Interactions between endoplasmic reticulum and nuclear membranes of different types of cells during repair of damaged Amoeba nuclei

Repair of amoeba nuclear envelopes that have been damaged microsurgically involves the association of pieces of endoplasmic reticulum with the damaged nuclear membranes. The capacity of endoplasmic reticulum of one type of cell to interact with the nuclear membranes of a different type was tested by placing the damaged nucleus of one kind of amoeba into the cytoplasm of another. Damaged nuclei from Amoeba proteus underwent repair in the cytoplasm of A. discoides or A. indica, as was the case in the reciprocal combinations of these nuclei and cytoplasms. In samples prepared 30 min after operation, heterologous endoplasmic reticulum was associated with holes in the nuclear membranes and appeared to fuse with the nuclear membranes at the margins of the holes. By 5 h after operation, almost all of the cells survived, and the nuclear membranes were largely intact, indicating that repair had occurred. In contrast, when an Amoeba dubia nucleus was damaged and placed in A. proteus cytoplasm there was no evidence of repair and many cells died within a few hours. The results indicate that endoplasmic reticulum and nuclear membranes from different types of cells can interact during repair of damaged nuclear membranes. There appears to be a specificity to this interaction, however, since in a combination of relatively dissimilar cells no association of endoplasmic reticulum with damaged nuclear envelopes was observed and repair did not occur.     

Role of the nuclear envelope in synthesis, processing, and transport of membrane glycoproteins

The outer nuclear membrane is morphologically similar to rough endoplasmic reticulum. The presence of ribosomes bound to its cytoplasmic surface suggests that it could be a site of synthesis of membrane glycoproteins. We have examined the biogenesis of the vesicular stomatitis virus G protein in the nuclear envelope as a model for the biogenesis of membrane glycoproteins. G protein was present in nuclear membranes of infected Friend erythroleukemia cells immediately following synthesis and was transported out of nuclear membranes to cytoplasmic membranes with a time course similar to transport from rough endoplasmic reticulum (t 1/2 = 5-7 min). Temperature-sensitive mutations in viral membrane proteins which block transport of G protein from endoplasmic reticulum also blocked transport of G protein from the nuclear envelope. Friend erythroleukemia cells and NIH 3T3 cells differed in the fraction of newly synthesized G protein found in nuclear membranes, apparently reflecting the relative amount of nuclear membrane compared to endoplasmic reticulum available for glycoprotein synthesis. Nuclear membranes from erythroleukemia cells appeared to have the enzymatic activities necessary for cleavage of the signal sequence and core glycosylation of newly synthesized G protein. Signal peptidase activity was detected by the ability of detergent-solubilized membranes of isolated nuclei to correctly remove the signal sequence of human preplacental lactogen. RNA isolated from the nuclear envelope was highly enriched for G protein mRNA, suggesting that G protein was synthesized on the outer nuclear membrane rather than redistributing to nuclear membranes from endoplasmic reticulum before or during cell fractionation. These results suggest a mechanism for incorporation of membrane glycoproteins into the nuclear envelope and suggest that in some cell types the nuclear envelope is a major source of newly synthesized membrane glycoproteins.     

In vivo reactivation of heat-denatured protein in the endoplasmic reticulum of yeast.

Saccharomyces cerevisiae cells grown at 24 degrees C acquire thermotolerance and survive exposure to 50 degrees C, but only if they are first incubated at 30 degrees C, the temperature where heat shock genes are activated. We show here that the enzymatic activity of a secretory beta-lactamase fusion protein, pre-accumulated at 37 degrees C in the endoplasmic reticulum, was abolished by exposure of the cells to 50 degrees C. When the cells were returned to 24 degrees C, beta-lactamase activity was resumed. Reactivation occurred in the endoplasmic reticulum, but not in the Golgi apparatus. It was dependent on metabolic energy, but did not require de novo protein synthesis. According to co-immunoprecipitation experiments, immuno-globulin-binding protein (BiP/Kar2p) was associated with the fusion protein. We suggest that recovery from thermal insult involves, in addition to cytoplasmic and nuclear events, refolding of heat-damaged proteins in the endoplasmic reticulum by a heat-resistant machinery, which forms part of a fundamental survival mechanism.     

Regulation of expression and intracellular distribution of calreticulin, a major calcium binding protein of nonmuscle cells

In the present study we have demonstrated the presence of calreticulin, a major Ca(2+)-sequestering protein of nonmuscle cells, in a variety of cell types in tissue culture. The protein localizes to the endoplasmic reticulum in most cell types and also to the nuclear envelope or nucleoli-like structures in some cell types. Calreticulin is enriched in the rough endoplasmic reticulum, suggesting a possible involvement in protein synthesis. Calreticulin terminates with the KDEL-COOH sequence, which is likely responsible for its endoplasmic reticulum localization. Unlike some other KDEL proteins, calreticulin expression is neither heat-shock nor Ca(2+)-shock dependent. Using a variety of metabolic inhibitors, we have shown that the pool of calreticulin in L6 cells has a relatively slow turnover and a stable intracellular distribution. In proliferating muscle cells in culture (both L6 and human skeletal muscle) calreticulin is present in the endoplasmic reticulum, and additional intranuclear staining is observed. When fusion of the L6 cells is inhibited with either a high serum concentration or TGF-beta or TPA, the nucleolar staining by anticalreticulin antibodies is diminished, although the presence of calreticulin in the endoplasmic reticulum remains unchanged. In contrast, in differentiated (i.e., fused) muscle cells neither intranuclear nor intracellular staining for calreticulin is present. We conclude, therefore, that calreticulin is abundant in the endoplasmic reticulum in proliferating myoblasts, while it is present in only small amounts in sarcoplasmic reticulum membranes in terminally differentiated myotubes. We propose a model for the domain structure of calreticulin that may explain the differential subcellular distribution of this protein. Because of its widespread distribution in nonmuscle tissues, we postulate that calreticulin is a multifunctional protein that plays an important role in Ca(2+) sequestering and thus that it is the nonmuscle analog of calsequestrin.     

Co-ordination between membrane phospholipid synthesis and accelerated biosynthesis of cytoplasmic ribonucleic acid and protein

1. The rate of synthesis of membrane phospholipid was studied in rat liver and seminal vesicles by following the incorporation of [(32)P]orthophosphate, [(14)C]choline and [(14)C]glycerol. Particular emphasis was laid on the endoplasmic reticulum, which was fractionated into smooth microsomal membranes, heavy rough membranes, light rough membranes and free polyribosomes. 2. Phospholipid labelling patterns suggested a heterogeneity in the synthesis and turnover of the different lipid moieties of smooth and rough endoplasmic membranes. The major phospholipids, phosphatidylcholine and phosphatidylethanolamine, were labelled relatively rapidly with (32)P over a short period of time whereas incorporation of radioisotope into the minor phospholipids, sphingomyelin, lysolecithin and phosphatidylinositol proceeded slowly but over a longer period of time. 3. The incorporation of orotic acid into RNA and labelled amino acids into protein of the four submicrosomal fractions was also studied. 4. Rapid growth of the liver was induced by the administration of growth hormone and tri-iodothyronine to hypophysectomized and thyroidectomized rats and by partial hepatectomy. Growth of seminal vesicles of castrated rats was stimulated with testosterone propionate. 5. The rate of labelling of membrane phospholipids was enhanced in all major subcellular particulate fractions (nuclear, mitochondrial and microsomal) during induced growth. However, it was in the rough endoplasmic reticulum that the accumulation of phospholipids, RNA and protein was most marked. The effect of hormone administration was also to accelerate preferentially the labelling with (32)P of sphingomyelin relative to that of phosphatidylcholine or phosphatidylethanolamine. 6. Time-course analyses showed that, in all four growth systems studied, the enhancement of the rate of membrane phospholipid synthesis coincided with the rather abrupt increase in the synthesis of RNA and protein of the rough endoplasmic reticulum. Growth hormone and tri-iodothyronine administered to hypophysectomized rats had additive effects in all the biosynthetic processes. The latent period of action of each hormone was maintained so that two waves of proliferation of endoplasmic reticulum occurred if the hormones were administered simultaneously. 7. It is concluded that there is some mechanism in the cell that tightly co-ordinates the formation of membranes, especially those of the endoplasmic reticulum, when an increased demand is made for protein synthesis.     

PRODUCTION OF MEMBRANE WHORLS IN RAT LIVER BY SOME INHIBITORS OF PROTEIN SYNTHESIS

Inhibitors of protein synthesis capable of differential effects on nascent peptide synthesis on membrane-bound and free polyribosomes were employed to investigate the structure and function of cellular membranes of liver. The formation of membranous whorls in the cytoplasm and distension of nuclear membranes were induced by inhibitors of protein synthesis (i.e., cycloheximide and emetine) which predominantly interfere with nascent peptide synthesis on membrane-bound polyribosomes in situ. Other inhibitors of protein synthesis such as puromycin and fusidic acid, which inhibit nascent peptide synthesis on both free and membrane-bound polyribosomes, and chloramphenicol, which inhibits mitochondrial protein synthesis, did not induce these alterations. Cycloheximide, puromycin, and chloramphenicol produce some common cellular lesions as reflected by similar alterations in morphology, such as swelling of mitochondria, degranulation of rough endoplasmic reticulum, and aggregation of free ribosomes. The process of whorl formation in the cytoplasm, the incorporation of [³H]leucine and of [³H]choline into endoplasmic reticulum and the total NADPH-cytochrome c reductase activity of the endoplasmic reticulum were determined. During maximum formation of membranous whorls, [³H]leucine incorporation into cytoplasmic membranes was inhibited, while [³H]choline incorporation into these structures was increased; maximum inhibition of protein synthesis and stimulation of choline incorporation into endoplasmic reticulum, however, preceded whorl formation. Cycloheximide decreased the activity of NADPH-cytochrome c reductase of rough endoplasmic reticulum, but increased NADPH-cytochrome c reductase activity of smooth endoplasmic reticulum. In addition, cycloheximide decreased the content of hemoprotein in both the microsomal and mitochondrial fractions of rat liver, and the activities of mixed function oxidase and of oxidative phosphorylation were impaired to different degrees. Succinate-stimulated microsomal oxidation was also inhibited. The possible mechanisms involved in the formation of membranous whorls, as well as their functions, are discussed.     

Synthesis and deposition of zein in protein bodies of maize endosperm

The origin of protein bodies in maize (Zea mays L.) endosperm was investigated to determine whether they are formed as highly differentiated organelles or as protein deposits within the rough endoplasmic reticulum. Electron microscopy of developing maize endosperm cells showed that membranes surrounding protein bodies were continuous with rough endoplasmic reticulum membranes. Membranes of protein bodies and rough endoplasmic reticulum both contained cytochrome c reductase activity indicating a similarity between these membranes. Furthermore, the proportion of alcohol-soluble protein synthesized by polyribosomes isolated from protein body or rough endoplasmic reticulum membranes was similar, and the alcohol-soluble or -insoluble proteins showed identical [¹⁴C]leucine labeling. These results demonstrated that protein bodies form simply as deposits within the rough endoplasmic reticulum.

Messenger RNA that directed synthesis of only the smaller molecular weight zein subunit was separated from mRNA that synthesized both subunits by sucrose gradient centrifugation. This result demonstrated that separate but similar sized mRNAs synthesize the major zein components. In vitro translation products of purified mRNAs or polyribosomes were approximately 2,000 daltons larger than native zein proteins, suggesting that the proteins are synthesized as zein precursors. When intact rough endoplasmic reticulum was placed in the in vitro protein synthesis system, proteins corresponding in molecular weight to the native zein proteins were obtained.

     

Metabolic processes involved in repair of Escherichia coli cells damaged by exposure to acid mine water

Escherichia coli was stressed by exposure to filter-sterilized acid mine water. Synthetic processes required for repair of sublethally injured survivors were studied by the addition of specific metabolic inhibitors to a resuscitation broth. Inhibitors of protein, RNA, DNA, lipid, and peptidoglycan synthesis as well as uncouplers and inhibitors of electron transport and ATPase activity were used. Acid mine water injury was severe, causing damage to the outer and cytoplasmic membranes. Repair of sublethally injured cells required protein, RNA, and lipid synthesis as well as a proton motive force.     

Metabolic processes involved in repair of Escherichia coli cells damaged by exposure to acid mine water.

Escherichia coli was stressed by exposure to filter-sterilized acid mine water. Synthetic processes required for repair of sublethally injured survivors were studied by the addition of specific metabolic inhibitors to a resuscitation broth. Inhibitors of protein, RNA, DNA, lipid, and peptidoglycan synthesis as well as uncouplers and inhibitors of electron transport and ATPase activity were used. Acid mine water injury was severe, causing damage to the outer and cytoplasmic membranes. Repair of sublethally injured cells required protein, RNA, and lipid synthesis as well as a proton motive force.     

The calcium pump of the liver nuclear membrane is identical to that of endoplasmic reticulum.

The envelope membrane of rat liver nuclei contains a P-type Ca(2+)-transporting pump, revealed by the presence of a Ca(2+)-stimulated phosphoenzyme. The level of the nuclear phosphoenzyme in autoradiographed polyacrylamide gels was decreased by lanthanum, as typically observed in the endoplasmic reticulum Ca2+ pump. It was also decreased by thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, two accepted inhibitors of the endoplasmic reticulum Ca(2+)-ATPase. Comparative proteolysis of the phosphorylated enzyme of liver microsomes (endoplasmic reticulum) and nuclear membranes revealed an identical cleavage pattern. In addition, antibodies raised against the endoplasmic reticulum Ca2+ pump cross-reacted with the pump in the nuclear membranes. The findings show that nuclear membranes contain a Ca(2+)-transporting pump closely related to that of the endoplasmic reticulum, if not identical to it. The pump is likely to be involved in the control of nuclear free calcium.     

Expression and characterization of hepatitis B virus surface antigen polypeptides in insect cells with a baculovirus expression system.

The baculovirus Autographa californica nuclear polyhedrosis virus was used as an expression vector to produce hepatitis B virus surface antigen with and without the pre-S domain. The S gene product was expressed as both fusion and nonfusion polypeptides. No difference was observed in the posttranslational modification of the fusion and nonfusion polypeptides. The S proteins were not secreted into the medium but were inserted into the endoplasmic reticulum, glycosylated, and partially extruded into the lumen of the endoplasmic reticulum as 22-nm lipoprotein particles. The oligosaccharide chains on the insect cell-derived S protein were of the N-linked high-mannose form, in contrast to the complex-type oligosaccharides detected on plasma-derived hepatitis B virus surface antigen. The pre-S-S polypeptides were inserted into the endoplasmic reticulum, glycosylated, and modified by fatty acid acylation with myristic acid. A procedure was developed to purify the S protein from cellular membranes by using detergent extraction and immunoaffinity chromatography. The purified S protein was in the form of protein-detergent micelles and was highly antigenic and immunogenic.     

Reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p involves the nuclear envelope

The reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p was examined. Using a Pex3-green fluorescent protein (Pex3-GFP) fusion protein, expressed under the control of an inducible promoter, it was observed that, initially on induction of Pex3-GFP synthesis, GFP fluorescence was localized to the endoplasmic reticulum and the nuclear envelope. Subsequently, a single organelle developed per cell that increased in size and multiplied by division. At these stages, GFP fluorescence was confined to peroxisomes. Fractionation experiments on homogenates of pex3 cells, in which the endoplasmic reticulum and nuclear envelope were marked with GFP, identified a small amount of GFP in peroxisomes present in the initial stage of peroxisome reassembly. Our data suggest a crucial role for the endoplasmic reticulum/nuclear envelope in peroxisome reintroduction on complementation of pex3 cells by the PEX3 gene.     

Studies on the amoebo-flagellate transformation inPhysarum polycephalum

Flagellar formation in the true slime mold,Physarum polycephalum, involves a sequence of events during which amoebae are changed into flagellate cells. In the present study a series of inhibitors thought to inhibit RNA and protein synthesis and microtubule assembly were added in an attempt to characterize the metabolic processes associated with this amoebo-flagellate transformation. Proflavin (inhibitor of cellular RNA synthesis), puromycin, cycloheximide and streptomycin (inhibitors of protein synthesis), blocked the transformation; however, actinomycin D (inhibitor of DNA-dependent RNA synthesis) did not block this transformation. On the other hand, 2-mercaptoethanol and dithiothreitol did block flagella formation, but even high concentrations of colchicine failed to have such an effect. Flagellate formation was more strongly inhibited by inhibitors of oxidative phosphorylation than by other respiratory inhibitors; this suggests that oxidative phosphorylation takes part in the energy metabolism of this transformation.     

Requirement for Golgi-localized PI(4)P in fusion of COPII vesicles with Golgi compartments

The role of specific membrane lipids in transport between endoplasmic reticulum (ER) and Golgi compartments is poorly understood. Using cell-free assays that measure stages in ER-to-Golgi transport, we screened a variety of enzyme inhibitors, lipid-modifying enzymes, and lipid ligands to investigate requirements in yeast. The pleckstrin homology (PH) domain of human Fapp1, which binds phosphatidylinositol-4-phosphate (PI(4)P) specifically, was a strong and specific inhibitor of anterograde transport. Analysis of wild type and mutant PH domain proteins in addition to recombinant versions of the Sac1p phosphoinositide-phosphatase indicated that PI(4)P was required on Golgi membranes for fusion with coat protein complex II (COPII) vesicles. PI(4)P inhibition did not prevent vesicle tethering but significantly reduced formation of soluble n-ethylmaleimide sensitive factor adaptor protein receptor (SNARE) complexes between vesicle and Golgi SNARE proteins. Moreover, semi-intact cell membranes containing elevated levels of the ER-Golgi SNARE proteins and Sly1p were less sensitive to PI(4)P inhibitors. Finally, in vivo analyses of a pik1 mutant strain showed that inhibition of PI(4)P synthesis blocked anterograde transport from the ER to early Golgi compartments. Together, the data presented here indicate that PI(4)P is required for the SNARE-dependent fusion stage of COPII vesicles with the Golgi complex.     

Influence of the cytoskeleton,energy supply,and protein synthesis on the structure of the endoplasmic reticulum

Summary FITC-Con A fluorescence was used to visualize rER arrangement of endothelial cells derived fromXenopus laevis tadpole hearts. In particular determinants of rER organization, intracellular localization and the interrelationships with other organelles were analysed. rER occurs in association with nucleus, mitochondria and microtubules.The structure of rER is strongly affected by energy metabolism and by microtubules. In order to elucidate the interdependence of structure and function we examined the influence of cellular respiration, net lactate production and protein synthesis on rER morphology, as well as the relationship between energy metabolism and protein synthesis. ER morphology is determined primarily by energy consuming intracellular transport mechanisms. Energy needed for protein synthesis is supplied by the respiratory chain while ATP from aerobic glycolysis only compensates when respiration is disturbed.Abbreviations ACM amphibian culture medium - ATP adenosine triphosphate - FITC-Con A fluoresceine-isothiocyanate-coupled con-canavalin A - MT microtubule - rER rough endoplasmic reticulum - sER smooth endoplasmic reticulum - TRITC-phalloidin tetramethyl-rhodaminyl-isothiocyanate-coupled phalloidin - pXTH primary cells fromXenopus laevis tadpole hearts - XTH-2 endothelial cell line derived fromXenopus laevis tadpole hearts     

Different concentrations of thyroid peroxidase and thyroglobulin in the nuclear envelope and the endoplasmic reticulum throughout the cytoplasm.

Thyroid peroxidase (TPO) and thyroglobulin (TG) represent two major glycoproteins of thyroid follicular cells performing biological functions such as iodination, transcytosis of thyroglobulin, and formation of thyroid hormones. They are involved in thyroid autoimmunity and thyroid inborn metabolic disorders. Studying these processes at a molecular level includes the determination of their precise intracellular distribution. An evaluation of the relative concentrations of TG and TPO in different subcellular compartments was carried out in stimulated human follicular cells using thin-frozen sections and the immunogold technique. It is documented that TG is transported from the endoplasmic reticulum and the Golgi apparatus to the follicular lumen by transport vesicles; most of it being present in the expanded endoplasmic reticulum throughout the cytoplasm. On the other hand, gold particles indicating TPO are adjacent to the membranes of the exocytotic pathway. They do not label the basolateral membrane but show the strongest density in the nuclear envelope and the apical membrane. The labeling density of TPO is about four times higher in the nuclear envelope than in the endoplasmic reticulum throughout the cytoplasm. In contrast, TG is concentrated three times higher in the rough endoplasmic reticulum throughout the cytoplasm than in the nuclear cisternae. Our results give the first quantitative evidence that TPO and TG are concentrated in different subcompartments of the endoplasmic reticulum. Because previous studies demonstrated the nuclear envelope as the site where the synthesis of endogenous peroxidase (Br?kelmann, J., D. W. Fawcett, Biol. Reprod. 1, 59-71 (1969)) begins, we suggest that synthesis of these functionally related proteins happens in specialized parts of the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Embryogenesis and germination in rye (Secale cereale L.)

Summary When rye embryos imbibe water they rapidly return to a condition of biochemical and structural complexity. Three stages of imbibition can be recognised: Phase I a short period (10 min) of physical wetting; Phase II a longer period (1 h) when little further imbibition occurs, followed by Phase III a continuous phase of active water uptake. The latter coincides with an increase in respiration rate and an increase both in the number of mitochondria and of cristae within them. Changes in fine structure become evident in all organelles in Phase III, after 2 h of imbibition. In the unimbibed embryo endoplasmic reticulum is present only as short crescents associated with electron lucent bodies, but in Phase III the endoplasmic reticulum proliferates to form many surrounding cirlets. After 6 h these circlets become fewer and instead the endoplasmic reticulum is seen in close association with the nuclear membrane. Concurrently incorporation of radioactive uridine and thymidine is first detectible. This suggests that the large increase in protein synthesis occurs on new ribosomes present on the reticulum associated with the nuclear membrane. For the first 6 h protein synthesis must occur either on polysomes within the dense packing of ribosomes or on these circlets of endoplasmic reticulum associated with electron lucent bodies.     

Formation of plant RNA virus replication complexes on membranes: role of an endoplasmic reticulum-targeted viral protein.

The mechanisms that direct positive-stranded RNA virus replication complexes to plant and animal cellular membranes are poorly understood. We describe a specific interaction between a replication protein of an RNA plant virus and membranes in vitro and in live cells. The tobacco etch virus (TEV) 6 kDa protein associated with membranes as an integral protein via a central 19 amino acid hydrophobic domain. In the presence or absence of other viral proteins, fluorescent fusion proteins containing the 6 kDa protein associated with large vesicular compartments derived from the endoplasmic reticulum (ER). Infection by TEV was associated with a collapse of the ER network into a series of discrete aggregated structures. Viral RNA replication complexes from infected cells were also associated with ER-like membranes. Targeting of TEV RNA replication complexes to membranous sites of replication is proposed to involve post-translational interactions between the 6 kDa protein and the ER.     

Mechanisms of protein yolk synthesis and deposition in crustacean oocytes

Summary Electron microscope studies on the oocytes of several crustacean species demonstrate that the protein yolk arises within vesicular and lamellar forms of the rough-surfaced endoplasmic reticulum. The vesicular form of the endoplasmic reticulum may have its origin from a blebbing process of the outer layer of the nuclear envelope. Disc-shaped granules, representing precursor elements of the yolk granules, appear within the vesicular and lamellar profiles of endoplasmic reticulum. Autoradiographic results suggest that the ribosomes attached to the endoplasmic reticulum take part in the biosynthesis of yolk proteins. Numerous disc-shaped granules accumulate within the cisternae of the endoplasmic reticulum, but eventually they undergo a transformation into a finely granular yolk granule. Thus, both the origin and growth of protein yolk granules occur within membranes constituting the endoplasmic reticulum. The results provide evidence that intra-ooplasmic synthesis of yolk protein occurs in these oocytes.This investigation was supported by research grants (HD-00699; GM-09229) and a Career Development Award (GM-11,524) from the National Institutes of Health, U.S. Public Health Service.     

NUCLEAR MEMBRANES FROM MAMMALIAN LIVER : I. Isolation Procedure and General Characterization

Nuclear membranes were isolated from rat and pig liver by sonication of highly purified nuclear fractions and subsequent removal of adhering nucleoproteins in a high salt medium. The fractions were examined in the electron microscope by both negative staining and thin sectioning techniques and were found to consist of nuclear envelope fragments of widely varying sizes. Nuclear pore complex constituents still could frequently be recognized. The chemical composition of the nuclear membrane fractions was determined and compared with those of microsomal fractions prepared in parallel. For total nuclei as well as for nuclear membranes and microsomes, various enzyme activities were studied. The results indicate that a similarity exists between both fractions of cytomembranes, nuclear envelope, and endoplasmic reticulum, with respect to their RNA:protein ratio and their content of polar and nonpolar lipids. Both membranous fractions had many proteins in common including some membrane-bound enzymes. Activities in Mg-ATPase and the two examined cytochrome reductases were of the same order of magnitude. The content of cytochrome b₅ as well as of P-450 was markedly lower in the nuclear membranes. The nuclear membranes were found to have a higher buoyant density and to be richer in protein. The glucose-6-phosphatase and Na-K-ATPase activities in the nuclear membrane fraction were very low. In the gel electrophoresis, in addition to many common protein bands, some characteristic ones for either microsomal or nuclear membranous material were detected. Significant small amounts of DNA and RNA were found to remain closely associated with the nuclear envelope fragments. Our findings indicate that nuclear and endoplasmic reticulum membranes which are known to be in morphological continuity have, besides a far-reaching similarity, some characteristic differences.