Analysis of polypeptide turnover rates in Chinese hamster ovary cell plasma membranes using two-dimensional electrophoresis

Coordination of protein synthesis and degradation

The degree of coordination between protein synthesis and protein degradation in developing and mature cels is considered. Studies on specific enzyme and general protein turnover in developing liver and differentiating mammary gland are presented. In the mature liver mitochondrion average protein degradation rates are higher for outer membrane and intermembrane space proteins than for matrix and inner membrane proteins. Significant heterogeneity of protein degradation rates was observed only in the outer mitochondrial membrane. During postnatal development the rates of degradation of proteins in many liver cellular fractions are increased. In the mitochondrion only the average rates of degradation of proteins in the outer membrane and intermembrane space fractions increase during development. Evidence for hormonally regulated changes in both protein synthesis and degradation during mammary cell differentiation is given. The data indicate that a transitory decrease in protein degradation accompanies the increase in protein synthesis on hormonal stimulation of the tissue. The results from the two model systems are collated and used to formulate a phenomenological hypothesis of protein degradation and its integration with protein synthesis in steady-state and non-steady-state conditions.

Surface expression and metabolic half-life of AMPA receptors in cultured rat cerebellar granule cells

The surface expression and metabolic turnover time of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are important factors in determining the involvement of these proteins in synaptic function. We used the membrane-impermeant chemical crosslinking reagent BS3 and subsequent immunoblot analysis to show that a large proportion of each of the AMPA receptor subunits GluR1-4 is surface expressed in cultured rat cerebellar granule cells. The apparent molecular mass of the individual subunits was 105 kDa and the surface expressed crosslinked complex was 530 kDa for each of the antibodies tested. These results are consistent with functional ionotropic glutamate receptors being pentameric subunit assemblies. We investigated the time course of AMPA receptor surface expression using the membrane-impermeant biotinylating agent NHS-SS-biotin. The half-life of surface expressed AMPA receptors was found to be 30 h. To estimate the mean degradation rate of AMPA receptors and total trichloracetic acid (TCA)-precipitable protein we used [35S]methionine/cysteine pulse-chase labelling. The half-life of AMPA receptors immunoprecipitated with anti-GluR1 antibody was approximately 48 h and the half-life of total TCA-precipitable protein from the same samples was 37 h.

A human oviduct-specific glycoprotein: synthesis, secretion, and localization during the menstrual cycle

The major objective of this study was to examine the hormonal regulation of a human oviduct-specific glycoprotein (huOGP) throughout the menstrual cycle and in all regions of the human oviduct. Regulation of synthesis and secretion was examined at both the protein (Western immunoblots and immunocytochemistry) and mRNA (Northern and slot blots) levels and correlated with changes in the morphological features of the oviductal epithelial cells throughout the cycle. Immunoblot analysis of oviductal fluid and explant culture media from all regions of the oviduct demonstrated that huOGP is primarily found during the follicular stage of the cycle and is not present in serum, follicular fluid, or uterine endometrium. Moreover, two-dimensional (2-D) immunoblots showed that all major isoelectric variants of huOGP observed on 2-D fluorographs are immunologically related. Light microscopic immunocytochemistry localized huOGP to oviductal secretory cells in both ampulla and isthmic regions, with the most intense immunoperoxidase staining seen in midcycle samples. Using an indirect immunogold technique at the electron microscopic level, huOGP was specifically localized to secretory granules of the ampullary and isthmic nonciliated epithelial cells. The ultrastructural characteristics of these secretory cells during the mid to late follicular phase of the cycle suggested elevated protein synthetic activity. In addition, mRNA expression for huOGP was elevated in all regions of the oviduct in midcycle specimens. Collectively, these data indicate that huOGP is a major tissue-specific, stage-specific secretory product of the human oviduct during the periovulatory stage of the cycle and support the hypothesis that huOGP synthesis and secretion may be regulated by fluctuations in the levels of estrogen and progesterone.

Rapid intramolecular turnover of N-linked glycans in plasma membrane glycoproteins. Extension of intramolecular turnover to the core sugars in plasma membrane glycoproteins of hepatoma

Plasma membrane glycoproteins of rat hepatocytes undergo a rapid terminal deglycosylation in that the terminal sugars of the oligosaccharide side chains are rapidly removed from the otherwise intact glycoproteins [Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl Acad. Sci. USA 80, 4026-4029]. The present paper demonstrates that this rapid intramolecular turnover of plasma membrane glycoproteins is not restricted to peripheral sugars but, in contrast to liver, in hepatoma the core sugars of the oligosaccharide chains are also involved. Intramolecular turnover was measured in Morris hepatoma 7777 in five plasma membrane glycoproteins with Mr of 85,000 (hgp85), 105,000 (hgp105), 115,000 (hgp115), 125,000 (hgp125), 175,000 (hgp175) (hgp = hepatoma glycoprotein) that were isolated and purified to homogeneity by concanavalin-A--Sepharose affinity chromatography and semipreparative SDS gel electrophoresis. Analysis of the carbohydrates of hgp85, hgp105, hgp115 and hgp125 revealed the presence of N-linked oligosaccharides containing L-fucose, D-galactose, D-mannose and N-acetyl-D-glucosamine, but only of trace amounts of N-acetyl-D-galactosamine; hgp175 additionally contained significant amounts of N-acetyl-D-galactosamine, indicating the presence of both N- and O-linked oligosaccharides. As shown by digestion with endoglucosaminidase H, the N-linked oligosaccharides of hgp105, hgp115, hgp125 and hgp175 were of the complex type, whereas hgp85 also contained oligosaccharides of the high-mannose type. Half-lives of the turnover of the oligosacharide chains and of the protein backbone of the five glycoproteins were measured in the plasma membrane in pulse-chase experiments in vivo, using L-[3H]fucose as a marker of terminal sugars, D-[3H]mannose as marker of a core sugar and L-[3H]leucine for labelling the protein backbone. Protein backbones of the five glycoproteins were degraded with individual half-lives ranging over 41-90 h with a mean of 66 h. Compared to the degradation of the polypeptide backbone, both the terminal sugar L-fucose and the core sugar D-mannose turned over with much shorter half-lives averaging about 20 h in the five glycoproteins. The data show that, conversely to liver, within plasma membrane glycoproteins of hepatoma not only peripheral sugars but also core sugars of the oligosaccharides are split off during the life-span of the protein backbone. It may therefore be assumed that this reprocessing of plasma membrane glycoproteins is sensitive to malignant transformation.

Inhibition of bovine peripheral blood mononuclear cell blastogenesis by fractionated mammary secretion

1. Bovine mammary secretion whey obtained during late involution markedly inhibited mitogen-induced blood mononuclear cell blastogenesis. 2. Whey proteins eluting in the first and second absorbance peaks following molecular exclusion chromatography were associated with greatest inhibition of mononuclear cell blastogenesis. 3. Greatest inhibition of concanavalin A-stimulated mononuclear cell blastogenesis was associated with high concentrations of whey proteins in absorbance peak 1. 4. Whole mammary secretion whey and whey proteins in absorbance peak 2 caused similar inhibition of concanavalin A- and phytohaemagglutinin-treated mononuclear cells. 5. Differential inhibition of mitogen-induced blastogenesis may reflect the presence of immunosuppressive substances in bovine mammary secretion whey which differ in specificity for bovine T-cell subsets.

A simple technique for sampling the uterine cavity of the baboon

The uterine environment within which the primate conceptus develops is poorly understood. This study was undertaken a) to develop a technique by which the uterine lumen could be sampled simply and efficiently and b) to analyze the proteins present in these uterine flushings throughout the menstrual cycle. The instrument described in this communication consists primarily of a double lumen cannula which permits one to inject and aspirate the flushing medium simultaneously. Volume recoveries usually exceeded 75% and the concentration of protein did not change significantly throughout the menstrual cycle. Two-dimensional polyacrylamide gel electrophoresis of the uterine flushings revealed the presence of one polypeptide (Mr congruent with 66,000; pI congruent with 5.7-6.0) that was not observed in serum and was presumed to be of uterine origin. The technique described here provides a rapid method by which baboon uterine secretions can be frequently collected in the lightly sedated animal.

Isolation and characterization of plasma membrane proteins of cultured human retinal pigment epithelium

Two-dimensional gel electrophoresis (2-D) was used to resolve the plasma membrane proteins from cultured human retinal pigment epithelial cells. The cells were metabolically labeled either with [35S]methionine to reveal proteins in general or with [3H]glucosamine or [3H]fucose which are more specific for glycoprotein visualization. The cell surface proteins were also iodinated, using the lactoperoxidase--glucose oxidase technique. These labeled membranes were separate into plasma membrane-enriched fractions by subjecting the water-shocked postnuclear supernatant to a discontinuous sucrose-density gradient. The five resulting membrane fractions were assayed for protein, RNA (microsomes), galactosyltransferase (Golgi membranes), 5'-nucleotidase (plasma membranes), and succinate dehydrogenase (mitochondrial membranes) and were examined by electron microscopy. The plasma membranes were enriched with minimal contamination at the 0.6-0.85 M (F2) and 0.85-1.0 M (F3) sucrose interfaces based on these biochemical and morphological criteria. Examination of 2-D autoradiographic profiles of F2 and F3 showed that approximately 180 proteins or protein subunits had incorporated [35S]methionine. Certain proteins were also labeled by [3H]glucosamine and [3H]fucose, and surface-labeled by iodination. This was especially true of 17 different high-molecular-weight (43-139 X 10(3) MW) very acidic glycoproteins which formed a constellation of spots. These glycoproteins, as well as others, were also seen in the whole-cell acidic glucosamine-labeled 2-D profiles, where about 150 proteins were detected. A total of 39 proteins were catalogued, of which 34 were detectable in the plasma membrane-enriched fractions. The results show that the use of subcellular fractionation, specific precursors, and labeling techniques aids in the detection and characterization of minor proteins in 2-D gels.

Role of prostaglandins in development of porcine blastocysts

Rapid elongation of porcine blastocysts between Days 11 to 12 of pregnancy coincides with an increase in uterine luminal content of prostaglandins. The present study evaluated the effect of two prostaglandin synthesis inhibitors (indomethacin and flunixin meglumine) on elongation of porcine blastocysts from spherical to filamentous forms between Day 11 to 12 of pregnancy. Gilts were hemi-hysterectomized on Day 11 of pregnancy. The excised uterine horn was flushed with 0.9% saline and diameter of blastocysts recovered were measured. Immediately following surgery, pregnant gilts were assigned to receive either: 1) vehicle every 4 h, 2) flunixin meglumine (banamine) every 4 h, or 3) indomethacin every 12 h. The remaining uterine horn was removed and flushed after the time of blastocyst elongation estimated for each gilt on basis of blastocyst development in the first horn. Uterine flushings were analyzed for total calcium, protein, acid phosphatase activity, estrone, estradiol-17 beta and prostaglandin F. Pretreatment blastocyst diameter was similar for all groups and ranged from 1 mm to 20 mm. Treatment of gilts with either banamine or indomethacin effectively inhibited (P less than 0.001) the increase in uterine luminal content of PGF. Total calcium, estrone and estradiol-17 beta were not influenced by treatment. Total uterine luminal protein and acid phosphatase activity were reduced (P less than 0.05) in banamine treated gilts compared to those receiving vehicle or indomethacin treatments. Although total PGF recovered in uterine flushings was reduced during the period of blastocyst elongation, treatment with PGF synthetase inhibitors failed to block rapid elongation of blastocysts from the spherical to filamentous forms.

Degradation of exogenous membrane proteins implanted into the plasma membrane of cultured hepatoma cells

The degradation of radiolabeled red cell band 3 and Sendai envelope proteins was studied after band 3 virosomes were fused with hepatoma cells as previously described (Hare, J E & Huston, M, Exp cell res 161 (1986) 317) [26]. 125I-band 3 (T1/2 = 13-14 h), Sendai HN (T1/2 = 37-40 h), and F (T1/2 = 21-23 h) envelope proteins were degraded by an apparent first-order process that was greater than 90% sensitive to 20 mM NH4Cl. 125I-Sendai envelope proteins were degraded at approximately similar rates when hepatoma cells were fused with intact virus, isolated viral membrane, or band 3 virosomes. There thus appears to be distinct heterogeneity among the degradation rates of implanted polypeptides dependent on structural aspects of each. To identify the subcellular site of membrane protein degradation, band 3 was labeled with membrane impermeant [14C]sucrose and implanted into hepatoma plasma membranes. After replating, trichloroacetic acid (TCA)-soluble label was found to accumulate in the lysosomal compartment of fractionated cells. The results identify the lysosome as the ultimate site of plasma membrane protein degradation, but suggest that plasma membrane proteins are selectively rather than non-selectively delivered to this compartment.

[Intracellular proteolysis]

This article is intended to give an overview of the most significant facts in the area of intracellular proteolysis. It begins with general considerations on the importance and nature of the intracellular proteolytic processes and examples are given of what takes place during both the extensive proteolysis and the limited cleavage of the cellular proteins. We have mentioned the intracellular proteases that have been identified and their established role since the knowledge of the proteases involved in important to understand the mechanisms of these processes.

Degradation of surface-labeled hepatoma membrane polypeptides: effect of inhibitors

When their membrane proteins were labeled with 125I by lactoperoxidase, dividing hepatoma cells lost radioactivity to the medium in a biphasic manner (T1/2 = 16-26 h, greater than 40 h). Lysosomotropic weak bases, chloroquine, and NH4Cl inhibited the rapid phase by 59%. More than 50% of the radioactivity which accumulates in the media from dividing cells during the first 4 h after labeling was trichloroacetic acid-soluble, and was identified as iodotyrosine. Iodotyrosine release from labeled membrane proteins was 60-71% inhibited by lysosomotropic agents chloroquine and NH4Cl as well as the sodium-proton ionophore, monensin. The inhibitory effect of NH4Cl and monensin was reversible. Inhibitors of microtubule and microfilament function and transglutamination had no effect on release of iodotyrosine to the medium, but trypsin-like protease inhibitors, p-aminobenzamidine, tosyl-L-lysine/chloromethylketone, and phenylmethylsulfonyl fluoride, as well as the cathepsin B inhibitor, leupeptin, inhibited by 21-24%. Iodotyrosine release showed a biphasic Arrhenius plot with an activation energy of 17 kcal/mol above but 27 kcal/mol below 20 degrees C. These results indicate that cell membrane polypeptides require a temperature-limiting event as well as passage through an ion-sensitive compartment prior to their complete degradation to constituent amino acids. In contrast to other lysosomal-mediated events, however, iodinated membrane proteins of dividing cells are degraded in a manner insensitive to agents which disrupt the cytoskeleton.

Modification of the intramolecular turnover of terminal carbohydrates of dipeptidylaminopeptidase IV isolated from rat-liver plasma membrane during liver regeneration

An intramolecular turnover of the terminal carbohydrates L-fucose, N-acetylneuraminic acid and D-galactose is a characteristic property of several liver plasma membrane glycoproteins, first demonstrated for dipeptidylaminopeptidase IV (EC 3.4.14.5., DPP IV). The core carbohydrates D-mannose and N-acetyl-D-glucosamine turn over like the polypeptide chain. The ratio of apparent half-lives of L-fucose and L-methionine of DPP IV is shifted from 0.17 in normal liver to 0.60 in regenerating liver. The ratio of half-lives of N-acetylneuraminic acid and L-methionine is only slightly changed from 0.43 in normal liver to 0.61 in regenerating liver. The ratio of apparent half-lives of D-mannose and L-methionine amounts to 0.80 in normal liver and 0.71 after partial hepatectomy. From this a drastic reduction of the intramolecular turnover of L-fucose on plasma membrane DPP IV in regenerating liver can be derived. The intramolecular N-acetylneuraminic acid turnover is affected to only a minor extent. D-Mannose turns over like the polypeptide in both normal and regenerating liver. The intramolecular L-fucose turnover may be involved in membrane glycoprotein recycling, which presumably is altered in regenerating liver. Additionally, L-fucose could regulate the rate of degradation of DPP IV, since core-fucosylated glycoproteins appear to be resistant to mammalian endo-N-acetylglucosaminidase.

Susceptibility of muscle soluble proteins to degradation by mast cell chymase

We investigated the in vitro susceptibility of muscle soluble proteins to the major alkaline proteinase (chymase) from skeletal muscle tissue, an enzyme originating from intramuscular mast cells, but also present in certain muscle fibers. Cytoplasmic proteins from rat skeletal muscle tissue were fractionated into four groups according to their different isoelectric points: fraction A (pI 9.5-7.0), B (pI 7.0-5.6), C (pI 5.5-4.5) and D (pI 5.3-3.5). Chromatography of these fractions on octyl-Sepharose CL-4B revealed the presence of a higher percentage of hydrophobic proteins in fraction C and D as compared to fraction A and B. In vitro degradation of these protein fractions by chymase, isolated from rat skeletal muscle tissue, was monitored (a) by measuring the ability of these proteins to bind Coomassie G-250, and (b) by analyzing the digestion mixture in isoelectric focusing gels. Both methods revealed fraction B proteins to be degraded very rapidly. While there was also a significant breakdown of fraction A proteins, fraction C and D proteins were degraded only very slowly, if at all. These differences in degradability are not due to the presence of a proteinase inhibitor in fraction C and D. The results suggest that mast cell chymase preferentially degrades those groups of muscle soluble proteins, the constituents of which have neutral to basic isoelectric points and a relatively low surface hydrophobicity.

Degradation of microinjected proteins: effects of lysosomotropic agents and inhibitors of autophagy

HeLa cells, injected with radioiodinated proteins by fusion with RBC ghosts, were exposed to inhibitors of lysosomal proteolysis and autophagy. The degradation of injected [125I]bovine serum albumin (BSA) was unaffected by chloroquine, NH4Cl, nocodazole, colcemid, puromycin, cycloheximide, or enucleation. Although degradation of [125I]lactate dehydrogenase (LDH) and [125I]pyruvate kinase (PK) was inhibited one-third by chloroquine or ammonia, their degradation was unaffected by the other compounds. In contrast, enhanced degradation of 125I-PK resulting from depriving injected HeLa cells of amino acids and serum was inhibited 70% by colcemid and abolished by chloroquine or ammonia. Similarly, degradation of [14C]sucrose-labeled BSA-polylysine conjugates that entered HeLa cells by endocytosis was inhibited as much as 80% by chloroquine and ammonia. Sensitivity of both enhanced proteolysis and degradation of exogenous proteins to ammonia or chloroquine indicates they are effective inhibitors of lysosomal proteolysis in HeLa cells. Failure of ammonia or chloroquine to inhibit degradation of injected 125I-BSA and the modest inhibition of degradation of injected 125I-LDH or 125I-PK indicates that virtually all BSA molecules and most PK or LDH molecules are degraded by a nonlysosomal proteolytic system. Components of this degradative system are present in vast excess or are long lived, since inhibition of protein synthesis for 20 hr had no effect on the degradation of injected proteins.

Modulation of human mononuclear cell responses by neutrophil-derived factors. II. Partial characterization of a neutrophil-derived lymphocyte-enhancing factor (N-LEF)

We previously reported that neutrophil (PMN)-derived factors modulated human mononuclear cell (MNC) responses. These interactions are important in understanding MNC localization, growth, and activation in inflammatory events such as occur in the rheumatoid synovium. We have partially identified and characterized a human neutrophil-derived, lymphocyte-enhancing factor (N-LEF). Highly purified PMN, devoid of MNC or platelets, were obtained by density gradient sedimentation. Supernatants (spt) were prepared from PMN phagocytosis of complement-coated zymosan particles. N-LEF was determined by [3H]TdR uptake or Ig secretion by stimulated MNC of cell lines. Neutral protease activity was quantitated by digestion of 125I-labeled Hb. Sodium dodecylsulfate (SDS) polyacrylamide gel electropheresis (PAGE) was carried out in 15% gels. We found (1) N-LEF biologic activity was cold stable, heat labile, nondialyzable, and diminished at acid pH (less than 4) or alkaline pH (greater than 10); (2) N-LEF biologic activity was abrogated by human plasma; (3) spt contained neutral protease and granule-associated but not cytoplasmic enzyme activities; (4) protease activities (for Hb, fibrin, ATEE, and Ac-Phe-BNa) were recovered from aprotinin-sepharose affinity columns and contained approximately 25,000 and approximately 28,000 molecular weight material on SDS-PAGE; (5) absorbtion of N-LEF spt on aprotinin-sepharose diminished both protease and biologic activities; and (6) N-LEF biologic activity and nonelastase neutral protease activity were found in approximately 13,000 mol wt fractions of Sephadex G100 (where cathepsin G chromatographs). Our observations suggest that phagocytosis of immune complexes by PMN induces release of a mononuclear cell enhancing factor, N-LEF, which may be an important mediator of inflammation, promoting transition from an acute to a chronic inflammatory response. Biologic properties of N-LEF are those of cathepsin G.

Computer applications in analysis, mapping and cataloging of proteins separated by two dimensional electrophoresis

Two dimensional electrophoretic separation of complex mixtures of proteins can only be exploited to its fullest potential using sophisticated computerized spot detection, quantification, pattern recognition, pattern normalization, data reduction and data storage. We present a discussion of some of the technical problems and of the options available which will ultimately lead toward full computerization of the data.

Relative stability of membrane proteins in Escherichia coli

The relative stability of membrane proteins in Escherichia coli was investigated to determine whether these proteins are degraded at heterogeneous rates and, if so, whether the degradative rates are correlated with the sizes or charges of the proteins. Cells growing in a glucose-limited chemostat with a generation time of 15 h were labeled with [(14)C]leucine. After allowing 24 h for turnover of (14)C-labeled proteins, the cells were labeled for 15 min with [(3)H]leucine. By this protocol, the rapidly degraded proteins have a high ratio of (3)H to (14)C, whereas the stable proteins have a lower ratio. The total cell envelope fraction was collected by differential centrifugation, and the proteins were separated by two-dimensional polyacrylamide gel electrophoresis. The relative ratio for each protein was determined by dividing its (3)H/(14)C ratio by the (3)H/(14)C ratio of the total membrane fraction. Although most of the 125 membrane proteins had relative ratios close to the average for the total membrane fraction, 19 varied significantly from this value. These differences were also observed when the order of addition of [(14)C]leucine and [(3)H]leucine was reversed. In control cultures labeled simultaneously with both isotopes, the relative ratios of these 19 proteins were similar to that of the total membrane fraction. Thirteen of these proteins had low relative ratios, which suggested that they were more stable than the average protein. An experiment in which the normal labeling procedure was followed by a 60-min chase period in the presence of excess unlabeled leucine suggested that the low relative ratios of 3 of these 13 proteins may be due to a slow post-translational modification step. Six membrane proteins had high relative ratios, which indicated that they were degraded rapidly. In contrast to the relationships found for soluble proteins in mammalian cells, there were no strong correlations between the degradative rates and either the isoelectric points or the molecular weights of membrane proteins in E. coli.

Isolation of a domain of the plasma membrane in Chinese hamster ovary cells which contains iodinatable, acidic glycoproteins of high molecular weight

A light vesicle fraction, apparently derived from the plasma membrane, was obtained following breakage of Chinese hamster ovary (CHO) cells by means of a fluid pump disrupting device. The final preparation was enriched approx. 40-fold over the homogenate in K+,Na+-stimulated ATPase and phosphodiesterase I, but only approx. 10-fold in 125I specific radioactivity after lactoperoxidase-catalyzed iodination. This preparation was compared with another plasma membrane fraction purified as large sheets via a two-phase centrifugation procedure. Two-dimensional polyacrylamide gel electrophoresis followed by Coomassie blue staining indicated that both fractions were fairly similar in polypeptide composition, although a few consistent differences were evident. However, staining of glycoproteins by the periodic acid-Schiff technique or by overlaying with 125I-labeled concanavalin A showed that the vesicle fraction was highly enriched in groups of high molecular weight, acidic glycoproteins which stain only weakly with Coomassie blue. These glycoproteins also bound 125I-labeled ricin I agglutinin and wheat germ agglutinin. They appear to be the major receptors for wheat germ agglutinin on the CHO cell surface. After surface labeling of cells by the 125I-lactoperoxidase technique, the membrane sheet fraction contained a large number of iodinated polypeptides, whereas labeling in the vesicle fraction was restricted almost entirely to the high molecular weight, acidic glycoproteins. It is proposed that the vesicle fraction constitutes a specific domain of the cell surface which is coated on its exterior by this group of glycoproteins. These components probably mask underlying proteins of the plasma membrane from external labeling.