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PubMed2005

Cellular functions of endoplasmic reticulum chaperones calreticulin, calnexin, and ERp57.

Glycosylated proteins destined for the cell surface or to be secreted from the cell are trafficked through the endoplasmic reticulum during synthesis and folding. Correct folding is determined in large part by the sequence of the protein, but it is also assisted by interaction with enzymes and chaperones of the endoplasmic reticulum. Calreticulin, calnexin, and ERp57 are among the endoplasmic chaperones that interact with partially folded glycoproteins and determine if the proteins are to be released from the endoplasmic reticulum to be expressed, or alternatively, if they are to be sent to the proteosome for degradation. Studies on the effect of alterations in the expression and function of these proteins are providing information about the importance of this quality control system, as well as uncovering other important functions these proteins play outside of the endoplasmic reticulum.

Animals↗
PubMed2001

Protein sorting upon exit from the endoplasmic reticulum.

It is currently thought that all secretory proteins travel together to the Golgi apparatus where they are sorted to different destinations. However, the specific requirements for transport of GPI-anchored proteins from the endoplasmic reticulum to the Golgi apparatus in yeast could be explained if protein sorting occurs earlier in the pathway. Using an in vitro assay that reconstitutes a single round of budding from the endoplasmic reticulum, we found that GPI-anchored proteins and other secretory proteins exit the endoplasmic reticulum in distinct vesicles. Therefore, GPI-anchored proteins are sorted from other proteins, in particular other plasma membrane proteins, at an early stage of the secretory pathway. These results have wide implications for the mechanism of protein exit from the endoplasmic reticulum.

Adenosine Triphosphatases↗
PubMed2002

Monitoring of free calcium in the neuronal endoplasmic reticulum: an overview of modern approaches.

The concentration of free calcium within the lumen of the endoplasmic reticulum ([Ca2+]L) fluctuates between 100 and 1000 microM. High [Ca2+]L provides an electro-driving force for Ca2+ release and supports high Ca2+ diffusion rate within the endoplasmic reticulum lumen. Fluctuations in [Ca2+]L also regulate numerous chaperones, responsible for postranslational protein processing. Thus, [Ca2+]L integrates various signalling events and establishes a link between fast signalling, associated with the endoplasmic reticulum Ca2+release/uptake, and long-lasting adaptive responses relying primarily on the regulation of protein synthesis. This paper overviews modern approaches for the direct monitoring of [Ca2+]L which rely on three classes of low-affinity Ca2+ probes: ER-targeted aequorin, synthetic fluorescent Ca2+ dyes and GFP-based ER-targeted Ca2+ probes. These techniques, especially as applied to neurones, may substantially widen our appreciation of the endoplasmic reticulum as a universal signalling organelle.

Aequorin↗
PubMed1999

The processing alpha1,2-mannosidase of Saccharomyces cerevisiae depends on Rer1p for its localization in the endoplasmic reticulum.

The yeast alpha1,2-mannosidase Mns1p is involved in N-linked oligosaccharide processing in Saccharomyces cerevisiae by converting Man9GlcNAc2 to a single isomer of Man8GlcNAc2. alpha1,2-Mannosidase is a 63 kDa type II resident membrane protein of the endoplasmic reticulum that has none of the known endoplasmic reticulum localization signals (HDEL/KDEL, KKXX, or RRXX). Using antibodies against recombinant alpha1,2-mannosidase, indirect immunofluorescence showed that alpha1,2-mannosidase localization is abnormal in rer1 cells and that the alpha1,2-mannosidase localizes in the vacuoles of rer1/deltapep4 cells whereas in wild-type and deltapep4 cells it is found in the endoplasmic reticulum. 35S-labeled cell extracts were subjected to double immunoprecipitation, first with antibodies to alpha1,2-mannosidase, then with either alpha1,2-mannosidase antibodies or antibodies to alpha1,6-mannose residues added in the Golgi. The labeled proteins were examined by autoradiography after sodium dodecyl sulfate polyacrylamide gel electrophoresis. A significant proportion of the labeled alpha1,2-mannosidase was immunoprecipitated by alpha1,6-mannose antibodies in wild-type, deltapep4 and rer1/deltapep4 cells with endogenous levels of alpha1,2-mannosidase, and in wild-type, deltapep4, rer1 and rer1/deltapep4 cells overexpressing alpha1,2-mannosidase. The alpha1,2-mannosidase of rer1/deltapep4 cells had a slower mobility on the gels than alpha1,2-mannosidase precipitated from wild-type or deltapep4 cells, indicating increased glycosylation due to transport through the Golgi to the vacuoles. It is concluded that the endoplasmic reticulum localization of alpha1,2-mannosidase in wild-type cells depends on Rer1p for retrieval from an early Golgi compartment.

Biological Transport↗
PubMed2001

Structure and function of Class I alpha 1,2-mannosidases involved in glycoprotein synthesis and endoplasmic reticulum quality control.

Class I alpha 1,2-mannosidases (glycosylhydrolase family 47) are conserved through eukaryotic evolution. This protein family comprises three subgroups distinguished by their enzymatic properties. The first subgroup includes yeast (Saccharomyces cerevisiae) and human alpha 1,2-mannosidases of the endoplasmic reticulum that primarily form Man(8)GlcNAc(2) isomer B from Man(9)GlcNAc(2). The second subgroup includes mammalian Golgi alpha 1,2-mannosidases, as well as enzymes from insect cells and from filamentous fungi, that trim Man(9)GlcNAc(2) to Man(8)GlcNAc(2) isomers A and/or C intermediates toward the formation of Man(5)GlcNAc(2). Yeast and mammalian proteins of the third subgroup have no enzyme activity with Man(9)GlcNAc(2) as substrate. The members of subgroups 1 and 3 participate in endoplasmic reticulum quality control and promote proteasomal degradation of misfolded glycoproteins. The yeast endoplasmic reticulum alpha 1,2-mannosidase has served as a model for structure-function studies of this family. Its structure was determined by X-ray crystallography as an enzyme-product complex. It consists of a novel (alpha alpha)(7) barrel containing the active site that includes essential acidic residues and calcium. The structures of the subgroup 1 human endoplasmic reticulum alpha 1,2-mannosidase and of a subgroup 2 fungal alpha 1,2-mannosidase were determined by molecular replacement. Comparison of the enzyme structures is providing some insight into the reasons for their different specificities.

Animals↗
PubMed1979

Role of glucocorticoids and progesterone in the development of rough endoplasmic reticulum involved in casein biosynthesis.

Hydrocortisone acetate injected into pseudopregnant rabbits induced casein synthesis and a parallel accumulation of casein mRNA. These effects were not accompanied by any enrichment of total RNA in the mammary cell. Hydrocortisone acetate did not favour the attachment of polysomes to endoplasmic reticulum. Casein mRNA concentration was enhanced in free and membrane-bound polysomes. After long treatments, the concentration of casein mRNA reached a plateau in membrane bound polysomes whereas it continued to be accumulated in free polysomes, suggesting that a substantial part of casein synthesis is then carried out by free polysomes. Progesterone injected with high doses of prolactin was unable to prevent the stimulatory action of prolactin on the synthesis of casein, the accumulation of casein mRNA and mammary gland growth, as judged by DNA content. By contrast, the increase in the total RNA content of mammary gland was still significantly reduced by progesterone. In addition, progesterone inhibited almost completely the formation of membrane-bound polysomes and the anchorage of casein mRNA to endoplasmic reticulum. From these data, it was concluded that the formation of the endoplasmic reticulum is not a prerequisite for the initiation of casein synthesis. Glucocorticoids do not play a major role in the formation of the endoplasmic reticulum and the Golai apparatus and in the binding of casein synthesizing polysomes to membranes. Progesteronne is capable of inhibiting preferentially and gradually the stimulation of cellular functions requiring the most potent prolactin stimulation.

Animals↗
PubMed1999

Smooth endoplasmic reticulum in fish Mauthner cells at different functional states.

The ultrastructure of Mauthner cells of goldfish fry and adult xenotoca in intact state and after prolonged natural stimulation has been studied qualitatively and quantitatively. Additionally, Mauthner cells of intact adult goldfish and adult rotan Percottus glehni were investigated. In all adult fish the dendroplasm of the two major dendrites was shown to contain a regular network of smooth endoplasmic reticulum, with cisterns and tubules arranged transversally to the dendrite stem. In the Mauthner cells of intact goldfish fry, the reticulum was not clearly expressed, the transversal cisterns occurred occasionally. After stimulation, however, it became more developed probably due to proliferation of additional transversal cisterns. The periodicity of transversally oriented cisterns in the dendrites of Mauthner cells in each fish species studied was nearly the same. However, the number of transversal cisterns per unit of dendrite length, and the total length of cisterns and tubules per unit of cross-section area varied both within and among the species. These parameters increased after stimulation. It is suggested that the proliferation of the transversal cisterns in the endoplasmic reticulum and the extent of their development depend on the functional state of the afferent synapses and the plasticity of the smooth reticulum reflects the involvement of postsynaptic mechanisms in regulation of Mauthner cell stability presumably via the regulation of calcium homeostasis under varying conditions of functioning.

Aging↗
PubMed2002

Molecular mechanisms of reduced sarcoplasmic reticulum Ca(2+) uptake in human failing left ventricular myocardium.

BACKGROUND: Human failing heart due to idiopathic dilated cardiomyopathy is associated with decreased sarcoplasmic reticulum Ca(2+) uptake. However, it is unknown as to which mechanism leads to this abnormality. METHODS: Immunodetectable sarcoplasmic reticulum proteins (phospholamban [PLB], phosphorylated PLB at serine-16 or threonine-17, calsequestrin and Ca(2+)-ATPase levels), the activities of Ca(2+)-calmodulin-dependent protein kinase and protein phosphatase and Ca(2+) uptake at varying Ca(2+) concentrations were determined in left ventricular specimens from the same 7 failing hearts (ejection fraction 20 +/- 2%) due to idiopathic dilated cardiomyopathy and 5 non-failing explanted control donor hearts. RESULTS: In failing hearts, compared with control donors, decreased maximal velocity and affinity of Ca(2+) uptake for Ca(2+) were found to be associated with reduced expression levels of Ca(2+)-adenosine triphosphatase (ATPase), PLB and phosphorylated PLB at serine-16, but not of calsequestrin and phosphorylated PLB at threonine-17. In contrast, protein phosphatase activity increased significantly and the activity and protein expression level of the delta isoform of Ca(2+)-calmodulin-dependent protein kinase remained unchanged in failing hearts compared with control donors. CONCLUSIONS: The impaired maximal velocity of sarcoplasmic reticulum Ca(2+) uptake may be due in part to reduced protein expression level of Ca(2+)-ATPase, whereas the reduced affinity may be due in part to the reduced ratio of Ca(2+)-ATPase to PLB and reduced PLB phosphorylation at serine-16 in failing hearts. The latter abnormality may be due in part to increased protein phosphatase activity. These results suggest that selective enhancement of Ca(2+) uptake into the sarcoplasmic reticulum by pharmaceutical agents, or by molecular tools that inhibit phosphatase activity, would be a valuable therapeutic approach for treating, or at least retarding, the process of heart failure.

Adolescent↗
PubMed1995

Transport of adenosine triphosphate into endoplasmic reticulum proteoliposomes.

We have reconstituted a partially purified extract from rat liver endoplasmic reticulum membrane proteins into phosphatidylcholine liposomes. The resulting proteoliposomes, of an average diameter of 58 nm, transport intact ATP into their lumen in a temperature-dependent manner; transport was saturable (apparent Km = 0.72 microM) and highly specific: CMP-sialic acid and GTP were transported very slowly or not at all. Transport of ATP was inhibited by DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) but not by carboxyatractyloside. Previously, we showed that vesicles derived from rat liver and dog pancreas endoplasmic reticulum translocate ATP into their lumen in vitro but in these studies, following incubations with ATP, most of the phosphate was transferred to proteins because of the many kinases, endogenous acceptors for phosphorylation, and ATP binding proteins present in the vesicle membranes and lumen. This reconstituted system, which yielded a highly functional ATP transporter, can be used for further characterization and purification of this and probably other nucleotide transporters of the endoplasmic reticulum membrane. Previously used reconstitution protocols which were successful for Golgi membrane nucleotide transporters did not yield a functional endoplasmic reticulum ATP transporter.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2000

Regulatory role of phospholamban in the efficiency of cardiac sarcoplasmic reticulum Ca2+ transport.

Phospholamban is an inhibitor of the sarcoplasmic reticulum Ca(2+) transport apparent affinity for Ca(2+) in cardiac muscle. This inhibitory effect of phospholamban can be relieved through its phosphorylation or ablation. To better characterize the regulatory mechanism of phospholamban, we examined the initial rates of Ca(2+)-uptake and Ca(2+)-ATPase activity under identical conditions, using sarcoplasmic reticulum-enriched preparations from phospholamban-deficient and wild-type hearts. The apparent coupling ratio, calculated by dividing the initial rates of Ca(2+) transport by ATP hydrolysis, appeared to increase with increasing [Ca(2+)] in wild-type hearts. However, in the phospholamban-deficient hearts, this ratio was constant, and it was similar to the value obtained at high [Ca(2+)] in wild-type hearts. Phosphorylation of phospholamban by the catalytic subunit of protein kinase A in wild-type sarcoplasmic reticulum also resulted in a constant value of the apparent ratio of Ca(2+) transported per ATP hydrolyzed, which was similar to that present in phospholamban-deficient hearts. Thus, the inhibitory effects of dephosphorylated phospholamban involve decreases in the apparent affinity of sarcoplasmic reticulum Ca(2+) transport for Ca(2+) and the efficiency of this transport system at low [Ca(2+)], both leading to prolonged relaxation in myocytes.

Adenosine Triphosphate↗
PubMed1989

Interaction of the local anesthetics dibucaine and tetracaine with sarcoplasmic reticulum membranes. Differential scanning calorimetry and fluorescence studies.

The local anesthetics dibucaine and tetracaine inhibit the (Ca2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum [DeBoland, A. R., Jilka, R. L., & Martonosi, A. N. (1975) J. Biol. Chem. 250, 7501-7510; Suko, J., Winkler, F., Scharinger, B., & Hellmann, G. (1976) Biochim. Biophys. Acta 443, 571-586]. We have carried out differential scanning calorimetry and fluorescence measurements to study the interaction of these drugs with sarcoplasmic reticulum membranes and with purified (Ca2+ + Mg2+)-ATPase. The temperature range of denaturation of the (Ca2+ + Mg2+)-ATPase in the sarcoplasmic reticulum membrane, determined from our scanning calorimetry experiments, is ca. 45-55 degrees C and for the purified enzyme ca. 40-50 degrees C. Millimolar concentrations of dibucaine and tetracaine, and ethanol at concentrations higher than 1% v/v, lower a few degrees (degrees C) the denaturation temperature of the (Ca2+ + Mg2+)-ATPase. Other local anesthetics reported to have no effect on the ATPase activity, such as lidocaine and procaine, did not significantly alter the differential scanning calorimetry pattern of these membranes up to a concentration of 10 mM. The order parameter of the sarcoplasmic reticulum membranes, calculated from measurements of the polarization of the fluorescence of diphenylhexatriene, is not significantly altered at the local anesthetic concentrations that shift the denaturation temperature of the (Ca2+ + Mg2+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Effects of potassium on lipid-protein interactions in light sarcoplasmic reticulum.

This study shows the effect of K+ on phospholipid-protein interactions in light sarcoplasmic reticulum (LSR) as measured by 31P NMR. In the presence of 110 mM K+, a substantial effect of the membrane protein on the behavior of the phospholipids was detected. Subtracting the spectrum of the LSR lipid extract from the spectrum of the intact LSR membrane produced a difference spectrum of much greater breadth than the normal phospholipid bilayer powder pattern. This powder pattern is indicative of a phospholipid domain considerably more motionally restricted than the phospholipids in a normal phospholipid bilayer. The apparent axially symmetric powder pattern is consistent with axial diffusion. In a reconstituted membrane containing the calcium pump protein at a lipid/protein ratio much less than in the light sarcoplasmic reticulum, the broad component was more prominent. The relative resonance intensity of the broad component appeared to be proportional to the lipid/protein ratio of the membrane. In 10 mM K+, no broad powder pattern is observed in the corresponding difference spectrum. Thus, in the absence of potassium, the membrane protein has much less influence on the phospholipid of the membrane, as measured by 31P NMR. In addition to the effects of K+ on the membrane structure of the sarcoplasmic reticulum, K+ modulated the function of the calcium pump. The rate of calcium-dependent ATP hydrolysis increased in light sarcoplasmic reticulum when [K+] increased from 10 to 110 mM. The rate of calcium transport was also stimulated by an increase in K+.

Adenosine Triphosphate↗
PubMed2001

Molecular mechanisms of regulation of the activity of sarcoplasmic reticulum Ca-release channels (ryanodine receptors), muscle fatigue, and Severin's phenomenon.

In this short review of the literature and our own data the characteristics of structural organization of sarcoplasmic reticulum Ca-release channels (ryanodine receptors) in different types of muscles, the participation of other sarcoplasmic reticulum proteins in excitation-contraction coupling and Ca-release channel operation, and the regulation of the channel activity by endogenous low molecular weight compounds are analyzed. Special attention is given to changes that occur in muscle cells during exhausting work and to the role of sarcoplasmic reticulum Ca-release channels in the loss of muscle contractile activity during the development of fatigue. It is concluded that the protection of muscle fibers against fatigue in the presence of the histidine-containing dipeptide carnosine, called in the literature "Severin's phenomenon", is primarily connected with modulation of sarcoplasmic reticulum Ca-release channel activity by carnosine.

Animals↗
PubMed2003

Role of lipids in the assembly of endoplasmic reticulum and dictyosomes in neuronal cells from the cerebral cortex of Yakutian ground squirrel (Citellus undulatus) during hibernation.

Phospholipids and cholesterol were assayed in homogenates and microsomal fractions from the cerebral cortex of summer-active, winter-torpid, and winter-active Yakutian ground squirrels (Citellus undulatus). Ultrastructural analysis of both microsomal fraction and intact neurons was performed by serial ultramicrotomy. The levels of sphingomyelin (SM), phosphatidylserine (PS), and phosphatidylethanolamine (PEA) were decreased in homogenates from the cerebral cortex of winter ground squirrels compared with the summer-active animals, while the levels of phosphatidylcholine (PC) and cardiolipin (CL) were increased. The level of cholesterol was decreased in the cerebral cortex of winter-torpid animals compared with both winter-active and summer-active animals, and the level of total phospholipids was decreased in comparison to the summer-active animals. Three-dimensional reconstruction of serial membrane profiles displayed the microsomal fraction to be an interconnected system of cisterns and vesicles, which corresponds to endoplasmic reticulum and dictyosomes (Golgi stacks) of intact neurons. In winter the content of PC was increased in the microsomal fraction, while the contents of lysophosphatidylcholine (LPC), PS, phosphatidylinositol (PI), and SM were decreased. In winter-torpid animals compared with the winter-active ones the contents of total phospholipids, PEA, LPC, and cholesterol were decreased. As for the winter-active ground squirrels, their lipid contents did not differ from those in the summer-active animals, but LPC content was decreased. The changes in microsomal lipid contents in intact pyramidal neurons throughout the hibernation were accompanied by disassembly of dictyosomes and endoplasmic reticulum (ER), including the decomposition of polyribosomes to monosomes. The ultrastructural analysis of nucleoli, ER, and dictyosomes of both winter-active and torpid ground squirrels showed a direct correlation between the increasing contents of both cholesterol and total phospholipids (mainly PEA and LPC) in microsomes and the structural recovery of endoplasmic reticulum, Golgi stacks, and nucleoli in intact pyramidal neurons. A role of seasonal variations in lipid contents of brain cells in their adaptation to low temperature is discussed. We also propose an involvement of cholesterol in the activation of protein-synthesizing function of endoplasmic reticulum and Golgi stacks in intact neurons.

Animals↗
PubMed1998

The cytoplasmic tail of the mouse brown locus product determines intracellular stability and export from the endoplasmic reticulum.

Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase related proteins. Human TRP-1/gp75 is sorted to melanosomes through the endoplasmic reticulum and Golgi complex to the endocytic pathway, directed by a sorting signal located in the cytoplasmic tail. This hexapeptide cytoplasmic sequence, which is conserved in the tyrosinase related protein family and through vertebrate evolution, was shown to act also as a sorting signal in mouse gp75, confirming that its sorting and cellular retention function is conserved between human and mouse. The cytoplasmic tail influenced the rate and efficiency of intracellular transport of gp75 from the endoplasmic reticulum to the cis-Golgi. Deletion of 33 or 27 amino acids from the carboxyl end of the 38 amino acid cytoplasmic tail of gp75 caused retention and rapid degradation of the truncated gp75 in the endoplasmic reticulum. This defective movement could be fully corrected by extending the truncated tail with the unrelated cytoplasmic tail of the low density lipoprotein receptor. Thus, the cytoplasmic tail of mouse gp75 not only determines sorting to the endocytic/melanosomal compartment, but also controls export from the endoplasmic reticulum to Golgi.

Amino Acid Sequence↗
PubMed2001

A role for smooth endoplasmic reticulum membrane cholesterol ester in determining the intracellular location and regulation of sterol-regulatory-element-binding protein-2.

Cellular cholesterol homoeostasis is regulated through proteolysis of the membrane-bound precursor sterol-regulatory-element-binding protein (SREBP) that releases the mature transcription factor form, which regulates gene expression. Our aim was to identify the nature and intracellular site of the putative sterol-regulatory pool which regulates SREBP proteolysis in hamster liver. Cholesterol metabolism was modulated by feeding hamsters control chow, or a cholesterol-enriched diet, or by treatment with simvastatin or with the oral acyl-CoA:cholesterol acyltransferase inhibitor C1-1011 plus cholesterol. The effects of the different treatments on SREBP activation were confirmed by determination of the mRNAs for the low-density lipoprotein receptor and hydroxymethylglutaryl-CoA (HMG-CoA) reductase and by measurement of HMG-CoA reductase activity. The endoplasmic reticulum was isolated from livers and separated into subfractions by centrifugation in self-generating iodixanol gradients. Immunodetectable SREBP-2 accumulated in the smooth endoplasmic reticulum of cholesterol-fed animals. Cholesterol ester levels of the smooth endoplasmic reticulum membrane (but not the cholesterol levels) increased after cholesterol feeding and fell after treatment with simvastatin or C1-1011. The results suggest that an increased cellular cholesterol load causes accumulation of SREBP-2 in the smooth endoplasmic reticulum and, therefore, that membrane cholesterol ester may be one signal allowing exit of the SREBP-2/SREBP-cleavage-regulating protein complex to the Golgi.

Acetamides↗
PubMed1969

The adenosine triphosphatase and calcium ion-transporting activities of the sarcoplasmic reticulum of developing musce.

1. The ATPase (adenosine triphosphatase) specific activity and the total nitrogen content of the myofibrillar fraction per g. wet weight of rabbit longissimus dorsi muscle increased steadily during the late foetal stages and the first few weeks after birth. 2. The ATPase specific activity of the sarcoplasmic-reticular fraction isolated by a sucrose-density-gradient procedure rose to a sharp peak 8-10 days after birth and then declined to the adult value, which was about 25% of the maximum. 3. The peak in ATPase activity was a feature of the sarcoplasmic reticulum isolated from muscle, and the time at which it occurred in relation to birth was related to the degree of development and the activity pattern of the muscle. 4. The peak in ATPase activity of the sarcoplasmic reticulum occurred at an earlier age if newborn animals were made to exercise earlier than was normal. 5. The ;extra' ATPase associated with the sarcoplasmic reticulum and the ability to concentrate Ca(2+) increased in a similar manner over the period of development studied. 6. It is postulated that the Ca(2+)-transport system of the sarcoplasmic reticulum consists of two components, namely the ATPase and the system coupling this enzyme to Ca(2+) transport. During development the ATPase develops first and has almost reached maximum activity in the longissimus dorsi muscle of the rabbit after 8-10 days. Subsequently the activity of the coupling system rises rapidly, leading to an increase in the capacity and efficiency of Ca(2+) transport.

Adenosine Triphosphatases↗
PubMed1976

Similarities in the polypeptide composition of glyoxysomal and endoplasmic-reticulum membranes from castor-bean endosperm.

Microsomal fractions, glyoxysomes and mitochondria were isolated from homogenates of germinating castor-bean (Ricinus communis) endosperm by sucrose-density-gradient centrifugation. Washed membrane preparations from these cellular fractions were examined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. At corresponding developmental stages the endoplasmic-reticulum and glyoxysomal membranes were strikingly similar in polypeptide composition, at least 16 polypeptides being present in membranes isolated from 3-day-old tissue. Supplying [35S]methionine to intact endosperm tissue resulted in the labelling of all membrane polypeptides, the specific radioactivity in the endoplasmic reticulum being greater than for equivalent polypeptides of the glyoxysomal membrane. Washing these membranes with sodium deoxycholate solution extensively solubilized protein components, with the exception of a predominant polypeptide of mol.wt. 55000. Mitochondrial membrane preparations differed from those of the endoplasmic reticulum and glyoxysomes in polypeptide molecular-weight distribution and the [35S]methionine-labelling pattern. The similarity in polypeptide composition between endoplasmic-reticulum and glyoxysomal membranes is discussed in relation to glyoxysome biogenesis.

Ricinus communis↗