Isolation and characterization of tunicamycin resistant mutants from Chinese hamster ovary cells

UDP-N-acetylglucosamine:dolichyl-phosphate N-acetylglucosamine-1-phosphate transferase is amplified in tunicamycin-resistant soybean cells

A tunicamycin-resistant soybean cell line was developed by gradually increasing the concentration of tunicamycin in the growth medium. At the final stage, the resistant cells could survive in media containing 60 micrograms/ml of tunicamycin, whereas normal cells show a greatly retarded growth rate at 0.5 microgram/ml of antibiotic. The tunicamycin-resistant cells had a greater than 40-fold increase in the activity of the enzyme UDP-GlcNAc:dolichyl-P GlcNAc1P transferase, a 2-3-fold increase in the activity of dolichyl-P-mannose synthase, but no increase in the activities of other enzymes of the lipid-linked saccharide pathway such as dolichyl-P-glucose synthase or mannosyl transferases. There was also no change in the activities of the glycoprotein-processing enzymes, glucosidase I or glucosidase II, as compared to wild-type cells. The increase in GlcNAc1P transferase was due to an increased production of enzyme, as seen by a dramatic increase in the amount of a 39-kDa protein, which is presumed to be this enzyme protein. The GlcNAc1P transferase from tunicamycin-resistant cells was equally sensitive to tunicamycin as was the wild-type enzyme, but was considerably more labile to temperatures above 30 degrees C. The activity in tunicamycin-resistant cells was greatly stimulated by exogenous dolichyl-P. The spectrum of oligosaccharides from labeled lipid-linked oligosaccharides was similar in wild-type and tunicamycin-resistant soybean cells, but the resistant cells had significantly greater amounts of the shorter and much lower amounts of the larger-sized oligosaccharides.

The 63-kilobase circular amplicon of tunicamycin-resistant Leishmania amazonensis contains a functional N-acetylglucosamine-1-phosphate transferase gene that can be used as a dominant selectable marker in transfection

Tunicamycin (TM)-resistant Leishmania amazonensis has been found previously to contain amplified chromosomal DNA, existing exclusively as extrachromosomal circles of 63 kb. Fragments of this DNA cloned into plasmids were functionally analyzed by transfection of wild-type cells. A clone with a 15-kb fragment of the 63-kb circle was initially found to confer TM resistance. A library of the 15-kb fragment was then prepared and used in toto to transfect wild-type cells. The transfectants that emerged after selection were found to contain a plasmid with an insert of 4.6 kb. Evidence from deletion experiments suggests that this is the minimal transfection-effective fragment. Sequencing of the 4.6-kb DNA revealed 1.4-kb homolog of N-acetylglucosamine-1-phosphate transferase genes. The L. amazonensis gene is similar to those from two other sources in their deduced peptide sequence by 65 to 70% and in hydropathic characteristics. The L. amazonensis gene is amplified by more than 128-fold over the wild type and overproduces a major transcript of 2.4 kb in all transfectants. The endogenous copy of this gene was amplified by polymerase chain reaction from the wild type and cloned into pX-NEO, a Leishmania expression vector. Amplification of this plasmid in the transfectants by selection with G418 simultaneously made them resistant to TM. Evidence provided thus indicates that the 1.4-kb DNA is an N-acetylglucosamine-1-phosphate transferase gene whose amplification is responsible for TM resistance in Leishmania variants and transfectants.

The 63-kilobase circular amplicon of tunicamycin-resistant Leishmania amazonensis contains a functional N-acetylglucosamine-1-phosphate transferase gene that can be used as a dominant selectable marker in transfection

Tunicamycin (TM)-resistant Leishmania amazonensis has been found previously to contain amplified chromosomal DNA, existing exclusively as extrachromosomal circles of 63 kb. Fragments of this DNA cloned into plasmids were functionally analyzed by transfection of wild-type cells. A clone with a 15-kb fragment of the 63-kb circle was initially found to confer TM resistance. A library of the 15-kb fragment was then prepared and used in toto to transfect wild-type cells. The transfectants that emerged after selection were found to contain a plasmid with an insert of 4.6 kb. Evidence from deletion experiments suggests that this is the minimal transfection-effective fragment. Sequencing of the 4.6-kb DNA revealed 1.4-kb homolog of N-acetylglucosamine-1-phosphate transferase genes. The L. amazonensis gene is similar to those from two other sources in their deduced peptide sequence by 65 to 70% and in hydropathic characteristics. The L. amazonensis gene is amplified by more than 128-fold over the wild type and overproduces a major transcript of 2.4 kb in all transfectants. The endogenous copy of this gene was amplified by polymerase chain reaction from the wild type and cloned into pX-NEO, a Leishmania expression vector. Amplification of this plasmid in the transfectants by selection with G418 simultaneously made them resistant to TM. Evidence provided thus indicates that the 1.4-kb DNA is an N-acetylglucosamine-1-phosphate transferase gene whose amplification is responsible for TM resistance in Leishmania variants and transfectants.

Tunicamycin-resistant variants from five species of Leishmania contain amplified DNA in extrachromosomal circles of different sizes with a transcriptionally active homologous region

Twelve independent variants were selected from five species of Leishmania for resistance to tunicamycin by exposure of cultured promastigotes to increasing concentrations of this antibiotic, an inhibitor of the microsomal N-acetylglucosamine-1-phosphate transferase in the dolichol pathway of N-glycosylation. All variants obtained from all species, as found previously with Leishmania amazonensis, contain amplified chromosomal DNA exclusively as extrachromosomal circles. These circular amplicons hybridize with amplified DNAs cloned previously from tunicamycin-resistant Leishmania amazonensis, but not with those from Leishmania resistant to other drugs. The amplicons from tunicamycin-resistant cells vary with different species in size from 30 to 70 kb, but all share a homologous region of 20 kb. Multiple independent transcripts are overexpressed from this region. Elevation of the microsomal glycosyltransferase activity is demonstrated in these variants from representative species. The results thus provide further evidence that this enzyme is overexpressed due to amplification of the gene in these cells. The consistent observation of this event in all cases studied also suggests that this is the predominant, if not the only mechanism of tunicamycin resistance in Leishmania.

Membrane mutants of animal cells: rapid identification of those with a primary defect in glycosylation

Membrane mutants of animal cells have been isolated by several laboratories, using a variety of selection protocols. The majority are lectin receptor mutants arising from altered glycosylation of membrane molecules. They have been obtained by selection for resistance to cytotoxic plant lectins or by alternative protocols designed, in many cases, to isolate different classes of receptor mutants. The identification of most membrane mutants expressing altered surface carbohydrates is rapidly achieved by determining their resistance to several lectins of different carbohydrate-binding specificities. For Chinese hamster ovary mutants, genetic novelty may subsequently be determined by complementation analysis with selected members of 10 recessive, glycosylation-defective complementation groups defined by this laboratory. In an attempt to identify new complementation groups, 11 Chinese hamster ovary membrane mutants independently isolated in different laboratories have been investigated for their lectin resistance and complementation properties. Only one new complementation group was defined by these studies. The remaining 10 mutants fell into complementation group 1, 2, 3, or 8. Although no evidence for intragenic complementation was observed, indirect evidence for different mutations within some genes was obtained. Seven of the independent isolates fell into complementation group 1, reflecting the high probability of isolating the Lec1 phenotype from Chinese hamster ovary populations. The results emphasize the importance of performing a genetic analysis before biochemical characterization of putative new membrane mutants.

Membrane mutants of animal cells: rapid identification of those with a primary defect in glycosylation

Membrane mutants of animal cells have been isolated by several laboratories, using a variety of selection protocols. The majority are lectin receptor mutants arising from altered glycosylation of membrane molecules. They have been obtained by selection for resistance to cytotoxic plant lectins or by alternative protocols designed, in many cases, to isolate different classes of receptor mutants. The identification of most membrane mutants expressing altered surface carbohydrates is rapidly achieved by determining their resistance to several lectins of different carbohydrate-binding specificities. For Chinese hamster ovary mutants, genetic novelty may subsequently be determined by complementation analysis with selected members of 10 recessive, glycosylation-defective complementation groups defined by this laboratory. In an attempt to identify new complementation groups, 11 Chinese hamster ovary membrane mutants independently isolated in different laboratories have been investigated for their lectin resistance and complementation properties. Only one new complementation group was defined by these studies. The remaining 10 mutants fell into complementation group 1, 2, 3, or 8. Although no evidence for intragenic complementation was observed, indirect evidence for different mutations within some genes was obtained. Seven of the independent isolates fell into complementation group 1, reflecting the high probability of isolating the Lec1 phenotype from Chinese hamster ovary populations. The results emphasize the importance of performing a genetic analysis before biochemical characterization of putative new membrane mutants.

Asparagine-linked glycosylation in Saccharomyces cerevisiae: genetic analysis of an early step

Asparagine-linked glycosylation is a form of covalent modification that distinguishes proteins that are either membrane bound or are in cellular compartments topologically outside of the cell from those proteins that remain soluble in the cytoplasm. This type of glycosylation occurs stepwise, with core oligosaccharide added in the endoplasmic reticulum and subsequent modifications occurring in the golgi. We used tunicamycin, an inhibitor of one of the earliest steps in the synthesis of N-linked oligosaccharide, to select for mutants that are resistant to this antibiotic. Genetic, biochemical, and physiological experiments led to the following conclusions. The synthesis of N-linked oligosaccharide is an essential function in cells. In contrast to mammalian cells, yeast cells do not transport tunicamycin by a glucosamine transport function. We identified a gene, ALG7, that is probably the structural gene for UDP-N-acetylglucosamine-1-P transferase, the enzyme inhibited by tunicamycin. Dominant mutations in this gene result in increased activity of the transferase and loss of the ability of the cell to sporulate. In addition, we identified another gene, TUN1, in which recessive mutations result in resistance to tunicamycin. The ALG7 and TUN1 genes both map on chromosome VII.

Isolation and characterization of mutator mutants from cultured mouse FM3A cells

A method to select mutator mutants was developed and 3 mutants were isolated from cultured mouse FM3A cells. Fluctuation analyses revealed that these mutator mutants have increased rates of spontaneous mutation at 3 genetic loci tested (resistance to ouabain, blasticidin S and tunicamycin). None of the 3 mutator mutants showed altered sensitivity to aphidicolin or arabinofuranosylcytosine, and so they differed from the mammalian mutator mutants reported previously. Also, all the mutator mutants had the same sensitivity as wild-type to UV or other DNA-damaging agents. Thus, these mutator mutants do not seem to have any deficiency in the DNA-repair process. To determine whether the mutator activity was due to the intracellular dNTP pool imbalance, 4 dNTPs in these mutator mutants were determined by high-pressure liquid chromatography and compared to that of the wild-type cells. The results show that there is no large dNTP pool imbalance in these mutator mutants. Since the mutator activity is not associated with the dNTP pool imbalance, these mutants may have altered protein(s) directly involved in DNA replication.

Asparagine-linked glycosylation in Saccharomyces cerevisiae: genetic analysis of an early step

Asparagine-linked glycosylation is a form of covalent modification that distinguishes proteins that are either membrane bound or are in cellular compartments topologically outside of the cell from those proteins that remain soluble in the cytoplasm. This type of glycosylation occurs stepwise, with core oligosaccharide added in the endoplasmic reticulum and subsequent modifications occurring in the golgi. We used tunicamycin, an inhibitor of one of the earliest steps in the synthesis of N-linked oligosaccharide, to select for mutants that are resistant to this antibiotic. Genetic, biochemical, and physiological experiments led to the following conclusions. The synthesis of N-linked oligosaccharide is an essential function in cells. In contrast to mammalian cells, yeast cells do not transport tunicamycin by a glucosamine transport function. We identified a gene, ALG7, that is probably the structural gene for UDP-N-acetylglucosamine-1-P transferase, the enzyme inhibited by tunicamycin. Dominant mutations in this gene result in increased activity of the transferase and loss of the ability of the cell to sporulate. In addition, we identified another gene, TUN1, in which recessive mutations result in resistance to tunicamycin. The ALG7 and TUN1 genes both map on chromosome VII.

Ia antigen expression is increased on tunicamycin-resistant human B-lymphoid cells

Cultured human B-lymphoid cells WIL-2 were mutated with ethylmethane sulfonate and selected for resistance to tunicamycin, an antibiotic that selectively inhibits N-linked glycosylation. Ultrastructural analysis of five isolated tunicamycin-resistant mutants (TMR) showed changes in surface microvilli, dilation of the endoplasmic reticulum, and an increase in surface myelin figures. Cytofluorometric analysis of TMR cells incubated with anti-HLA monoclonal antibodies showed a normal density of HLA-A, B antigens and an increased density of Ia antigens. The properties of these TMR mutants have remained stable for at least 18 months. These TMR cells may serve as a useful model to study the biochemical events in the processing and expression of Ia antigens.

Establishment of a murine cell line resistant to arabinosyladenine and devoid of adenosine deaminase activity

A population of cells stably resistant to a concentration of arabinosyladenine 100-fold greater than that tolerated by wild-type cells was selected over a period of 15 months. The cells are not cross resistant to arabinosylcytosine and may have at least one kinase with altered substrate specificity.

Tunicamycin-resistant mutations in mouse FM3A cells

Tunicamycin is an antibiotic that inhibits the oligosaccharide synthesis of glycoproteins. It greatly suppressed the growth of cultured mouse mammary carcinoma FM3A cells, when added to growth medium at concentrations of more than 0.1 microgram/ml. We have developed a single-step selection system for quantitatively detecting mutations resistant to the antibiotic in FM3A cells. Mutant colonies resistant to 1-1.2 micrograms tunicamycin per ml (the optimal concentration of the selecting agent) appeared at a frequency of 10(-4) to 10(-5) in an unmutagenized population, but they increased over 50-fold in the population mutagenized with 0.5 microgram N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) per ml for 2 h and selected under optimal conditions for the time of mutation expression and cell density in selective medium. Fluctuation analysis, by the method of Luria and Delbrück, revealed that tunicamycin-resistant mutations occurred at random during proliferation in normal medium at a rate of 1.2 x 10(-6) per cell per generation. So far 45 spontaneous and MNNG-induced mutant lines have been isolated and serially passaged in the absence of tunicamycin. These mutant lines all inherited their resistance for more than 60 generations. The mutants examined in detail were 12- to 26-fold more resistant than wild-type cells in terms of the D10 value, the concentration of tunicamycin reducing the plating efficiency to 10% of the control. In the hybrids between wild-type and mutant cells the tunicamycin resistance behaved in a co-dominant manner. Tunicamycin inhibited the incorporation of [3H]mannose into the acid-insoluble cell fraction; in this respect, mutant cells were over 30-fold more resistant than wild-type cells. Possible mechanisms of tunicamycin resistance are discussed.

Selection of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosaminyltransferase activity

Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) have been isolated by a stepwise selection procedure with progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from UDP-N-acetylglucosamine to the lipid carrier. The TM-resistant cells exhibited a 200-fold increase in their LD50 for TM and were morphologically distinct from the parental cells. The rate of asparagine-linked glycoprotein biosynthesis was the same for wild-type and TM-resistant cells. Membrane preparations from TM-resistant cells cultured for 16 d in the absence of TM had a 15-fold increase in the specific activity of the UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-phosphate transferase as compared to membranes of wild-type cells. The products of the in vitro assay were N-acetylglucosaminylpyrophosphoryl-lipid and N,N'-diacetylchitobiosylpyrophosphoryl-lipid for membranes from both TM-resistant and wild-type cells. The transferase activity present in membrane preparations from wild-type of TM-resistant cells was inhibited by comparable levels of TM. The data presented are consistent with overproduction of enzyme as the mechanism of resistance in these variant CHO cells.

Isolation and characterization of Chinese hamster ovary cell mutants with altered sensitivity to high doses of tunicamycin

A mutant, CTM422, resistant to low dose of tunicamycin (TM) was isolated from Chinese hamster ovary (CHO) cells, and it showed 7- to 10-fold higher resistance to TM than CHO. We further mutagenized CTM422, to isolate TM-high-resistant mutants which were resistant to about 100-fold higher dose of TM than CHO. The TM-high-resistant mutants (N101 and N102) acquired about 4-fold higher cross-resistance to 2-deoxy-d-glucose than CHO or CTM422, while both CHO and CTM422 showed similar sensitivity to 2-deoxy-d-glucose. TM-low-resistance appeared to be codominant, and TM-high-resistance was partially codominant against TM sensitivity, respectively. The transfer activity of N-acetylglucosamine from UDP-N-acetylglucosamine into the lipid fraction with CHO and CTM422 cell extracts was inhibited by TM to a similar extent, while the extract of N102 cells showed about 10-fold higher resistance to TM than CHO or CTM422.