorotic acid inhibition of serine dehydratase induction

Administration of glucagon to rats fed a protein-free diet caused a significant induction of the liver enzyme, serine dehydratase. This effect of glucagon is inhibited by the concomitant administration of fluoroorotic acid. This inhibition was enhanced by pretreatment with glucosamine or galactosamine, probably through depletion of the intracellular uridine pools. Although less than a doubling of enzyme activity was observed after glucagon plus fluoroorotic acid administration, the amount of protein precipitable by antisera specifically reactive against serine dehydratase increased 4.5 times. Ouchterlony double-diffusion analysis showed a completely cross-reacting single precipitin band from liver extracts of untreated animals and rats treated with the analog. Analysis of the antigen-antibody complex by Na dodecyl sulfate-gel electrophoresis indicated that a single protein was being immunochemically precipitated from both the glucagon- and glucagon plus fluoroorotic acid-treated rats. In the latter, the precipitated protein had a molecular weight similar to purified serine dehydratase. These results are consistent with the concept that the incorporation of fluoroorotic acid into mRNA results in the synthesis of a protein with characteristics similar to authentic serine dehydratase but without normal enzymatic activity. Other possible mechanisms to explain the production of this abnormal protein are discussed.     

Ribosomes mask cytochrome b5 on rough microsomal vesicles

Cytochrome b₅ is unmasked on the removal of ribosomes by chemical degranulation of rat liver microsomes. Reattachment of ribosomes to stripped membranes remasks this enzyme on the membrane surface. This haemoprotein may be involved either in the attachment of ribosomes to reticular membranes or in protein biosynthesis by membrane-bound ribosomes. © 1998 John Wiley & Sons, Ltd.     

Identification of a membrane protein responsible for ribosome binding in rough microsomal membranes

A membrane protein fraction was obtained from rat liver rough microsomes by affinity chromatography on a concanavalin A-Sepharose column and then a chelating-Sepharose column. This protein fraction comprised about 2% of the total membrane proteins of rough microsomes and the ribosome-binding activity of ribosome-stripped rough microsomes was predominantly found in this protein fraction, as determined with a liposome assay system. To identify the essential components responsible for the ribosome binding, two approaches were employed. Trypsin treatment of liposomes reconstituted with this protein fraction resulted in the loss of the ribosome-binding activity in parallel with the loss of a dominant band, estimated Mr 34,000, in SDS-polyacrylamide gels. Next, the direct interaction between the binding sites on the membrane of reconstituted liposomes and 60S ribosomal subunits was investigated by photocrosslinking using sulfosuccinimidyl 2-(m-azido-o-nitrobenzamido)-ethyl-1,3'-dithiopropionate (SAND). The photocrosslinked complex was formed between 60S ribosomal subunits pretreated with SAND and binding-site proteins on the membrane of the liposomes. Then, after the liposomes were solubilized, the complex was isolated by sucrose gradient centrifugation of the binding mixture. The crosslinked proteins were released from 60S ribosomal subunits by cleavage of of crosslinks with beta-ME and analyzed by SDS-polyacrylamide gel electrophoresis and 125I-autoradiography. The 34-kDa protein (p34) was the predominant component that crosslinked to the 60S ribosomal subunits and was found in proportion to the amount of 60S ribosomal subunits added to the system. The p34 was distinguishable by immunoblot analysis from urate oxidase, which is the 34-kDa protein of peroxisomal cores contaminating rough microsomes. These results suggest that the present p34 is a likely candidate molecule for the ribosome-binding activity of rough microsomes.     

Non-protein-mediated transfer of phosphatidic acid between microsomal and mitochondrial membranes

The transfer of phosphatidic acid between rat liver microsomes loaded with [32P]-phosphatidic acid and rat liver mitochondria was studied in the absence of added lipid transfer proteins. It was found that during 1 h at 37 degrees C in the medium containing 100 mM KCl, 20-30% of phosphatidic acid but only 2.5% of phosphatidylcholine were transferred. This spontaneous transfer of phosphatidic acid remained the same after pretreatment of microsomes and mitochondria with 125 mM KCl or microsomes alone with 1 mM Tris, pH 8.6, procedures reported to remove adsorbed lipid transfer proteins. This transfer was insensitive to thiol-blocking reagents. The initial rate of this non-protein-mediated transfer of phosphatidic acid was virtually independent of the concentration of the acceptor membranes (mitochondria), thus indicating that it occurs by diffusion of the phospholipid through the aqueous phase rather than by membrane collision. About 80% of phosphatidic acid synthesized in the outer mitochondrial membrane was recovered in the inner membrane after a 1-h incubation, pointing to a high rate of the intermembrane transfer of this phospholipid within intact mitochondrion.     

Heparin releases monosomes and polysomes from rough endoplasmic reticulum

Incubation of membrane-bound polyribosomes isolated from murine myeloma cells with heparin caused release of material which sedimented in the polysome, monosome and ribosomal subunit regions of linear sucrose gradients. The released material corresponded to approximately one half that which could be released by treatment with heparin plus Triton X-100. The action of heparin appeared to be related to its polyanionic nature. The use of heparin as a ribonuclease inhibitor in the separation and isolation of free and membrane-bound polysomes could cause artificial accumulation of detached polysomes in the free polysome fraction.