Mechanisms for the incorporation of proteins into the plasma membrane

Biosynthesis and transport of plasma membrane glycoproteins in the rat intestinal epithelial cell: studies with sucrase-isomaltase

Sucrase-isomaltase (SI), an integral heterodimeric glycoprotein of the intestinal microvillus membrane, is synthesized as a single enzymically active precursor protein (pro-SI) of high relative molecular mass. After glycosylation in the Golgi complex pro-SI is transferred to the microvillus membrane where it is cleaved into the two subunits by pancreatic elastase. Pro-SI was purified by monoclonal antibody-affinity chromatography from microvillus membranes of fetal intestinal transplants in which SI is found exclusively in the non-cleaved precursor form. The N-terminal amino acid sequence of pro-SI was identical to that of the isomaltase subunit of SI which anchors the mature enzyme complex to the lipid bilayer, but it differed from the N-terminal sequence of the sucrase subunit of SI. This structural comparison indirectly gave insight into the mechanisms of membrane insertion and assembly of pro-SI during its biosynthesis. Subcellular fractionation studies indicate transient structural association of newly synthesized pro-SI with the basolateral membrane on its transfer from the Golgi complex to the microvillus membrane, suggesting that part of the basolateral membrane or its associated structures might be involved in the sorting-out processes of microvillar membrane proteins. This concept may have general relevance for the mechanisms of membrane insertion, intracellular transport and sorting of other microvillar membrane glycoproteins in the intestinal epithelial cell.

Na(+)-K(+)-ATPase alpha 1- and beta 1-subunit degradation: evidence for multiple subunit specific rates

Na(+)-K(+)-ATPase is a heterodimeric plasma membrane protein consisting of an alpha-catalytic and a beta-glycoprotein subunit. Because these two subunits are derived from two separate genes, they may not be synthesized with stoichiometric equivalence. The aim of this study was to estimate relative rates of synthesis and degradation of nascent and mature Na(+)-K(+)-ATPase alpha- and beta-subunits to determine whether either of the nascent subunits accumulates in excess and, if so, the fate of the excess subunits. We studied a pig kidney cell line (LLC-PK1/Cl4) that expresses only alpha 1- and beta 1-subunits. Relative synthesis and degradation rates of nascent subunits were first estimated by pulsing cells for 10 min with [35S]methionine followed by chase periods of up to 120 min and by immunoprecipitation. We found that directly after labeling, beta-subunits were present in threefold excess over alpha-subunits and that nearly 50% of this beta-subunit pool was degraded by 60 min. Nascent alpha-subunits were not degraded during the chase period. In a second strategy to examine relative rates of nascent alpha- vs. beta-subunit accumulation, cells were pulsed for 5-60 min and immunoprecipitated directly (without chase). The rate of accumulation of labeled alpha was greater than that of beta between 5 and 60 min, consistent with the results of the pulse-chase strategy, demonstrating a significant component of degradation of beta during this period. Despite the very different degradation rates of newly synthesized alpha- vs. beta-subunits, the degradation rates of alpha- and beta-subunits beyond 4 h after synthesis were indistinguishable (t0.5 = 10-12 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Tunicamycin reduces Na(+)-K(+)-pump expression in cultured skeletal muscle

The purpose of this study was to examine effects of tunicamycin (TM), which inhibits core glycosylation of the beta-subunit, on functional expression of the Na(+)-K+ pump in primary cultures of embryonic chick skeletal muscle. Measurements were made of specific-[3H]-ouabain binding, ouabain-sensitive 86Rb uptake, resting membrane potential (Em), and electrogenic pump contribution to Em (Ep) of single myotubes with intracellular microelectrodes. Growth of 4-6-day-old skeletal myotubes in the presence of TM (1 microgram/ml) for 21-24 hr reduced the number of Na(+)-K+ pumps to 60-90% of control. Na(+)-K+ pump activity, the level of resting Em and Ep were also reduced significantly by TM. In addition, TM completely blocked the hyperpolarization of Em induced in single myotubes by cooling to 10 degrees C and then re-warming to 37 degrees C. Effects of tunicamycin were compared with those of tetrodotoxin (TTX; 2 x 10(-7) M for 24 hr), which blocks voltage-dependent Na+ channels. TM produced significantly greater decreases in ouabain-binding and Em than did TTX, findings that indicate that reduced Na(+)-K+ pump expression was not exclusively secondary to decreased intracellular Na+, the primary regulator of pump synthesis in cultured muscle. Similarly, effects of TM were significantly greater than those of cycloheximide, which inhibits protein synthesis by 95%. These findings demonstrate that effects were not due to inhibition of protein synthesis. We conclude that glycosylation of the Na(+)-K+ pump beta-subunit is required for full physiological expression of pump activity in skeletal muscle.

Expression of a cell surface immobilization antigen during serotype transformation in Tetrahymena thermophila

A temperature shift from 40 to 28 degrees C rapidly induced expression of a specific immobilization antigen at the cell surface in Tetrahymena thermophila. This transformation was inhibited by actinomycin D and cycloheximide but not by colchicine or cytochalasin B. The major surface antigen expressed at 28 degrees C in cells homozygous for the SerH3 allele was partially purified, and an antiserum against this preparation was raised in rabbits. Electrophoresis, immunoblot, and [35S]methionine incorporation studies are reported which support the conclusion that the H3 antigen is an acidic protein with an Mr of approximately 52,000 daltons. An induced synthesis of the H3 immobilization antigen was detected within 30 min after a shift from 40 to 28 degrees C. This protein appeared to be synthesized in the microsomal fraction and transferred without cleavage to the cell surface, where it was inserted first into nonciliated regions.

Primary structure of the beta-subunit of Torpedo californica (Na+ + K+)-ATPase deduced from the cDNA sequence

DNA complementary to the Torpedo californica electroplax mRNA coding for the beta-subunit of (Na+ + K+)-ATPase has been cloned by screening a cDNA library with an oligodeoxyribonucleotide probe. Nucleotide sequence analysis of the cloned cDNA has revealed that this polypeptide consists of 305 amino acid residues (including the initiating methionine). The transmembrane topology and the potential N-glycosylation sites of this polypeptide are discussed.

Expression of a cell surface immobilization antigen during serotype transformation in Tetrahymena thermophila

A temperature shift from 40 to 28 degrees C rapidly induced expression of a specific immobilization antigen at the cell surface in Tetrahymena thermophila. This transformation was inhibited by actinomycin D and cycloheximide but not by colchicine or cytochalasin B. The major surface antigen expressed at 28 degrees C in cells homozygous for the SerH3 allele was partially purified, and an antiserum against this preparation was raised in rabbits. Electrophoresis, immunoblot, and [35S]methionine incorporation studies are reported which support the conclusion that the H3 antigen is an acidic protein with an Mr of approximately 52,000 daltons. An induced synthesis of the H3 immobilization antigen was detected within 30 min after a shift from 40 to 28 degrees C. This protein appeared to be synthesized in the microsomal fraction and transferred without cleavage to the cell surface, where it was inserted first into nonciliated regions.

Labeling of the glycoprotein subunit of (Na,K)ATPase with fluorescent probes

Sodium plus potassium activated adenosinetriphosphatase [(Na,K)ATPase] is composed of a catalytic subunit (alpha) and a glycoprotein subunit (beta) of unknown function. A method has been developed to label the beta subunit of purified dog kidney (Na,K)ATPase with fluorescent probes. The method consists of oxidation of beta-subunit oligosaccharides, reaction of the resulting aldehydes with fluorescent hydrazides, and reduction of the hydrazones and unreacted aldehydes with NaBH4. Two oxidation methods were compared. Simultaneous treatment with neuraminidase and galactose oxidase did not inhibit significantly (Na,K)ATPase activity and allowed insertion of up to 11 mol of probe per mol of beta. In contrast, oxidation of (Na,K)ATPase oligosaccharides with periodate resulted in 50-80% inhibition of the (Na,K)ATPase activity with low or undetectable labeling. Eleven commercial probes and two novel hydrazides were tested for labeling of (Na,K)ATPase treated with galactose oxidase and neuraminidase. Eight probes did not label (Na,-K)ATPase but labeled red cell ghosts oxidized with periodate. Four probes labeled beta specifically but either adsorbed to the membrane tightly, or cross-linked the beta subunits, or formed unstable adducts. Lucifer yellow CH labeled beta specifically without membrane adsorption. Labeling stoichiometries from 1 to 11 mol of lucifer yellow CH per mol of beta were obtained without inhibition of (Na,K)ATPase activity and without significant alteration of the anthroylouabain binding capacity or its association and dissociation kinetics. Anthroylouabain specifically bound to the lucifer-labeled (Na,K)ATPase had a decreased quantum yield, probably due to resonance energy transfer. This suggests that the sites of lucifer attachment on beta are within energy transfer distance from the cardiac glycoside site on alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Amplification of sodium- and potassium-activated adenosinetriphosphatase in HeLa cells by ouabain step selection

A multistep selection for ouabain resistance was used to isolate a clone of HeLa S3 cells that overproduces the plasma membrane sodium, potassium activated adenosinetriphosphatase (Na+,K+-ATPase). Measurements of specific [3H]ouabain-binding to the resistant clone, C+, and parental HeLa cells indicated that C+ cells contain 8-10 X 10(6) ouabain binding sites per cell compared with 8 X 10(5) per HeLa cell. Plasma membranes isolated from C+ cells by a vesiculation procedure and analyzed for ouabain-dependent incorporation of [32P]phosphate into a 100,000-mol-wt peptide demonstrated a ten- to twelvefold increase in Na+,K+-ATPase catalytic subunit. The affinity of the enzyme for ouabain on the C+ cells was reduced and the time for half maximal ouabain binding was increased compared with the values for the parental cells. The population doubling time for cultures of C+ cells grown in dishes was increased and C+ cells were unable to grow in suspension. Growth of C+ cells in ouabain-free medium resulted in revertant cells, C-, with biochemical and growth properties identical with HeLa. Karyotype analysis revealed that the ouabain-resistant phenotype of the C+ cells was associated with the presence of minute chromosomes which are absent in HeLa and C- cells. This suggests that a gene amplification event is responsible for the Na+,K+-ATPase increase in C+ cells.