Two sensitive, convenient, and widely applicable assays for marker enzyme activities specific to endoplasmic reticulum

Proteomics of Saccharomyces cerevisiae Organelles

Knowledge of the subcellular localization of proteins is indispensable to understand their physiological roles. In the past decade, 18 studies have been performed to analyze the protein content of isolated organelles from Saccharomyces cerevisiae. Here, we integrate the data sets and compare them with other large scale studies on protein localization and abundance. We evaluate the completeness and reliability of the organelle proteomics studies. Reliability depends on the purity of the organelle preparations, which unavoidably contain (small) amounts of contaminants from different locations. Quantitative proteomics methods can be used to distinguish between true organellar constituents and contaminants. Completeness is compromised when loosely or dynamically associated proteins are lost during organelle preparation and also depends on the sensitivity of the analytical methods for protein detection. There is a clear trend in the data from the 18 organelle proteomics studies showing that proteins of low abundance frequently escape detection. Proteins with unknown function or cellular abundance are also infrequently detected, indicating that these proteins may not be expressed under the conditions used. We discuss that the yeast organelle proteomics studies provide powerful lead data for further detailed studies and that methodological advances in organelle preparation and in protein detection may help to improve the completeness and reliability of the data.

Cell surface display and intracellular trafficking of free glycosylphosphatidylinositols in mammalian cells

In addition to serving as membrane anchors for cell surface proteins, glycosylphosphatidylinositols (GPIs) can be found abundantly as free glycolipids in mammalian cells. In this study we analyze the subcellular distribution and intracellular transport of metabolically radiolabeled GPIs in three different cell lines. We use a variety of membrane isolation techniques (subcellular fractionation, plasma membrane vesiculation to isolate pure plasma membrane fractions, and enveloped viruses to sample cellular membranes) to provide direct evidence that free GPIs are not confined to their site of synthesis, the endoplasmic reticulum, but can redistribute to populate other subcellular organelles. Over short labeling periods (2.5 h), radiolabeled GPIs were found at similar concentration in all subcellular fractions with the exception of a mitochondria-enriched fraction where GPI concentration was low. Pulse-chase experiments over extended chase periods showed that although the total amount of cellular radiolabeled GPIs decreased, the plasma membrane complement of labeled GPIs increased. GPIs at the plasma membrane were found to populate primarily the exoplasmic leaflet as detected using periodate oxidation of the cell surface. Transport of GPIs to the cell surface was inhibited by Brefeldin A and blocked at 15 degrees C, suggesting that GPIs are transported to the plasma membrane via a vesicular mechanism. The rate of transport of radiolabeled GPIs to the cell surface was found to be comparable with the rate of secretion of newly synthesized soluble proteins destined for the extracellular space.

Oleic acid stimulates rapid translocation of cholinephosphate cytidylyltransferase in type II cells

Activity of cholinephosphate cytidylyltransferase, the rate-limiting enzyme in phosphatidylcholine synthesis, increases rapidly in the lung after birth predominantly due to an increase in membrane-associated activity. Although there is strong evidence that enzyme translocation is a major regulatory mechanism in other cells, this mechanism has not been conclusively demonstrated in intact alveolar type II cells. In this study, we show that oleic acid stimulates rapid translocation of cytidylyltransferase activity and protein from cytosol to microsomes in both primary cultured fetal and adult type II cells and MLE12 cells, a cell line derived from murine distal respiratory epithelial cells. Shifts in subcellular distribution occurred within 5 min of exposure to 200 microM oleic acid. The magnitude of the increases in microsomal enzyme activity and immunoreactive protein levels was several-fold greater in d21 fetal cells than adult type II cells. Oleic acid-induced translocation was confirmed in in vitro translocation experiments. After incubating MLE12 cell postmitochondrial supernatants at 37 degrees C with oleic acid and separation of enzyme isoforms on glycerol density gradients, enzyme activity was decreased in gradient fractions corresponding to both cytosolic isoforms and microsomal activity increased 7.9-fold compared to the distribution of enzyme activity in postmitochondrial supernatants incubated at 4 degrees C without oleic acid. The increase in microsomal activity was associated with an increased incorporation of [14C]oleic acid in the membrane free fatty acid fraction. Developmental changes in type II cell membrane lipid composition may induce the rapid translocation/activation of cytidylyltransferase in the lung after birth.

Developmental changes in cholinephosphate cytidylyltransferase activity and microsomal phospholipid fatty acid composition in alveolar type II cells

Cholinephosphate cytidylyltransferase is the rate-limiting enzyme in the choline pathway of phosphatidylcholine synthesis in fetal and adult lung. To examine the developmental changes in cytidylyltransferase activity, subcellular fractions were prepared from freshly isolated type II cells from 19 and 21 d gestation fetal rats, newborn (0 h, 3 h, 3-12 h, +1, +2, +7 postnatal day), and adult rats and the fractions assayed for cytidylyltransferase activity. Cytidylyltransferase activity per cell was low during late fetal gestation, but rose rapidly during the first 3 h after birth, predominantly due to an increase in microsomal enzyme activity. Microsomal and cytosolic enzyme specific activity increased 6 and 3.9 fold, respectively, from birth (0 h) to +1 postnatal day. The subcellular distribution of enzyme activity changed from 40% microsomal in fetal type II cells to 85% microsomal at 3-12 h, and 60% in this compartment in adult type II cells. Although evidence suggests that membrane lipid may affect enzyme activity, developmental changes in type II cell membrane phospholipid fatty acid composition have not been previously studied. Capillary gas chromatography analysis of phospholipid fatty acids extracted from microsome fractions revealed a 3-fold increase in both total saturated and unsaturated phospholipid fatty acids from day 19 to day 21 gestation. There was a further 62% increase in total saturated fatty acids during the first postnatal day, concomitant with the peak in microsomal cytidylyltransferase activity.

Early lipid intermediates in glycosyl-phosphatidylinositol anchor assembly are synthesized in the ER and located in the cytoplasmic leaflet of the ER membrane bilayer

Glycosylated phosphoinositides serve as membrane anchors for numerous eukaryotic cell surface glycoproteins. Recent biochemical and genetic studies indicate that the glycolipids are assembled by sequential addition of components (monosaccharides and phosphoethanolamine) to phosphatidylinositol. The biosynthetic steps are presumed to occur in the ER, but formal proof of this is lacking. We describe experiments designed to establish the subcellular location of the initial steps in glycosyl-phosphatidylinositol (GPI) anchor biosynthesis and to define the transmembrane distribution of early biosynthetic lipid intermediates. The experiments were performed with the thymoma cell line BW5147.3. A subcellular fractionation protocol was used to show that early biosynthetic steps in GPI assembly, i.e., synthesis and deacetylation of N-acetylglucosaminyl phosphatidylinositol, occur in the ER. GPI biosynthetic intermediates were synthesized by incubating the microsomes with UDP-[3H]GlcNAc, and the transmembrane distribution of the labeled lipids was probed with phosphatidylinositol-specific phospholipase C (PI-PLC). Treatment of the radiolabeled microsomes with PI-PLC showed that > 70% of the N-acetylglucosaminyl phosphatidylinositol and glucosaminyl phosphatidylinositol could be hydrolyzed, indicating that the two lipids were primarily distributed in the cytoplasmic (outer) leaflet of the microsomes. Similar cleavage results were obtained using Streptolysin O-permeabilized thymoma cells. When permeabilized cells were incubated with UDP-[3H]GlcNAc and treated with PI-PLC, approximately 85% of the radiolabeled N-acetylglucosaminyl phosphatidylinositol and glucosaminyl phosphatidylinositol could be cleaved, indicating that they were accessible to the enzyme. The cumulative data indicate that early GPI intermediates are primarily located in the cytoplasmic leaflet of the ER, and are probably synthesized from PI located in the cytoplasmic leaflet and UDP-GlcNAc synthesized in the cytosol.

Reconstitution of a T cell receptor-stimulated plasma membrane calcium transporter: lack of dependence on inositol phosphates

The activation of T lymphocytes, like many cells, requires a rapid rise in intracellular Ca2+ secondary to both an influx and a release from intracellular stores. The latter is activated by inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. It is controversial if inositol phosphates can also stimulate a plasma membrane Ca2+ channel in T cells. We have studied the human T cell line HPB-ALL which, upon stimulation of its antigen receptor, does not generate detectable levels of Ins(1,4,5)P3 or internal Ca2+ release, but does have a Ca2+ influx. We have reconstituted a receptor-activated Ca2+ transporter from plasma membranes from these cells which has properties similar to the transporter observed in vivo and does not require inositol phosphates for activation. These data show that mitogens may activate more than one type of ligand-gated Ca2+ transport mechanism in T lymphocytes.

Expression of recombinant soluble and membrane-bound catechol O-methyltransferase in eukaryotic cells and identification of the respective enzymes in rat brain

The rat and human recombinant soluble and membrane-bound catechol O-methyltransferase (S- and MB-COMT, respectively) were expressed using mammalian and baculovirus vectors. Low levels of rat and human S-COMT polypeptides were detected by immunoprecipitation in K-562 cell lines transfected with the S-COMT vectors. From K-562 cells transfected with the rat MB-COMT construct, both S- and MB-COMT recombinant proteins were detected by a rat COMT-specific anti-serum. Infection of lepidopteran Spodoptera frugiperda cells with recombinant S- or MB-COMT baculovirus constructs yielded high amounts of enzymically active and immunoreactive S- or MB-COMT proteins, respectively. Pulse/chase experiments with [35S]methionine-labelled insect cells infected with the MB-COMT baculovirus showed that the 30-kDa recombinant human MB-COMT polypeptide was not processed into the 25-kDa S-COMT form. Subcellular fractionations of insect cells, followed by immunoblotting with COMT antiserum, showed that recombinant S-COMT was found only in the soluble, cytoplasmic fraction, whereas MB-COMT resided both in soluble and membrane fractions. The recombinant MB-COMT sedimented in Percoll gradients at the density of 1.042 g/ml cosedimenting with the plasma-membrane marker. Fractionation and immunoblotting experiments on homogenized total rat brains indicated that the rat S-COMT (24 kDa) and some of the rat MB-COMT (28 kDa) was recovered in soluble fractions, whereas the microsomal material having COMT activity contained the MB-COMT polypeptide. The rat brain microsomal MB-COMT had a density of 1.042 g/ml in Percoll gradients, cosedimenting with the plasma-membrane and rough-endoplasmic-reticulum marker enzymes. The meta/para methylation ratio of dihydroxybenzoic-acid substrate by different recombinant and rat brain COMT-containing subcellular fractions was analysed.