Membrane flow and intercoversions among endomembranes

Heterogeneity of cyclic nucleotide phosphodiesterases in liver endoplasmic reticulum

A microsomal fraction from rat liver was subfractionated into three rough endoplasmic reticulum fractions RIII, RII and RI, together with a smooth endoplasmic reticulum plus Golgi fraction. Cyclic nucleotide phosphodiesterase activity was found in all fractions. Subsequently it was shown that Golgi fractions were essentially devoid of cyclic AMP phosphodiesterase activity and the activity resided in the smooth endoplasmic reticulum fraction. The activity of the endoplasmic reticulum constituted some 20% of the homogenate activity, with the major fraction of this being associated with the RII fraction and the least with the RI fraction. With the exception of the activity of the RI fraction, which was a peripheral enzyme, all of the other enzyme activities were integral, requiring detergent or repeated freeze-thawing to effect solubilization. All of the activities appeared to be exposed at the external surface of the endoplasmic reticulum, as they were inactivated by trypsin under conditions where glucose 6-phosphatase was not. All of these activities displayed distinct sensitivities to both thermal and trypsin inactivation, yielding activity decays consistent with a single enzyme species being present in each case. The freeze-thaw-solubilized enzymes yielded single symmetrical peaks on sucrose-density-gradient centrifugation and polyacrylamide-gel electrophoresis. The sedimentation coefficients for the enzymes in the smooth-endoplasmic-reticulum-plus-Golgi, RIII, RII and RI fractions were 3.2S, 4.2S, 4.5S and 4.5S respectively. Whereas the activity in the smooth-endoplasmic-reticulum-plus-Golgi fraction exhibited normal Michaelis kinetics, those in the other fractions yielded kinetics indicative of apparent negative co-operativity. All of the enzymes exhibited low Km values towards cyclic AMP. The enzymes did not appear to be regulated by Ca2+ or calmodulin. ZnCl2 was found to be a potent non-competitive inhibitor of the enzyme in all fractions. NaF was a weak non-competitive inhibitor. The bilayer fluidizing agent benzyl alcohol exerted dissimilar effects on the enzyme activities. It is concluded that the endoplasmic reticulum displays lateral heterogeneity, with single, rather distinct, cyclic AMP phosphodiesterases being found in the different fractions.

Butyrophilin, an apical plasma membrane-associated glycoprotein characteristic of lactating mammary glands of diverse species

Lipid globule membranes were isolated from human and bovine milk and from the milk of sheep, goat, pig, rat and guinea pig, and their polypeptide compositions were analyzed. The major polypeptides with molecular weights similar to that of bovine butyrophilin were separated by gel electrophoresis, isolated and characterized with respect to isoelectric point, molecular weight, immunological cross-reactivity and peptide composition after proteolytic cleavage. We show that in all species examined these proteins are similar to bovine butyrophilin in (i) their relative insolubility in buffers of low and high ionic strength and in non-denaturing detergents, (ii) the occurrence of several isoelectric variants, and (iii) patterns of peptides obtained by protease digestion. It is concluded that closely related proteins are major constituents of the cytoplasmic coat structures associated with milk lipid globule membranes of many species, and we propose the name butyrophilins for this group of proteins. Bovine and human butyrophilins are glycosylated with relatively large amounts of glucosamine, mannose, glucose and galactose but little fucose, sialic acids or galactosamine. Most if not all of the sugar residues are associated with an acetone-soluble peptide fragment of Mr 12000-16000 focusing at about pH 4.0. We suggest that this fragment contains a membrane-spanning peptide sequence and is involved in the attachment of the cytoplasmic coat to the membrane of the milk lipid globule.

Membrane differentiation in the Golgi apparatus of mammalian urinary bladder epithelium

The endomembrane system in superficial and intermediate epithelial cells of mammalian urinary bladder was studied by cytochemistry, thin-section and freeze-fracture electron microscopy to determine the sites where special forms of membrane differentiation first appear. Glutaraldehyde-resistant NADH-ferricyanide reductase, distinctive 11-12 nm intramembrane particles (IMP), and asymmetry of membrane leaflets served as markers of membrane maturation. The three markers were specifically associated with the maturing face of Golgi apparatus and were absent from the remainder of the endomembrane system. Activity of this enzyme was associated with the lateral regions of the maturing face, fusiform vesicles, and the plasmalemma. Asymmetric unit membrane (AUM) plaques were not observed in the Golgi apparatus per se but were present in immature fusiform vesicles that had not detached from the maturing face. When freeze-fracture replicas and thin sections were compared, randomly arranged 11-12 nm IMP first appeared in maturing face membranes that were adjacent to clusters of "free" polyribosomes in the Golgi apparatus region. The proximity of these polyribosomes suggests that they may be related to the coincident appearance of the 11-12 nm IMP in the maturing face membrane. Our observations support the hypothesis that membranes undergo differentiation during "flow" through compartments of the endomembrane system. The lateral regions of the maturing face of the Golgi apparatus appear to be a critical location for the morphogenesis of plasma membranes in urinary bladder.

Translocation of newly synthesized gangliosides to the cell surface

A new method was developed to follow the translocation of gangliosides from their site of synthesis within the cell to the plasma membrane. Cultured mouse neuroblastoma N18 and rat glioma C6 cells were labeled for increasing times with D- [1-3H]galactose and then subjected to mild oxidation with NaIO4. Under the conditions chosen, oxidation was essentially restricted to cell-surface sialic acid residues, which were converted to derivatives with an aldehyde function. The labeled gangliosides were isolated from the cells and reacted with dinitrophenylhydrazine to form dinitrophenyl (DNP) derivatives of the oxidized gangliosides. The DNP-gangliosides then were separated from their unmodified counterparts by thin-layer chromatography. Thus, the rate of labeling of surface gangliosides was distinguished from the rate of labeling of total gangliosides. Our results indicated that the transfer of gangliosides from the site of synthesis to the cell surface required approximately 20 min and that newly synthesized gangliosides appeared to be transported to the plasma membrane at a constant rate. No essential differences were found in the rates of translocation of different ganglioside species by N18 cells or between gangliosides of N18 and C6 cells.

Exocytosis of pinocytic contents by Chinese hamster ovary cells

The extent of exocytosis of pinocytic vesicle contents was studied in suspension-cultured Chinese hamster ovary (CHO) cells using horseradish peroxidase (HRP) as a pinocytic content marker. HRP was shown to be internalized via fluid-phase pinocytosis in CHO cells. After an HRP pulse of 2.5-10 min a rapid decrease of 30-50% in cell-associated HRP activity was observed within 10-20 min at 37 degrees C. During this time the loss of cell-associated HRP was accompanied by an equivalent increase in extracellular HRP. After this rapid exocytosis of HRP, the remaining peroxidase activity decreased with a t1/2 of 6-8 h, the known lysosomal half-life of HRP. In pulse-chase experiments HRP was chased into a nonexocytic compartment. Based on cell fractionation and electron microscopic experiments, this nonexocytic compartment was identified as a lysosome and the compartment from which exocytosis occurs as a pinosome. The occurrence of pinocytic content exocytosis in cultured fibroblasts suggests that exocytosis of pinocytic vesicle contents is a general phenomenon.

Immunocytochemical localization of galactosyltransferase in HeLa cells: codistribution with thiamine pyrophosphatase in trans-Golgi cisternae

An affinity-purified, monospecific rabbit antibody against soluble human milk galactosyltransferase was used to localize the enzyme in HeLa cells by immunofluorescence and by the protein A-gold technique at the electron microscope level. Specific immunofluorescence was observed in a juxtanuclear cytoplasmic region which was identified, on immunostained thin sections of low-temperature Lowicryl K4M-embedded HeLa cells, as Golgi apparatus. Label by gold particles was limited to two to three trans cisternae of the Golgi apparatus, indicating a compartmentalization of galactosyltransferase in the cisternal stack. Combination of preembedding thiamine pyrophosphatase cytochemistry, with postembedding immunostaining for galactosyltransferase proved codistribution of the two enzymes. However, the acid phosphatase-positive, trans-most cisterna was negative for galactosyltransferase. The close topological association of both galactosyltransferase and thiamine pyrophosphatase (or nucleoside diphosphatase) suggests a concerted action of both enzymes in glycosylation.

Human erythrocyte galactosyltransferase. Characterization, membrane association and sidedness of active site

Human erythrocyte UDPgalactose : 2-acetamido-2-deoxy-alpha-D-galactopyranosylpeptide galactose beta(1 lead to 3) transferase (Galactosyltransferase) has been characterized in terms of detergent and metal ion requirements. Michaelis constants for donor and acceptor substrates, inhibition constant for N-acetylgalactosamine, pH optimum and ionic strength effects. The assay thus optimized permits initial velocity measurements. Galactosyltransferase was shown to be membrane-bound by demonstrating its association with erythrocyte ghosts after high and low ionic strength treatments to remove weakly-associated proteins. In the absence of detergents, no activity was detectable in sealed ghosts and inside-out vesicles derived from erythrocyte membranes. Enzyme activation by detergents paralleled solubilization of membrane proteins. Both latency and solubilization studies indicated a substrate inaccessible active site for the enzyme in situ in the membrane. Galactosyltransferase activity in resealed ghosts, leaky ghosts and inside-out vesicles was resistant to the action of trypsin, chymotrypsin or pronase applied as single agents. A mixture of these proteases, however, strongly reduced the enzyme activity in inside-out vesicles and leaky ghosts, indicating a cytosolic orientation for the active site of the galactosyltransferase.

Heterogeneous distribution of filipin-sterol complexes in nuclear membranes

Filipin, a sterol-specific polyene antibiotic, has been shown by electron microscopy to form complexes in membranes of mouse urinary bladder cells. Following instillation of a glutaraldehyde-filipin-dimethylsulfoxide solution into the bladder lumen, filipin-cholesterol complexes appear as membrane corrugations in thin sections and as 20-25 nm protuberances and depressions on PF and EF faces in freeze-fracture replicas. The complexes are observed in plasmalemma, Golgi membrane, rough endoplasmic reticulum and nuclear membrane of five different cell types (urothelial, endothelial, mesothelial, smooth muscle and fibroblasts). In the present report, we direct particular attention to the localization of numerous filipin-cholesterol complexes present in the nuclear envelopes of these cells. Our results suggest that enrichment of cell membranes with cholesterol occurs at an earlier stage in the flow-differentiation process than previously suspected. In addition, the unequal distribution of complexes in favor of the outer nuclear membrane suggests that it has a higher cholesterol content than the inner membrane.

Disposition of gangliosides and sialosylglycoproteins in neuronal membranes

Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP-[3H]N-acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H]sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 M-KCl, 125 mM-EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of th neuraminidase-labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase-labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labelled in vivo by an intracerebral injection of N-[3H]acetylmannosamine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with behaviour of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

The golgi apparatus: two organelles in tandem

The Golgi apparatus consists of distinct cis and trans compartments that may act sequentially to refine the protein export of the endoplasmic reticulum by removing escaped endoplasmic reticulum proteins. Refinement may be a multistage process akin to fractional distillation; the stack of cisternae comprising the cis Golgi may be the plates in this distillation tower. The trans Golgi, consisting of the last one or two cisternae, may be the receiver that collects from the cis Golgi only its most refined fraction for later distribution to specific locations throughout the cell.

Compartmentalized assembly of oligosaccharides on exported glycoproteins in yeast

Temperature-sensitive secretory mutants (sec) of S. cerevisiae have been used to evaluate the stages and localization of glycoprotein oligosaccharide synthesis. At the nonpermissive growth temperature (37 degrees C), the sec mutants accumulate secretory organelles and glycoproteins. Histochemical staining and thin-section electron microscopy reveal that the secreted glycoprotein, acid phosphatase, is contained within one of three distinct organelles that accumulates in different mutants: ER; Golgi-like structures called Berkeley bodies; and 80--100 nm vesicles. When produced at 37 degrees C, invertase and acid phosphatase have less carbohydrate in the mutants that accumulate ER than in other mutants, or than in the wild-type strain. External invertase migrates on SDS-polyacrylamide gels as a heterogeneous species with an apparent molecular weight of 100 to 140 kd. Radiolabeled invertase, immunoprecipitated from extracts of ER-accumulating mutant cells, migrates as a set of three discrete protein species with apparent molecular weights of 79, 81, and 83 kd; the other mutants produce a form more like the secreted enzyme. In each case, removal of N-glycosidically linked oligosaccharides by treatment with endoglycosidase H produces a discrete species that migrates as a protein of 61 kd. Immunochemical analysis of bulk glycoprotein accumulated in the mutants suggests that a major portion of the N-linked oligosaccharide, the outer chain, is added after material passes from the ER.

Exfoliation of membrane ecto-enzymes in the form of micro-vesicles

Cultures from various normal and neoplastic cell lines exfoliated vesicles with 5'-nucleotidase activity which reflected the ecto-enzyme activity of the parent monolayer culture. The ratio of 5'-nucleotidase to ATPase activity in the microvesicles indicated that cellular ecto-ATPase was conserved in the exfoliative process. Phospholipids of the microvesicles contained significantly increased amounts of sphingomyelin and total polyunsaturated fatty acids. It was concluded that the shedded vesicles constituted a select portion of the plasma membrane. Examination by electron microscopy showed the vesicles had an average diameter of 500 to 1000 nm and often contained a second population of vesicles about 40 nm in diameter. As much as 70% of the plasma membrane ecto-5'-nucleotidase activity of a culture was released into the medium over a 24-h period. Phosphoesterhydrolases from C-6 glioma or N-18 neuroblastoma microvesicles dephosphorylated cell surface constituents when in contact with monolayer cultures. Exfoliated membrane vesicles may serve a physiologic function; it is proposed that they be referred to as exosomes.

A morphological study of the uptake and orthograde axoplasmic transport of horseradish peroxidase by damaged autonomic neurons in vitro

A preparation comprising the guinea-pig inferior mesenteric ganglion (IMG) and ligated hypogastric nerves was maintained in vitro in a twin-chamber apparatus. Horseradish peroxidase (HRP), added to the ganglion compartment, was taken up via coated pinocytotic vesicles into the neuronal perikarya, and subsequently accumulated in many polymorphic cytoplasmic organelles, but it did not enter the Golgi complex. After 24 h incubation, HRP was also localized in membrane-bounded organelles in the nonmyelinated axons of the hypogastric nerves in the ligated nerve compartment. HRP-labelled organelles accumulated in swollen nonmyelinated axons immediately proximal to the ligation along with other organelles known to undergo fast axoplasmic transport. It is suggested that HRP taken up by the neuronal perikarya subsequently underwent fast axoplasmic transport without passing through the Golgi complex. The organelles in which HRP was transported in an orthograde direction were very similar to those in which the enzyme is known to be transported retrogradely in the same neuronal system. The advantages of this in vitro system for studying the orthograde transport of exogenous protein are discussed.

Analytical study of microsomes and isolated subcellular membranes from rat liver VIII. Subfractionation of preparations enriched with plasma membranes, outer mitochondrial membranes, or Golgi complex membranes

Preparations enriched with plasmalemmal, outer mitochondrial, or Golgi complex membranes from rat liver were subfractionated by isopycnic centrifugation, without or after treatment with digitonin, to establish the subcellular distribution of a variety of enzymes. The typical plasmalemmal enzymes 5'-nucleotidase, alkaline phosphodiesterase I, and alkaline phosphatase were markedly shifted by digitonin toward higher densities in all three preparations. Three glycosyltransferases, highly purified in the Golgi fraction, were moderately shifted by digitonin in both this Golgi complex preparation and the microsomal fraction. The outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the out mitochondrial membrane preparation, in agreement wit its behavior in microsomes. With the exception of NADH cytochrome c reductase (which was concentrated in the outer mitochondrial membrane preparation), typical microsomal enzymes (glucose-6-phosphatase, esterase, and NADPH cytochrome c reductase) displayed low specific activities in the three preparations; except for part of the glucose-6-phosphatase activity in the plasma membrane preparation, their density distributions were insensitive to digitonin, as they were in microsomes. The influence of digitonin on equilibrium densities was correlated with its morphological effects. Digitonin induced pseudofenestrations in plasma membranes. In Golgi and outer mitochondrial membrane preparations, a few similarly altered membranes were detected in subfractions enriched with 5'-nucleotidase and alkaline phosphodiesterase I. The alterations of Golgi membranes were less obvious and seemingly restricted to some elements in the Golgi preparation. No morphological modification was detected in digitonin-treated outer mitochondrial membranes. These results indicate that each enzyme is associated with the same membrane entity in all membrane preparations and support the view that there is little overlap in the enzymatic equipment of the various types of cytomembranes.

Ouabain-resistant mechanism of volume control and the ultrastructural organization of liver slices recovering from swelling in vitro

We have studied the net extrusion of water by liver slices recovering from swelling at 1 degree C and have attempted to relate this to ultrastructural alterations. Special attention was paid to the ouabain-resistant extrusion of water. The restoration of many details of intracellular architecture was dissociated from the net loss of water, since an osmotic stimulus (produced by 5% inulin) caused a passive withdrawal of water with little recovery of structure. Also, a similar recovery of structure was produced during active extrusion of water in the absence and presence of ouabain, even though ouabain reduced the water extrusion by 50%. The time-course of water extrusion in the presence of ouabain was correlated with the formation of cytoplasmic vesicles. Incubation without K+ in the medium had similar effects to those caused by ouabain. Colchicine had little effect on water extrusion in presence or absence of ouabain except at concentrations which reduced tissue ATP levels and caused much necrosis. Cytochalasin B alone had little effect on water extrusion, but led to the accumulation of many vesicles in the cytoplasm and appeared to abolish the access of such vesicles to the canaliculi. In the presence of ouabain, cytochalasin B had a similar effect on ultrastructure, and totally prevented the ouabain-resistant water extrusion. Ni2+ had rather similar effects to cytochalasin B both in the presence and absence of ouabain, although to a smaller degree. The results support our previous suggestion that the ouabain-resistant water extrusion proceeds by secretion of water into cytoplasmic vesicles, followed by the exocytotic expulsion of the vesicular contents into the bile canaliculi. Microfilaments appear to play an important role in the process.

Membrane dynamics in the parotid acinar cell during regranulation: a stereological study following isoprenaline-induced secretion

Recently weaned male rabbits were injected either with 150 micrograms/kg isoprenaline in saline containing 0.01 M ascorbic acid or simply with the drug vehicle. Groups of drug-injected animals were killed at various time after injection. Parotid gland tissue samples from all animals were fixed, embedded and thin sectioned, and micrographs were prepared at standard magnification. Estimations of membrane areas of each membrane type in parotid acinar cells were made. It was found that in animals killed 2 hours after induced secretion apical area was larger than in controls. In animals killed at successively later times the apical area was progressively less. No elevation of any internal smooth membrane areas was ascertained at any sampling time, though the areas of rough endoplasmic reticulum in 2-12 hour samples were larger. It is suggested that excess apical membrane, though probably removed by interiorization, is afterwards disassembled in side the cell to create fresh macromolecular building units (protein molecules), perhaps after passing through the Golgi apparatus. This cryptic pool of building units can provide about 900 micrometers2 of secretion granule membrane per cell, the supply apparently being exhausted in the first eight hours after degranulation, whilst granule numbers are being increased. Thereafter, apparently, limited granule fusion occurs, so that ultimately the cellular complement of secretion granule membrane comes to enclose a greater volume of secretory product, though the average granule number per cell is small.

In vivo intermembrane transfer of phospholipids in the photosynthetic bacterium Rhodopseudomonas sphaeroides

The kinetics of accumulation of phospholipids into the intracytoplasmic membrane of Rhodopseudomonas sphaeroides have been examined. We have previously demonstrated that accumulation of phospholipids in the intracytoplasmic membrane is discontinuous with respect to the cell cycle. In this study we demonstrated a sevenfold increase in the rate of phospholipid incorporation into the intracytoplasmic membrane concurrent with the onset of cell division. Pulse-chase labeling studies revealed that the increase in the rate of phospholipid accumulation into the intracytoplasmic membrane results from the transfer of phospholipid from a site other than the intracytoplasmic membrane, and that the transfer of phospholipid, rather than synthesis of phospholipid, is most likely subject to cell cycle-specific regulation. The rates of synthesis of the individual phospholipid species (phosphatidylethanolamine, phosphatidyglycerol, and an unknown phospholipid) remained constant with respect to one another throughout the cell cycle. Similarly, each of these phospholipid species appeared to be transferred simultaneously to the intracytoplasmic membrane. We also present preliminary kinetic evidence which suggested that phosphatidylethanolamine may be converted to phosphatidycholine within the intracytoplasmic membrane.

Flow kinetics of mouse histocompatibility antigens

Major histocompatibility antigens of the mouse (H-2 antigens) are found on a variety of different cell types and constitute a class of integral membrane-bound glycoproteins involved in tissue graft rejection and immune surveillance. We used monospecific alloantibodies directed against mouse H-2 antigens and standard pulse-chase technique to investigate the flow kinetics of delivery of newly synthesized membrane constituents to the cell surface. [35S]Methionine was injected intraperitoneally, and, after a chase with unlabeled methionine, livers were excised and fractionated into endoplasmic reticulum, Golgi apparatus, and plasma membrane fractions. Label first appeared in H-2 antigens located within the endoplasmic reticulum. Maximum specific activity observed between 5-7 min after injection of label was followed by a rapid loss of label. H-2 antigens of Golgi apparatus also were labeled early. Peak specific activity observed 15-25 min after injection of label was again followed by rapid loss of label. H-2 antigens of the plasma membrane were labeled last and appeared to accumulate radioactivity with no evidence of rapid turnover. These results provide evidence for a precursor-product relationship between H-2 antigens located within the cell on membranes of endoplasmic reticulum and Golgi apparatus and those on the plasma membrane. Flow of individual membrane-bound glycoproteins from their sites of synthesis and insertion into the membrane of the endoplasmic reticulum to the plasma membrane through the Golgi apparatus is indicated.

Heterogeneous distribution of filipin--cholesterol complexes across the cisternae of the Golgi apparatus

The Golgi apparatus is a key element in the ordered movement of secretory polypeptides from the rough endoplasmic reticulum to the plasma membrane during secretion. It has been shown that cisternae that receive membranes from the reticulum are morphologically similar to the latter and that cisternae liberating secretory granules resemble that plasma membrane. By using an ultrastructural probe for membrane cholesterol, filipin, on freeze-fractured and thin-sectioned exocrine and endocrine pancreatic cells, we have shown that an enrichment in filipin-cholesterol complexes takes places across the stacked cisternae of the Golgi apparatus; the reticulum-related (forming) cisternae are poor in such complexes, but the secretory granule-related (maturing) cisternae contain numerous complexes. Secretory granule membrane is also richly labeled with filipin-cholesterol complexes. The heterogeneous cholesterol distribution in the membranes of the Golgi apparatus, as shown by filipin, emphasizes the polarity of this organelle, in agreement with its role in organizing the traffic of the secretory polypeptides from the rough endoplasmic reticulum to the plasma membrane.

Phosphorylation of caseins, present evidence for an amino acid triplet code posttranslationally recognized by specific kinases

The fifty of so phosphorylated hydroxyamino acid residues hitherto investigated in caseins from different species have been found to occur in tripeptide sequences -Ser/Thr-X-A- where X represents any amino acid residue and A is an acidic residue. This strongly suggests that phosphorylation of caseins involves basically the stepwise enzymatic recognition of primary and secondary anionic amino acid triplets where the determinants are dicarboxylic residues and phosphoseryl residues, respectively. Studies of genetic variants of bovine caseins have provided clear-cut evidence for the actual occurrence of the former recognition sites. The occurrence of the above tripeptide sequences is a necessary but not a sufficient condition for phosphorylation of caseins to occur. Possible factors of constraint such as different intrinsic properties of both phosphate acceptor residues and acidic determinants, the characteristics of the local environment in terms of overall charge and hydrophilicity, secondary structure and steric hindrance, an insufficient available pool of casein kinase(s)... are discussed. All evidence now available supports the concept that phosphorylation of caseins is a posttranslational event and it is suggested that the process may occur during the transfer of completed polypeptide chains from the smooth endoplasmic reticulum to the Golgi apparatus where most of phosphate incorporation is presumably carried out. This organelle is rich in membrane-bound specific cyclic AMP-independent kinase(s) able in vitro to rephosphorylate specifically although not completely phosphatase-treated caseins and caseinophosphopeptides.

Biogenesis of hepatocyte plasma-membrane domains. Incorporation of (3H)fucose into plasma-membrane and golgi-apparatus glycoproteins

1. Rats were injected intracaudally with [3H]fucose and its rate of incorporation into the fucoproteins of serum, Golgi and plasma-membrane subfractions was followed for up tp 2h. 2. Incorporation into the Golgi dictyosome and secretory-vesicular fractions reached a maximum at 15 min or less, but most of the radioactivity was associated with classes of secretory glycoproteins. Incorporation into sinusoidal plasma-membrane fractions reached a maximum at 30 min, coinciding with the maximum release of fucoproteins into the serum. Contiguous and canalicular plasma-membrane fractions were labelled slightly later and at a lower rate and specific radioactivity. 3. Fluorography of fucoproteins separated by polyacrylamide-gel electrophoresis helped to distinguish between the major secretory and membrane-bound glycoproteins. The results show that a major biogenetic sequence is probably from Golgi dictyosomes to Golgi secretory elements to a sinusoidal plasma membrane. 4. The kinetics of incorporation make it unlikely that there is rapid and direct insertion of glycoproteins into the bile-canalicular plasma membrane. A route involving direct transfer of glycoproteins via a membrane-mediated intracellular path from the blood sinusoidal to the bile-canalicular plasma membranes is proposed.

Plasma membranes from intestinal microvilli and erythrocytes contain cytochromes b5 and P-420

The presence of cytochromes b5, P-450 and P-420 and activities of NADH- and NADPH-cytochrome c redutases were determined in plasma membranes isolated from microvilli of the chick and rat intestinal epithelium and erythrocyte membranes from chick, rat and man. The results are compared with the amounts of these components found in microsomal fractions from intestinal epithelium and in nuclear membranes from chick erythrocytes. Plasma membranes from intestinal microvilli and from erythrocytes contained significant amounts of NADH-cytochrome c reductase activity and of a pigment spectrophotometrically indistinguishable from rat liver microsomal cytochrome b5. In addition, cytochrome b5 fragments were prepared from the membranes by limited trypsin digestion and consisted of two to four components with Mr values in the range 10 000-13 500. In low-temperature difference spectra, the presence of a second cytochrome was noted which was similar to cytochrome P-420. Cytochrome P-450 and NADPH-cytochrome c reductase activities were not detected in plasma membrane fractions in significant concentrations but were present in the corresponding endomembrane fractions. These findings in highly purified, well defined plasma membrane fractions, in which contamination by endomembranes is minimal, strengthen the evidence for the existence of cytochrome-containing redox systems in plasma membranes of various cells and suggest that such redox components are general components of the cell surface. Possible functions and origins of these redox components in plasma membranes are discussed.

Relation of somal lipid synthesis to the fast axonal transport of protein and lipid

The role of somal lipid synthesis in the fast axonal transport of protein and lipid was examined in vitro utilizing spinal/sciatic nerve preparations of bullfrog. Inhibition of phospholipid synthesis in dorsal root ganglia by the amphiphilic cation, fenfluramine (0.1-2.0 mM) was monitored as decreased incorporation of [3H]choline into phosphatidyl choline. This inhibition was directly proportional to a decrease in the amount of [3H]protein undergoing fast axonal transport, the two variables being related by a slope close to unity. [3H]Choline-labeled lipid undergoing fast transport in the axon was unaffected by inhibition of somal phospholipid synthesis. Levels of fenfluramine up to 1.0 mM had no effect on uptake or incorporation of [3H]leucine. Selective exposure of desheathed nerve trunks to 1.0 mM fenfluramine had no effect on [3H]protein translocation, indicating that local phospholipid synthesis is not required to maintain ongoing transport in the axon. Inhibition of cholesterol synthesis in the ganglia with the analog 20,25-diazacholesterol also resulted in depression of [3H]protein transport. Since synthesis of both phospholipid and cholesterol are required at the level of the ganglion, it is suggested that the initiation of fast axonal transport of protein is dependent on the assembly of lipoprotein structures in the soma.

Subcellular compartmentalization of saccharide moieties in cultured normal and malignant cells

We studied subcellular localization of saccharide moieties in cultured normal and malignant cells fixed in paraformaldehyde and treated with a nonionic detergent, using lectins specific for various surgar residues as probes in fluorescence microscopy. In normal cells, concanavalin A and Lens culinaris agglutinin, specific for mannose-rich carbohydrate cores in glycoproteins, labeled the endoplasmic reticulum as a wide perinuclear region. Other lectins, on the other hand, stained the Golgi apparatus as a juxtanuclear reticular structure. A similar compartmentalization was also seen in all malignant cells studied, although the Golgi apparatus in these cells was distinctly vesicular in appearance. Our results indicate that saccharide moieties in both normal and malignant cells are similarly compartmentalized, and thus speak in favor of a unidirectional subcellular flow for both membrane and secreted glycoconjugates.

Polarized distribution of viral envelope proteins in the plasma membrane of infected epithelial cells

The surface distribution of the envelope glycoproteins of influenza, Sendai and Vesicular Stomatitis viruses was studied by immunofluorescence and immunoelectromicroscopy in infected epithelial cell monolayers, from which these viruses bud in a polarized fashion. It was found that before the onset of viral budding, the envelope proteins are exclusively localized into the same plasma membrane domains of the epithelial cells from which the virions ultimately bud: the glycoproteins of influenza and Sendai were detected at the apical surface, while the G protein of Vesicular Stomatitis virus was concentrated at the basolateral region. On the other hand, Sendai virus nucleocapsids, which can be easily identified in the cytoplasm before viral assembly, could be observed throughout the cell, not showing any preferential localization near the surface that the virions utilize for budding. These results are consistent with a model in which the asymmetric distribution of viral envelope proteins, rather than a polarized delivery of nucleocapsids, directs the polarity of viral budding. Furthermore, the asymmetric surface localization of viral glycoproteins suggests that these proteins share with intrinsic surface proteins of epithelial cells common biogenetic mechanisms and informational features or "sorting out" signals that determine their compartmentalization in the plasma membrane.

Mechanism of exchange of cytochrome b5 between phosphatidylcholine vesicles

The intervesicle exchange of cytochrome b5 has been studied by fluorescence quenching. The binding of cytochrome b5 to 1,2-bis(9.10-dibromostearoyl)-sn-glycerol-3-phosphorylcholine vesicles results in a quenching of cytochrome b5 fluorescence whereas the fluorescence is enhanced upon binding to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphorylcholine vesicles. This difference in cytochrome b5 fluorescence upon binding was used to study the kinetics of cytochrome b5 intervesicle exchange between the "quenching" and "enhancing" vesicles. Separation of the two cytochrome b5-vesicle complexes by density gradient centrifugation provided direct evidence for cytochrome b5 intervesicle exchange. Both the fluorescence assay and the density gradient assay yield the same value for the extent of cytochrome b5 exchange, obtained after equilibration, between the two types of vesicles. Both experiments also indicate that cytochrome b5 binds in a reversible fashion and has an equal affinity for the two types of vesicles. The kinetics of the exchange process are consistent with a mechanism involving the transfer of cytochrome b5 through the aqueous phase and rule out a mechanism involving vesicle collision.