Isolation of plasma membrane, golgi apparatus, and endoplasmic reticulum fractions from single homogenates of mouse liver

Interactions of calreticulin with proteins of the endoplasmic and sarcoplasmic reticulum membranes

The ability of [125I]calreticulin to bind to membrane fractions isolated from different muscle and non-muscle tissues was examined by a protein overlay technique. Specific [125I]calreticulin binding proteins were detected in rat liver smooth and rough endoplasmic reticulum and Golgi, in canine pancreatic microsomes, and in rabbit skeletal muscle sarcoplasmic reticulum. These proteins were confined only to membranes that contain calreticulin; they were not found in rat liver mitochondria or cytosol. [125I]Calreticulin binds to a 50-kDa protein and a number of lower M(r) (20,000-38,000) endoplasmic reticulum membrane proteins and to 30-kDa protein in skeletal muscle sarcoplasmic reticulum. Full-length calreticulin and the carboxyl-terminal region (C-domain) of the protein both competed with [125I]calreticulin for binding to the membrane proteins. Binding of [125I]calreticulin to pancreatic microsomes was also partially inhibited by the N-domain and to a lesser extent by the P-domain of the protein. We conclude that calreticulin interacts with the endoplasmic reticulum membrane proteins mainly through its carboxyl-terminal domain and that the endoplasmic and sarcoplasmic reticulum membranes may contain different calreticulin binding proteins.

Fractionation of primary cultured cerebellar neurons: distribution of sialyltransferases involved in ganglioside biosynthesis

Primary cultured neurons were fractionated using sucrose density gradients. The activities of four sialyltransferases (GM3, GD3, GD1a, and GT1a synthase) involved in ganglioside biosynthesis were assayed in the collected fractions. The distribution of GM3 synthase coincided with that of mannosidase II, an enzyme assumed to be a cis-Golgi marker. Both enzymes were mainly associated with the more dense fraction. GD1a and GT1a synthase activities, on the other hand, were mainly recovered in the less dense fraction. Moreover, they were colocalized with thiamine pyrophosphatase, an enzyme assumed to be a marker of the late Golgi (trans-Golgi and trans-Golgi network). GD3 synthase activity was equally distributed between both fractions. These results are integrated in a model of ganglioside biosynthesis.

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Mouse liver contains a Pseudomonas aeruginosa exotoxin A-binding protein

The opportunistic pathogen Pseudomonas aeruginosa produces several potential virulence factors, including the ADP-ribosylating toxin, exotoxin A (PE). Studies using a burned mouse model have shown that PE consistently inhibits protein synthesis and depletes elongation factor 2 in mouse liver and variably in other organs. One reason for toxin sensitivity could be the presence of a PE receptor on the surface of cells. Therefore we examined detergent extracts of mouse tissues for the presence of toxin-binding proteins. Proteins which specifically bind PE were present in extracts from liver, kidney, lung, spleen, and heart. Because liver appears to be a prominent target for the toxin in a burned animal, we choose to isolate the PE-binding protein from mouse liver and compare this protein to the recently characterized toxin-binding protein from toxin-sensitive mouse LM fibroblasts. The toxin-binding proteins from both sources have a molecular mass of approximately 350 kDa, share similar protease digestion profiles, and are glycosylated. However the glycosylation patterns for the two species are quite different. Both glycoproteins bind toxin with high avidity. The toxin-binding moiety is located, at least in part, on the plasma membrane and thus could represent the receptor involved in internalization of toxin molecules responsible for cell death.

Processing of prothrombin in the secretory pathway

Antibodies raised against plasma prothrombin and the prothrombin propeptide were used to identify prothrombin precursors in rough and smooth microsomes and in the Golgi apparatus. The data demonstrate that the propeptide is part of the prothrombin molecule when undergoing a variety of modifications in the Golgi apparatus. It is shown that these modifications result in an increase in the apparent molecular mass of the prothrombin precursor from 78 kDa in early processing to 83 kDa in late processing. The 83 kDa prothrombin precursor was not recognized by the anti-propeptide antiserum and most likely represents the final product of the precursor in the secretory pathway. Evidence is presented that the propeptide is released from the parent molecule in the Golgi apparatus by a membrane-bound Ca(2+)-dependent serine proteinase(s) with characteristics similar to those of the proalbumin-to-albumin-converting enzyme. Vitamin K-dependent carboxylase activity was measured in membrane fragments obtained from the Golgi apparatus preparation. Sucrose-density-gradient centrifugation and the use of marker enzymes showed that carboxylase activity was highest in fractions enriched in cis-Golgi cisternae. Two different synthetic peptides were used as substrates for the carboxylase. These peptides were from the N-terminal and the C-terminal part of the gamma-carboxyglutamic acid (Gla) region of prothrombin. It is shown that the N-terminal and the C-terminal peptides were preferred as substrates for the carboxylase in the microsomal and the Golgi apparatus preparations respectively. It is also shown that the prothrombin precursor acquires negative charges in the Golgi apparatus that do not result from addition of sugars in late processing. These negative charges could be eliminated by thermal decarboxylation, suggesting that Gla residues may also be synthesized in late processing.

Development of monoclonal antibodies against different protein and carbohydrate epitopes of dipeptidyl peptidase IV from rat liver plasma membranes

Dipeptidyl peptidase IV (DPP IV) is a serine exopeptidase expressed at high levels in rat kidney, liver and lung. We established eight monoclonal antibodies against partially purified DPP IV from rat liver plasma membranes. By means of a competitive dot blot assay with purified DPP IV, these antibodies were shown to recognize four different epitopes of the glycoprotein, designated A - D. The epitopes are located on the extracellular domain of DPP IV, as shown by papain digestion of liver plasma membranes. Treatment of DPP IV with neuraminidase and glycopeptide N-glycosidase F, as well as incubation of hepatocytes with the alpha-mannosidase I inhibitor deoxymannojirimycin, revealed that epitope A may be formed by a mannose-rich sugar chain and epitope D might represent a complex carbohydrate structure in the mature glycoprotein, while the epitopes B and C are formed by the protein moiety. Concanavalin A reduced the binding of monoclonal antibody to epitope A by 78%. Binding to epitope D was blocked by 73% with wheat germ lectin, and by more than 99% with sialic acid; epitopes B and C were unaffected by any of the lectins or sugars tested. The immunological cross-reactivity with DPP IV from Morris hepatoma 7777 was demonstrated with monoclonal antibodies against epitopes A-C. Epitope D was not recognized on hepatoma DPP IV. However, in addition to DPP IV, four hepatoma plasma membrane glycoproteins were precipitated by the monoclonal antibody against the epitope D, indicating that this epitope is not uniquely restricted to DPP IV.

Food restriction in female Wistar rats. III. Thermotropic transition of membrane lipid and 5'-nucleotidase activity in hepatocytes

The effect of diet restriction was measured on the anisotropy parameter of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 5'-nucleotidase enzyme activity in liver plasma membrane preparates. Diet restriction was applied to rats 3.5 months old on an every-other-day schedule (EOD) and the rats were killed at the age of 28-29 months. Six months and 24 months rats, fed ad libitum (AL), were used as controls. The Arrhenius plots of anisotropy parameter of liver membranes from young, old AL and old EOD animals exhibited well defined breakpoints at 16.3 degrees C, 19.5 degrees C and 16.7 degrees C, respectively. The breakpoint temperature of 5'-nucleotidase activity was lower in samples from young rats as compared to those from old AL rats, whereas no difference was observed comparing young and EOD fed rats. Present results support the hypothesis that diet restriction modifies lipid composition of liver plasma membranes in such a way that the appearance of age-dependent alterations is delayed.

Subcellular biosynthesis and transport of gangliosides formed from exogenous lactosylceramide in rat liver

In order to clarify the mechanisms of ganglioside biosynthesis and transport we intravenously administered a liposomal dispersion of radiolabelled lactosylceramide (LacCer) to rats and then followed the time course of the individual gangliosides which became radioactive in the Golgi-apparatus and plasma-membrane fractions prepared from the liver. After administration of radiolabelled LacCer the liver retained a substantial amount of radioactivity, which was distributed among an organic phase (mainly residual LacCer), a fraction containing low-Mr substances (mainly 3H2O) and a ganglioside fraction. The hepatocytes were found to provide the bulk of gangliosides biosynthesized from exogenous LacCer. After subcellular fractionation, the total radioactive gangliosides increased in the Golgi apparatus up to 8 h, to then decrease and practically disappear at 24 h; in the plasma membranes they were progressively concentrated, accounting for high absolute values. Ganglioside patterns were greatly modified with time in both the Golgi apparatus and plasma membrane, but without significant differences between them. Biosynthesis in the Golgi apparatus and accumulation in the plasma membrane of each individual ganglioside followed a precursor-product relationship. The obtained results indicated that once a ganglioside is biosynthesized in the Golgi apparatus, it is in part made available for translocation to the plasma membrane, which rapidly occurs, and is in part retained in the Golgi apparatus, where it acts as a precursor for the biosynthesis of more glycosylated gangliosides.

Subcellular localization of iodinated thyroid tubulin

Subcellular fractions enriched in mitochondria, plasma membranes, microsomes and Golgi apparatus were obtained from thyroid glands of rats injected with I125. Autoradiography of SDS-polyacrylamide gels revealed the presence of a number of radiolabelled proteins in each membrane fraction. One polypeptide, with the same electrophoretic mobility as brain tubulin, was found in all fractions except the plasma membranes and was immunoprecipitated with commercial anti-tubulin monoclonal antibodies. Hydrolysis of Asp-Pro linkages of I125 labelled tubulin with formic acid indicated that there were iodination sites in both the carboxy terminal one third and the amino terminal two thirds of the molecule. These results, together with the absence of iodinated tubulin from the cytosolic fraction, are consistent with the idea that a population of thyroid membrane tubulin is iodinated at multiple sites either just before or after insertion into intracellular membranes where it may act as an anchorage point for microtubule-membrane interactions.

Endolyn-78, a membrane glycoprotein present in morphologically diverse components of the endosomal and lysosomal compartments: implications for lysosome biogenesis

A monoclonal antibody (2C5) raised against rat liver lysosomal membranes was used to identify a 78-kD glycoprotein that is present in the membranes of both endosomes and lysosomes and, therefore, is designated endolyn-78. In cultures of rat hepatoma (Fu5C8) and kidney cells (NRK), this glycoprotein could not be labeled with [35S]methionine or with [32P]inorganic phosphate but was easily labeled with [35S]cysteine and [3H]mannose. Pulse-chase experiments and determinations of endoglycosidase H (endo H) sensitivity showed that endolyn-78 is derived from a precursor of Mr 58-62 kD that is processed to the mature form with a t1/2 of 15-30 min. The protein has a 22-kD polypeptide backbone that is detected after a brief pulse in tunicamycin-treated cells. During a chase in the presence of the drug, this is converted into an O-glycosylated product of 46 kD that despite the absence of N-linked oligosaccharides is effectively transferred to lysosomes. This demonstrates that the delivery of endolyn-78 to this organelle is not mediated by the mannose-6-phosphate receptor (MPR). Immunocytochemical experiments showed that endolyn-78 is present in the limiting membranes and the interior membranous structures of morphologically identifiable secondary lysosomes that contain the lysosomal hydrolase beta-glucuronidase, lack the MPR, and could not be labeled with alpha-2-macroglobulin at 18.5 degrees C, a temperature which prevents appearance of endocytosed markers in lysosomes. Endolyn-78 was present at low levels in the plasma membrane and in peripheral tubular endosomes, but was prominent in morphologically diverse components of the endosomal compartment (vacuolar endosomes and various types of multivesicular bodies) which acquired alpha-2-macroglobulin at 18.5 degrees C, and frequently contained substantial levels of the MPR and variable levels of beta-glucuronidase. On the other hand, the MPR was very rarely found in endolyn-containing structures that were not labeled with alpha-2-macroglobulin at the low temperature. Thus, the process of lysosomal maturation appears to involve the progressive delivery of lysosomal enzymes to various types of endosomes that may have already received some of the lysosomal membrane proteins. Although endolyn-78 would be one of the proteins added early to endosomes, other lysosomal membrane proteins may be added only to multivesicular endosomes that represent very advanced stages of maturation.

Biochemical properties of dipeptidyl peptidase IV in liver and hepatoma plasma membranes

In the present investigation we compared the glycoprotein DPP IV from rat liver and Morris hepatoma 7777 by means of biochemical and immunological methods. For that purpose nine monoclonal anti-DPP IV-antibodies recognizing four different epitopes and a monospecific anti-DPP IV-antiserum were applied. In the homogenates of both tissues a plasma membrane-bound and a soluble form were detected. The immunological cross-reactivity of both forms was demonstrated with the antiserum and the monoclonal antibodies against the epitopes A, B and C while epitope D was restricted to liver plasma membrane. Differences of the distinct DPP IV forms were exhibited in the molecular weights, isoelectric points and peptide maps. In the hepatoma homogenate only 10% of DPP IV activity was found compared to normal liver but the ratio of soluble to membrane-bound form is higher in the hepatoma than in the liver. The fractionation of the homogenates into different cell components revealed for the liver a continuous increase of DPP IV activity from the endoplasmic reticulum fractions to the Golgi apparatus and finally to the plasma membranes. By contrast, in hepatoma the flow from the Golgi apparatus to plasma membrane was greatly reduced. The loss of DPP IV from the surface of cultured hepatoma cells was concomitant with a decrease of cell-substratum adhesion. DPP IV was found to be inserted into the liver plasma membrane by two different mechanisms, a phospholipase C-sensitive and a papain-sensitive one. In the hepatoma the phospholipase C-sensitive anchorage was not expressed. Besides liver and hepatoma the distribution of DPP IV was characterized in various rat organs by enzyme activity, histochemistry and immunohistochemistry with the anti-DPP IV-antibodies.

Genetic control of ganglioside biosynthesis in mice

The enzymatic basis for the differences in hepatic ganglioside patterns in the mouse strains C57Bl/6 and Swiss White (SW) was investigated. SW has a "Swiss-type" ganglioside profile, expressing GM1- and GD1a- in addition to GM2- as major hepatic gangliosides, whereas C57Bl/6 shows a "GM2-type" profile, expressing only GM2- as the major hepatic ganglioside. The enzyme UDP-galactose:GM2 ganglioside galactosyltransferase (GM2-GalT), which catalyzes the synthesis of GM1 ganglioside, showed a four- to fivefold elevation in intact and solubilized liver Golgi membrane fractions of the SW strain compared to C57Bl/6. Crosses between C57Bl/6 and SW produced an F1 generation with a hepatic ganglioside and enzymatic phenotype intermediate between those of the two parental strains. All three genotypic groups show two forms of the Golgi apparatus enzyme with isoelectric points of 6.5-6.8 and 8.3-9.0. The simplest mode of action of genes which control the enzymatic phenotype that would be consistent with these findings are one or two structural genes or one or two cis-regulatory genes affecting the rate of enzyme synthesis.

Long-term study of serum glycosyl-transferase levels in pregnant women and their newborn infants

1. The authors have studied changes of glucosyl-, galactosyl- and sialyl-transferase activities in the sera of women during pregnancy and the early post-partum period, and in neonates from birth to 1 yr of age. 2. Galactosyl- and glucosyl-transferase activities throughout pregnancy were high in comparison to control values, and increased with advancing gestation. In the early post-partum period, the activities of these enzymes decreased markedly, reaching values similar to those of control subjects. 3. There were no variations, however, in serum sialyl-transferase activity at any stage of pregnancy or the post-partum period. In newborn infants, serum glucosyl- and sialyl-transferase activities were identical to control values. 4. Galactosyl-transferase activity was high at birth, decreased progressively and returned to the normal range only after 1 yr. 5. The physiological significance of these glycosyl-transferases in the sera of pregnant women and newborn infants is discussed.

Subcellular distribution and biosynthesis of rat liver gangliosides

Gangliosides have generally been assumed to be localized primarily in the plasma membrane. Analysis of gangliosides from isolated subcellular membrane fractions of rat liver indicated that 76% of the total ganglioside sialic acid was present in the plasma membrane. Mitochondria and endoplasmic reticulum fractions, while containing only low levels of gangliosides on a protein basis, each contained approx. 10% of total ganglioside sialic acid. Gangliosides also were present in the Golgi apparatus and nuclear membrane fractions, and soluble gangliosides were in the supernatant. Individual gangliosides were non-homogeneously distributed and each membrane fraction was characterized by a unique ganglioside composition. Plasma membrane contained only 14 and 28% of the total GD1a and GD3, respectively, but 80-90% of the GM1, GD1b, GT1b and GQ1b. Endoplasmic reticulum, when corrected for plasma membrane contamination, contained only trace amounts of GM1, GD1b, GT1b and GQ1b, but 11 and 5% of the total GD1a and GD3, respectively. The ganglioside composition of highly purified endoplasmic reticulum was similar. Ganglioside biosynthetic enzymes were concentrated in the Golgi apparatus. However, low levels of these enzymes were present in the highly purified endoplasmic reticulum fractions. Pulse-chase experiments with [3H]galactose revealed that total gangliosides were labeled first in the Golgi apparatus, mitochondria and supernatant within 10 min. Labeled gangliosides were next observed at 30 min in the endoplasmic reticulum, plasma membrane and nuclear membrane fractions. Analysis of the individual gangliosides also revealed that GM3, GM1, GD1a and GD1b were labeled first in the Golgi apparatus at 10 min. These studies indicate that gangliosides synthesized in the Golgi apparatus may be transported not only to the plasma membrane, but to the endoplasmic reticulum and to other internal endomembranes as well.