Structural aspects of the membrane of the endoplasmic reticulum

Extensive exchange of rat liver microsomal phospholipids

Liver microsomal fractions were prepared from rats injected with a single dose of choline [14C]methylchloride or with single or multiple doses of 32Pi. Exchangeability of microsomal phospholipids was determined by incubation with an excess of mitochondria and phospholipid exchange proteins derived from beef heart, beef liver or rat liver. Labeled phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol were found to act as a single pool and were 85--95% exchangeable in 1--2h. High latencies of mannose-6-phosphate phosphohydrolase activities and impermeability of microsomes to EDTA proved that phospholipid exchange proteins did not have access to the intracisternal space. If microsomal membranes are largely composed of phospholipid bilayers, the experiments suggest that one or more of the phospholipid classes in microsomal membranes undergo rapid translocation between the inner and outer portions of the bilayer.

Recognition of different pools of phosphatidylglycerol in intact cells and isolated membranes of Acholeplasma laidlawii by phospholipase A2

Phospholipase A2 (EC 3.1.1.4) from pig pancreas hydrolyzes phosphatidylglycerol in intact cells and isolated membranes of Acholeplasma laidlawii. Complete degradation of phosphatidylglycerol in intact cells at 37 degrees C does not result in lysis as shown by the retention of intracellular K+ ions and the cytoplasmic glucose-6-phosphatase, as well as the inability to detect activity of membrane-bound intracellular NADH-oxidase. A. laidlawii was grown on linoleic acid. Phospholipase A2 treatment of these cells at 5 degrees C, at which temperature the lipids are still in the liquid-crystalline state, results in a rapid breakdown of 50% of the phosphatidylglycerol. The residual phosphatidylglycerol can be hydrolyzed only at elevated temperatures and at much smaller rates, depending strongly on the incubation temperature. When membranes isolated from these cells are incubated at 5 degrees C, 70% of the phosphatidylglycerol is hydrolyzed immediately. The hydrolysis of the residual 30% is again strongly temperature dependent. Cells were grown on palmitate, elaidate, or oleate to investigate possible effects of the lipid phase transition on the accessibility of phosphatidylglycerol for phospholipase A2. Under conditions in which all the lipid is in the solid state, no hydrolysis occurs. When solid and liquid-crystalline lipid phases coexist, a limited hydrolysis of phosphatidylglycerol can be observed. The results demonstrate the disposition of phosphatidylglycerol in three different pools in the membrane of A. laidlawii. Phospholipase A2 has been used to discriminate between these pools and to estimate the amount of phosphatidylglycerol which is present in the liquid-crystalline phase. The present data, however, do not allow a definite localization of the phosphatidylglycerol pools.

Redox constituents in milk fat globule membranes and rough endoplasmic reticulum from lactating mammary gland

Milk fat globule membranes (MFGM) and rough endoplasmic reticulum (RER) membranes were isolated from milk and lactating mammary gland from the cow and were characterized by biochemical and electron microscope methods in terms of gross composition (proteins, phospholipids, neutral lipids, cholesterol, RNA, and DNA) and purity. Both fractions contained significant amounts of a b-type cytochrome with several properties similar to those of cytochrome b5 from liver, as well as a rotenone-insensitive NADH- and NADPH-cytochrome c reductase. The b-type cytochrome content in the apical plasma membrane-derived MFGM was of the same order of magnitude as it was in RER membranes. It was characterized by a high resistance to extraction by low- and high-salt concentrations and nonionic detergents. MFGM contained much more flavin and much higher activities of xanthine oxidase than the RER membranes. The same redox components were found in MFGM and mammary RER from women, rats, mice, and goats, but in absolute contents great differences between the species were noted. The cytochromes described here differed from liver cytochrome b5 in some spectral properties. The alpha-band of the reduced hepatic cytochrome b5 is asymmetric with a maximum at 555 nm that is split into two distinct peaks at low temperatures. The alpha-band of the b-type cytochromes from MFGM and mammary RER appears as one symmetrical peak at about 560 nm that is not split at low temperatures. When treated with cyanide, MFGM and mammary microsomes showed difference spectra of a reduced b-type cytochrome. Under the same conditions, liver microsomes gave a completely different spectrum. These findings demonstrate the presence of a b-type cytochrome and associated redox enzymes in MFGM, i.e., a derivative of the apical cell surface membrane that is regularly used for envelopment of the milk fat globule during secretion.

Lateral enzyme topology in the rough endoplasmic reticulum of rat liver

Rough microsomes were subfractionated on the basis of different properties in order to investigate the nature and extent of the enzyme heterogeneity of these vesicles. A discontinuous gradient, containing monovalent cations allowed the separation of a ribosome-poor membrane fraction which was enriched in electron transport enzymes and relatively poor in phosphatases. Zonal centrifugation on a stabilizing gradient separated 3 fractions characterized by enrichment of electron transport enzymes, glucose-6-phosphatase and adenosinetriphosphatase, respectively. An essentially similar pattern was seen when ribosomes were removed with EDTA and the denuded vesicles subfractionated on a sucrose gradient. Rough microsomes from phenobarbital-treated rats exhibited the same pattern both qualitatively and quantitatively. It appears that electron transport enzymes and two types of phosphatases are heterogeneously distributed among rough microsomal vesicles.

Cyanide sensitivity and induction of the microsomal oleoyl-CoA desaturase of potato tuber

1. The properties of oleoyl-CoA desaturase, induced in microsomal fractions by the 'ageing' treatment of potato tuber slices (aeration of slices for 3-18 h), were investigated to study the effect of cyanide on desaturation and cycloheximide on the induction of the desaturase. 2. The electrons needed for the desaturation can be supplied either by NADH or NADPH, but ascorbate can also drive the reaction; experiments with CO suggested that cytochrome P-450 was not involved in the desaturation. 3. A strong inhibition of the desaturation by potassium cyanide was observed with each of the electron donors; total inhibition was noticed with 1 mM KCN; low concentrations (0.1 mM) caused 50% inhibition of the desaturation. 5. The variation of the oleoyl-CoA desaturase activity during the ageing process and the drop in this activity in aged slices treated by cycloheximide indicate that the enzyme undergoes an active turnover.

Membrane asymmetry

The components of biological membranes are asymmetrically distributed between the membrane surfaces. Proteins are absolutely asymmetrical in that every copy of a polypeptide chain has the same orientation in the membrane, and lipids are nonabsolutely asymmetrical in that almost every type of lipid is present on both sides of the bilayer, but in different and highly variable amounts. Asymmetry is maintained by lack of transmembrane diffusion. Two types of membrane proteins, called ectoproteins and endoproteins, are distinguished. Biosynthetic pathways for both types of proteins and for membrane lipids are inferred from their topography and distribution in the formed cells. Note added in proof. A cell-free system has now been developed which permits the mechanisms of membrane protein assembly to be studied (108). The membrane glycoprotein of vesicular stomatitis virus has been synthesized by wheat germ ribosomes in the presence of rough endoplasmic reticulum from pancreas. The resulting polypeptide is incorporated into the membrane, spans the lipid bilayer asymmetrically, and is glycosylated (108). The amino terminal portion of this transmembrane protein is found inside the endoplasmic reticulum vesicle, while the carboxyl terminal portion is exposed on the outer surface of the vesicle. Furthermore, addition of the glycoprotein to membranes after protein synthesis does not result in incorporation of the protein into the membrane in the manner described above (108). Consequently, protein synthesis and incorporation into the membrane must be closely coupled. Indeed, using techniques to synchronize the growth of nascent polypeptides, it has been shown (109) that no more than one-fourth of the glycoprotein chain can be made in the absence of membranes and still cross the lipid bilayer when chains are subsequently completed in the presence of membranes. These findings demonstrate directly that the extracytoplasmic portion of an ectoprotein can cross the membrane only during biosynthesis, and not after.

Evidence for the preferential interaction of glycophorin with negatively charged phospholipids

Glycophorin, extracted from the erythrocyte membrane after treatment with lithium-diiodo-salicylate, still contains a significant amount of phospholipid, consisting predominantly of phosphatidylserine. Methods are described which lead to a full delipidation of the protein. After treatment with neuraminidase, delipidated glycophorin shows a preferential interaction with monolayers of negatively-charged phospholipids. This lipid-protein interaction is decreased by the presence of cholesterol in the lipid film.

On the involvement of cytochrome P-450 in the binding of ribosomes to a subfraction of rat-liver rapidly sedimenting endoplasmic reticulum

Rat liver endoplasmic reticulum has been separated into four ribosome-containing subfractions, two from rapidly sedimentation endoplasmic reticulum and two from the microsomes, by differential centrifugation and sucrose density centrifugation. Ribosomes from one of the rapidly sedimenting subfractions were extracted by Trion X-100 as a complex with cytochrome P-450, optimally at a detergent protein ratio of 2/1 (w/w). Upon extraction approximately 50% of the cytochrome P-450 in the membrane appeared complex-bound to ribosomes, and, maximally, 6-7 subunit molecules of the cytochrome were attached per ribosome. The specific concentration of cytochrome P-450 on these ribosomes was 2.5-times higher than in the parent membrane. Cytochrome b5, glucose-6-phosphatase, NADPH-cytochrome c reductase, NADH-ferricyanide reductase, cytochrome oxidase and phospholipids were present in small or trace amounts on the ribosomes in relation to cytochrome P-450. Ribosomes extracted from other subfractions contained much less bound cytochrome P-450. Phenobarbital treatment induced an increase in the cytochrome P-450 content that was different for the various subfractions. This increase could not be correlated with changes in the amounts of cytochrome-ribosome complexes released by detergent. We propose that cytochrome P-450 is part of a specific binding site in the membrane for a fraction of the ribosomes attached to the endoplasmic reticulum. The ribosomes may be anchored to cytochrome P-450 via nascent chain proteins.

Preparation and characterization of total, rough and smooth microsomes from the lung of control and methylcholanthrene-treated rats

Optimal conditions for the preparation of relatively pure microsomes and microsomal subfractions from rat lung have been determined. The most importnat of these conditions is homogenization of a 20% (w/v) suspension of lung tissue in 0.44 M sucrose/1% (w/v) bovine serum albumin with four up-and-down strokes at 440 rev./min in a Potter-Elvehjem homogenizer. The 10000 X g supernatant prepared from this homogenate can be centrifuged at 105000 X g to obtain total microsomes or subfractionated into rough and smooth microsomes on a Cs+-containing discontinuous sucrose gradient. The total, rough and smooth microsomes have been characterized in terms of their chemical composition, enzymatic activity, and morphology. These preparations should prove useful in studies of various enzymes in lung (e.g. benzpyrene monooxygenase, epoxide hydrase, enzymes of phospholipid and ascorbic acid synthesis) and in subfractionations designed to reveal heterogeneites in the lateral plane of the lung endoplasmic reticulum.

Localization of enzymes in specialized regions of the microsomal membrane

Microsomal vesicles were centrifuged through sucrose density gradients containing deoxycholate. With 0.15% detergent electron transport enzymes and phosphatases could be separated. Increasing the deoxycholate concentration to 0.19% resulted in separation of the microsomal material into five bands containing (in order from the top of the gradient) adenosine monophosphatase, inosine diphosphatase and some glucose-6-phosphatase (band 1); NADH-linked (band 2) and NADH-linked (band 3) electron transport enzymes; and glucose-6-phosphatase (bands 4 and 5). It appears that enzymes are arranged in specialized patches in the microsomal membrane.

Distribution of protein-bound sugar residues in microsomal subfractions and Golgi membranes

Liver microsomal subfractions and Golgi membranes free from adsorbed and secretory proteins have a characteristic sugar composition. The ratio of mannose to galactose is largest in rough microsomes, smaller in smooth I microsomes, still smaller in smooth II microsomes, and smallest in Golgi membranes. There is about twice as much glucosamine in Golgi membranes and 3 times as much in smooth II microsomes as in the other microsomal subfractions. Golgi membranes are rich in sialic acid in comparison to rough microsomes and it is present at even higher levels in the two smooth microsomal subfractions. Increasing concentrations of deoxycholate preferentially remove protein-bound mannose and glucosamine, while releasing significantly less galactose. About half of the microsomal mannose and galactose can be liberated from the surface of intact microsomal vesicles by treatment with trypsin. When trypsin is added to permeable vesicles where the inside surface can be also attacked, an additional 20% of the total mannose but no additional galactose is liberated.