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R S Gupta

Publications and source records attributed to R S Gupta.

At least 181 records · Page 10Linked to original sources
PubMed1986

Cellular basis for the species differences in sensitivity to cardiac glycosides (digitalis).

The relative toxicity of numerous cardiotonic steroids (viz. ouabain, digitoxin, digoxin, convallatoxin, SC4453, bufalin, gitaloxin, digoxigenin, actodigin, oleandrin, digitoxigenin, gitoxin, strophanthidin, gitoxigenin, lanatosides A, B and C, alpha- and beta-acetyl digoxin, alpha- and beta-methyl digoxin) and related compounds towards a number of independent cell lines established from human, monkey, mouse, Syrian hamster, and Chinese hamster have been determined. All cardiac glycosides and their genins, as well as the cardiotonic alkaloid cassaine, exhibited greater than 100-fold higher toxicity towards cultured human and monkey cells in comparison to the cell lines of mouse, Syrian hamster, and Chinese hamster origins. These differences are species-related as all cell lines (both normal as well as transformed) from any one species, as well as cells from the closely related species (e.g., man and monkey or mouse, Chinese hamster, and Syrian hamster), showed similar sensitivity towards these drugs. The failure to see any significant differences in cellular toxicity for a larger number of other compounds which either bear limited structural resemblance to cardiac glycosides (viz. estradiol 17-beta-acetate, testosterone propionate, 21-acetoxy pregnenolone, beta-estradiol, digitonin, tigogenin, and tomatine) or interact with the Na+/K+ ATPase in a different manner (viz. veratridine, sanguinarine nitrate, penicillic acid, vanadium pentoxide, harmaline-HCI,5,5'-diphenyl hydantoin, quindonium bromide, and methyl quinolizinum bromide) provides strong evidence that the observed species-related differences are highly specific for cardiotonic steroids. Studies on the binding of [3H]ouabain show that, in comparison to human and monkey cell lines, no significant binding of the drug is observed in cells derived from the resistant species (i.e., mouse and Chinese hamster). The Na+/K+ ATPase from cells of the resistant species is inhibited at much higher concentrations of ouabain and digitoxin in comparison to the enzyme from human cells, and a good correlation is observed between these concentrations and those reported for inhibition of the enzyme from isolated heart muscles of the same species. These results provide strong evidence that the species-related differences in sensitivity to digitalis have a cellular basis and that the cultured cells from various mammalian species provide a useful model system for investigating the mechanism of action of cardiac glycosides.

Animals↗
PubMed1986

Novel mutants of CHO cells resistant to adenosine analogs and containing biochemically altered form of adenosine kinase in cell extracts.

Stable mutants which are approximately five- and eightfold resistant to an inosine analog, formycin B (Fomr) have been selected in a single-step from Chinese hamster ovary cells at a frequency of approximately 10(-6). Cross-resistance studies with these mutants show that the Fomr mutants exhibit increased resistance to all adenosine analogs (N-and C-nucleosides) examined and, in accordance with their cross-resistance pattern, the mutants exhibited decreased cellular uptake and phosphorylation of formycin B and various adenosine analogs. In cell hybrids formed with sensitive cells, the drug-resistant phenotype of these mutants behaved recessively. However, unlike mutants resistant to adenosine analogs that have been obtained previously, which contain no measurable activity of adenosine kinase (AK) in cell extracts, the two Fomr mutants studied contained about 60 and 110% of the enzyme activity (compared to the parental cells) in their cell extracts. Biochemical studies with AK from the mutant cells show that in comparison to the wild-type enzyme, the mutant enzymes required much higher concentrations of the adenosine analog N7-(delta 2-isopentenyl) formycin A for similar inhibition of [3H]adenosine phosphorylation. These results indicate that AK from the Fomr mutants has lower affinity for phosphorylation of adenosine analogs in comparison to the enzyme from the parental cells. The genetic lesion in the Fomr mutants may thus be directly affecting the structural gene for AK.

Adenosine↗
PubMed1986

Ouabain-resistant mutants of Chinese hamster ovary cells are not directly affected in Na+, K+-ATPase.

The nature of biochemical lesions in a number of independent single-step ouabain-resistant (OuaR) mutants of Chinese hamster ovary cells has been examined. The cellular uptake of 86Rb in the mutant cells was found to be resistant to inhibition by ouabain and correlated with the degree of resistance of the mutants to the drug. However, the plasma membrane Na+, K+-ATPase from all five of the OuaR mutants examined showed similar sensitivity to inhibition by ouabain as seen for the enzyme from the parental sensitive cells. These results provide evidence that, contrary to the general assumption, OuaR mutants of Chinese hamster ovary cells, which are widely used for genetic and quantitative mutagenesis studies, are not directly affected in the Na+, K+-ATPase.

Animals↗
PubMed1986

Immunological studies with different classes of mutants affected at the adenosine kinase locus in CHO cells.

Adenosine kinase (AK) from CHO cells has been purified to homogeneity and specific antibodies to it have been raised in rabbits. Using this antibody, the presence of a specific cross-reacting protein (CRP) in cell extracts of different classes of mutants resistant to purine nucleoside analogs which are affected in AK has been investigated by the immunoblotting technique. Results of our studies show that 31 of the 32 independently selected class A AK- mutants (obtained at high frequency in presence of adenosine analogs toyocamycin, tubercidin, 6-methylmercaptopurine riboside, or pyrazofurin and containing no measurable activity of AK in cell extracts) contained similar amounts of a specific CRP as seen in the parental AK+ cells. The CRP in the parental and different mutant cell lines has the same relative molecular mass as purified AK. Similar results were obtained with two mutants each of the class B and C type (selected in presence of C-nucleosides formycin A and formycin B), which are also affected in AK but show novel properties. The presence of equivalent amounts of the CRP in the vast majority of the class A mutants strongly indicates that the high frequency of those mutants in CHO cells is not a result of an epigenetic or deletion type of event, but that such mutants may contain missense types of mutations at a presumed "mutational hot spot" within the structural gene for adenosine kinase.

Adenosine Kinase↗
PubMed1986

Cross-resistance of nocodazole-resistant mutants of CHO cells toward other microtubule inhibitors: similar mode of action of benzimidazole carbamate derivatives and NSC 181928 and TN-16.

Stable mutants which are between 1.6- and 2.2-fold resistant to the microtubule inhibitor, nocodazole (NocR mutants) have been isolated in Chinese hamster ovary cells after a single-step selection. The different NocR mutants exhibit specific changes in their cross-resistance or collateral sensitivity toward various microtubule inhibitors (viz. colchicine, podophyllotoxin, taxol, vinblastine, and maytansine), but they show no change in resistance toward unrelated compounds, indicating that the genetic lesions in these mutants are microtubule related. The set of NocR mutants examined and a second-step podophyllotoxin-resistant cell line, PodRII6 (which is highly resistant to nocodazole) were found to exhibit proportionately increased cross-resistance toward various benzimidazole carbamate derivatives (viz. mebendazole, fenbendazole, carbendazim, parbendazole, oxibendazole, albendazole, benomyl, and cambendazole) as well as two additional microtubule inhibitors, NSC 181928 (ethyl 5-amino-1,2-dihydro-3[N-methylanilino)methyl]pyrido [3,4-b]-pyrazin-7-ylcarbamate) and TN-16 [3-(1-anilinoethylidene)-5-benzyl-pyrrolidine-2,4-dione]. These results indicate that the mechanism of action of these latter two inhibitors is very similar to that of nocodazole. The lack of cross-resistance of the NocR and Podrii6 cell lines to thiabendazole provides evidence that the mechanism of action of this compound is different from that of other benzimidazole carbamate derivatives. Based on structure-activity relationship studies between various nocodazole-like compounds, a number of structural features of these compounds which appear important/essential for this type of biological activity have been identified.

Animals↗
PubMed1985

Involvement of adenosine kinase in the phosphorylation of formycin B in CHO cells.

In Chinese hamster ovary cells, [3H]formycin B is metabolized into formycin B-5'-monophosphate, formycin A-5'-monophosphate and higher phosphorylated derivatives of formycin A which are incorporated into RNA. Mutants of CHO cells independently selected for resistance to various adenosine analogs viz. toyocamycin, tubercidin, 6-methylmercaptopurine riboside, which contain no detectable activity of adenosine kinase (AK) in cell extracts, all exhibited between 2- to 3-fold increased resistance to formycin B. Formycin B-resistant mutants of CHO cells are also affected in AK, as indicated by the absence of AK activity in cell extracts. Both types of AK- mutants showed reduced uptake and phosphorylation of [3H]formycin B in comparison to the parental (AK+) cells. In addition, toxicity of formycin B towards CHO cells was reduced in presence of adenosine in a concentration dependent manner. These observations strongly indicate that in CHO cells, formycin B is phosphorylated via AK and that like other nucleoside analogs its phosphorylation may be essential for the drugs cellular toxicity.

Adenosine↗
PubMed1985

Cross-resistance and biochemical studies with two classes of HeLa cell mutants resistant to cardiac glycosides. The unusual behavior of cardenolide SC4453.

In HeLa cells two different types of mutants resistant to the cardiac glycoside ouabain (OuaR mutants) or erythrophleum alkaloid cassaine (CasR mutants) have been obtained. One type of mutants resistant to these compounds (designated as group A) are highly resistant (between 50 and 2000-fold) to various cardiac glycosides and their genins such as ouabain, oleandrin, digitoxin, digitoxigenin, strophanthidin, convallatoxin, gitoxin, gitoxigenin, gitaloxin, bufalin, and digoxigenin, but exhibit no cross-resistance to SC4453, a digoxin analog which contains a pyridazine ring in place of the lactone ring in the C-17 position. The second type of mutants (group B) exhibit cross-resistance to all of the cardiac glycosides including SC4453, but their level of resistance is at least 5-10-fold less than that of group A mutants. Interestingly, both groups of mutants exhibited similar degree of cross-resistance towards digoxin and actodigin (AY22241), indicating some differences in their behavior from other cardiac glycosides. Both classes of mutants exhibit no cross-resistance to a wide variety of other structurally and functionally related compounds, e.g. sanguinarine nitrate, ethacrynic acid, penicillic acid, veratridine, harmaline hydrochloride, 5,5'-diphenylhydantoin, quindonium bromide, methyl quinolizinum bromide, estradiol 17 beta-acetate, 21-acetoxy-pregnenolone, vanadium pentoxide, digitonin, and adriamycin, indicating that the genetic lesions in both groups of mutants are specific for cardiac glycosides. This inference is supported by the observation that both group A and B mutants show reduced binding of [3H]ouabain. In group A mutants, a part of the Na+/K+-ATPase activity is highly resistant to inhibition by ouabain, indicating that the genetic lesion in these mutants directly affects Na+/K+-ATPase. In contrast, the Na+/K+-ATPase from the group B mutants showed similar resistance towards ouabain and SC4453 as observed for the parental HeLa cells, indicating that these mutants are affected in a cellular component, other than Na+/K+-ATPase, which is involved in the interaction of cardiac glycosides with the cells. The lack of cross-resistance of the group A mutants to SC4453 and normal sensitivity of their Na+/K+-ATPase to this compound provides strong evidence that the mechanism of interaction of SC4453 with Na+/K+-ATPase differs from that of other cardiac glycosides.

Abietanes↗
PubMed1985

Autoradiographic detection of diphtheria toxin resistant mutants in human diploid fibroblasts.

An autoradiographic procedure for the detection of diphtheria toxin (DT) resistant (DipR) mutants in human diploid fibroblast (HDF) cells has been developed. The assay is based on the observation that when HDFs from confluent cultures are seeded in medium containing 0.01 flocculating units/ml or higher concentration of DT, protein synthesis in sensitive cells is severely inhibited by 4-6 hr. If at this or later time, a radiolabeled protein precursor (eg, 3H-leucine) is added to the culture, it is almost exclusively incorporated into the resistant cells, which are then readily identified by autoradiography. The DipR cells can also be identified by labeling in the presence of 3H-thymidine, although a higher background is observed in these experiments. Reconstruction experiments using DipS and DipR HDFs show that the frequency of heavily labeled cells that are detected by autoradiography show an excellent correlation with the number of DipR cells added and to the number of DipR cells as detected by conventional colony forming assay. These studies provide strong evidence that the labeled cells identified by autoradiography are bona fide DipR mutants. The detection of DipR cells by autoradiography is apparently not affected by the presence of the sensitive cells in the mixtures. The spontaneous frequency of DipR cells in HDFs has been found to be in the range of 1-5 X 10(-6), and this increases in a dose dependent manner upon treatment with the mutagen ethyl methanesulfonate. These results indicate that the autoradiographic assay could be used for quantitative mutagenesis. Since the autoradiographic assay does not depend on cell division, it may prove useful in estimating the incidence of pre-existing mutations in cell populations that either do not divide or have very limited growth potential (eg, lymphocytes, muscle cells, neurons, senescent fibroblasts, etc).

Autoradiography↗
PubMed1985

Inhibition of metabolic cooperation by phorbol esters in a cell culture system based on adenosine kinase deficient mutants of V79 cells.

In Chinese hamster V79 cells, stable mutants which are greater than 1000-fold resistant to the adenosine analog, tubercidin (Tubr mutants), and which exhibit high degree of cross-resistance to various other adenosine analogs, viz. toyocamycin, formycin A, 6-methylmercaptopurine riboside and 8-azaadenosine, have been isolated. The inability of the mutant cells to phosphorylate [3H]tubercidin and lack of adenosine kinase activity (AK- phenotype) in their cell extracts provide evidence that the mutant cells are unable to convert adenosine analogs into their toxic phosphorylated derivatives. When AK- cells are co-cultured with increasing numbers of parental V79 (AK+) cells in medium containing tubercidin, then due to metabolic cooperation between AK+ and AK- cells, a cell density-dependent decline in recovery of the resistant cells is observed. However, diphtheria toxin resistant (Dipr) mutants of V79 cells, which are altered in elongation factor-2, showed no similar cell density effect. Addition of 0.01 microgram/ml of phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to growth medium in these experiments markedly enhanced the recovery of the Tubr mutants, but it had no effect on the recovery of Dipr mutants, which suggests that TPA was enhancing recovery of AK- mutants by inhibiting metabolic cooperation between AK- and AK+ cells. Maximum effect of TPA on the recovery of AK- mutants (3- to 5-fold enhancement) was observed at a density of 6 X 10(5) V79 cells/60 mm diameter dish and it was independent of the particular adenosine analog that was used as selective agent. Studies with a number of different phorbol derivatives show that only those phorbol esters which show tumor promoting activity in the mouse skin system inhibited metabolic cooperation between AK+/AK- cells in a dose-dependent manner. An excellent correlation was observed in these studies between the relative tumor promoting activity of various phorbol esters, their relative binding affinities to the cell surface receptors, and the concentrations at which they inhibited metabolic cooperation in the AK-/AK+ cell system. The AK-/AK+ cell system thus provides a new system for examining the effect of tumor promoters on metabolic cooperation between cells.

Adenosine↗
PubMed1985

Genetic and biochemical studies with mutants of mammalian cells affected in microtubule-related proteins other than tubulin: mitochondrial localization of a microtubule-related protein.

In Chinese hamster ovary cells, stable mutants exhibiting specific resistance or collateral sensitivity towards the various microtubule inhibitors podophyllotoxin, colchicine, griseofulvin, taxol, nocodazole, vinblastine, and maytansine have been isolated. A number of independent mutants selected for resistance to podophyllotoxin and colchicine contain electrophoretically altered forms of two proteins, P1 and P2, of relative molecular masses of approximately 63 000 and 69 000, respectively. The proteins P1 and P2 have been shown to be microtubule related by a number of different genetic and biochemical criteria and are among the major proteins of Chinese hamster ovary cells, being present in approximately equimolar amounts with tubulin. In addition, a griseofulvin-resistant mutant contains a novel mutation (presumably nonsense) which reduces the relative molecular mass of a protein, P5 (relative mass congruent to 200 000), by about 20 000. Specific antibodies to protein P1 have been raised and cross-reactivity studies show that a similar protein is also present in cells from all vertebrate species examined, viz. man, monkey, mouse, Chinese hamster, Syrian hamster, and chicken. Immunofluorescence studies with anti-P1 and anti-tubulin antibodies show that, in interphase cells from various species, the P1 antibody reacts specifically with mitochondria whose overall cellular distribution is strikingly similar to the distribution of microtubules. The mitochondrial localization of the microtubule-related protein P1 provides strong suggestive evidence regarding the existence of a chemical and functional linkage between these two structures, with protein P1 playing an important role in this linkage. Some implication of these results are discussed and it is suggested that mitochondria play an important role in the dynamics of microtubules.

Animals↗
PubMed1985

Chinese hamster ovary cell mutants specifically affected in the phosphorylation of C-purine nucleosides.

In Chinese hamster ovary cells, two different types of mutants resistant to purine nucleoside analogs have been isolated. One type of mutants selected in presence of C-nucleosides formycin A and formycin B (FomR mutants) exhibited a high degree of cross resistance to different C-nucleosides but showed very slight to no cross resistance to various N-nucleosides. In contrast, mutants selected in presence of toyocamycin (Toyr mutants) exhibited a high degree of cross resistance to all C- and N-adenosine analogs. Studies on the cellular uptake and phosphorylation of [3H]adenosine, [3H]tubercidin, and [3H]formycin A show that, unlike the Toyr mutants which show reduced phosphorylation of all three compounds, the FomR mutants show reduced cellular phosphorylation of only [3H]formycin A. It is suggested that the genetic lesion in the FomR mutants affects adenosine kinase in a novel manner that specifically affects the phosphorylation of C-purine nucleosides.

Adenosine↗
PubMed1985

Cross-resistance of vinblastine- and taxol-resistant mutants of Chinese hamster ovary cells to other anticancer drugs.

Stable mutants resistant to the anticancer drug vinblastine (VinR mutants) have been isolated after a single selection step from Chinese hamster ovary cells. Of the two types of VinR mutants which are obtained, one class exhibits specific cross-resistance to only some of the microtubule inhibitors. However, the second type of mutants, which is affected in membrane permeability, exhibits cross-resistance to a wide variety of unrelated compounds. Both classes of VinR mutants showed codominant expression in cell hybrids formed between resistant and sensitive cells. The cross-resistance patterns of the VinR mutants and a single-step mutant resistant to the anticancer drug taxol (TaxR mutant) toward various anticancer drugs have been determined. The mutants resistant to both of these drugs showed significantly increased resistance toward aclarubicin, dactinomycin, doxorubicin, bisantrene, bruceantin, chromomycin A3, demecolcine, colchicine, daunorubicin, ellipticine, emetine, ethidium bromide, maytansine, mithramycin, mitoxantrone, nitidine chloride, olivomycin, podophyllotoxin, puromycin, taxol, vinblastine, vincristine, vindesine, teniposide, and etoposide. Interestingly, either one or both of the above mutants exhibited somewhat enhanced sensitivity toward vidarabine, acivicin, bleomycin, cisplatin, cytarabine, alpha-difluoromethyl-ornithine, 5-FU, tegafur, and tiazofurin. For a number of other anticancer drugs which were examined (chlorambucil, mitolactol, IMPY, hexamethylmelamine, hydroxyurea, diglycoaldehyde, methotrexate, mitoguazone, mitomycin, nocodazole, and 6-thioguanine), the level of resistance of these mutants was found to be unaltered. The information regarding cross-resistance and collateral sensitivity patterns of VinR and TaxR mutants should prove to be very useful in the design of drug combinations which could prove more effective in cancer chemotherapy.

Alkaloids↗
PubMed1985

Species-specific differences in the toxicity and mutagenicity of the anticancer drugs mithramycin, chromomycin A3, and olivomycin.

Three structurally related anticancer drugs, mithramycin, chromomycin A3, and olivomycin, exhibited large differences (greater than 100-fold) in their toxicity towards cultured cells from various species. These differences are species related, as all cell lines from any one species showed similar sensitivity to the three drugs. Of the three species examined, namely, human, mouse, and Chinese hamster, human cells were found to be most sensitive to these drugs. However, no significant difference in toxicity was observed between normal human diploid fibroblasts and heteroploid cell lines established from tumors. The above drugs were found to induce mutants at the hypoxanthine-guanine phosphoribosyl-transferase locus (i.e., resistance to 6-thioguanine) and produced DNA strand breaks, in a dose-dependent manner, in cells from all three species. However, the concentrations of these drugs which produced similar mutagenic or DNA strand break responses differed greatly for cells from the three species, and a good correlation was observed between the toxic and the mutagenic concentrations of these drugs for cells from the three species examined. These studies provide strong evidence that the toxic and mutagenic concentrations of different substances could differ greatly between cells from human and other species and indicate that the results of such studies cannot always be extrapolated from animal to human situations. It is suggested that a knowledge of the relative toxicity of any chemical towards cultured cells from human versus test animal should prove of value in extrapolating the results from animal systems to humans.

Animals↗
PubMed1985

Species-specific differences in toxicity of antimitotic agents toward cultured mammalian cells.

Toxicities of various microtubule inhibitors, namely, colchicine, podophyllotoxin, maytansine, vinblastine, nocodazole, griseofulvin, and steganacine, toward numerous independently established cell lines from three different species, namely, human, mouse, and Chinese hamster, were examined. Some of these inhibitors (namely, colchicine, vinblastine, taxol, and maytansine) were found to exhibit large (between tenfold and fiftyfold) differences in their toxic and antimitotic concentrations toward various cell lines and these differences appeared to be species related inasmuch as all cell lines from a particular species showed similar sensitivities toward these inhibitors. Of the three species examined, cells of human origin exhibited maximum sensitivity toward these inhibitors while Chinese hamster cells were found to be most resistant. The reduced cellular transport of [3H]colchicine and [3H]vinblastine in Chinese hamster cells as compared to the cellular transport in human cells and the equivalent binding of [3H]colchicine and [3H]vinblastine to microtubule proteins in cell extracts from both these lines provided strong evidence that the observed differences in toxicity to these inhibitors were most likely caused by differences in the cellular transport of these drugs. In contrast to the toxicities of the above compounds, the toxicities of other microtubule inhibitors such as podophyllotoxin, steganacine, griseofulvin, and nocodazole were found to be very similar for cells from all three species, indicating that the cellular transport of these 2 groups of microtubule inhibitors differed in some important respect. Some implications of the observed species-specific differences in drug toxicity to clinical studies are discussed.

4-Butyrolactone↗
PubMed1984

Genetic and biochemical studies on mutants of CHO cells resistant to 7-deazapurine nucleosides: differences in the mechanisms of action of toyocamycin and tubercidin.

From mutants of Chinese hamster ovary cells which are resistant to toyocamycin and tubercidin, second-step mutants which exhibit a further 8- to 9-fold increase in resistance to toyocamycin (ToyrII mutants) but no change in resistance to tubercidin have been isolated. The ToyrII mutants are similar to the first-step mutants in their levels of adenosine kinase activity (congruent to 1%), as well as cellular uptake and phosphorylation of adenosine and its analogs. The increased resistance of the ToyrII mutants to toyocamycin but not to tubercidin provides strong evidence that the mechanism of cellular toxicity of these two analogs is different from each other and suggests that these mutants may be affected in a cellular component which is specifically involved in the toxicity of toyocamycin. The ToyrII mutants also exhibit increased resistance to sangivamycin and the tricyclic nucleoside pentaaza-acenaphthylene riboside (TCN, NSC 154020) indicating that the mechanisms of cellular toxicity of these two analogs may be similar to that of toyocamycin.

Animals↗
PubMed1984

Mutants of chinese hamster ovary cells affected in two different microtubule-associated proteins. Genetic and biochemical studies.

Several colchicine-resistant (ColR) mutants of a Chinese hamster ovary cell line, PodRII6, which has earlier been shown to be affected in a microtubule-associated protein (referred to as P1 in these studies), have been isolated. Gel electrophoretic analyses show that one of the ColR mutants (ColR22) contains an additional new protein spot (designated as M2; Mr congruent to 70,000) that is derived by a mutational alteration in a neighboring more acidic protein P2 present in both PodRII6 and ColR22 cell lines. Studies on the relative amounts of the M2 and P2 proteins (and also the M1 and P1) in the ColR22 mutant and hybrid (i.e. ColR22 X PodS) cells are in accordance with the codominant behavior of the mutations and provide evidence that in the ColR22 mutant only one of the two copies of the genes for these proteins has been mutated. The two proteins which are affected in this mutant have been shown to constitute microtubule-associated proteins by a number of criteria. (i) These proteins along with tubulins are specifically released upon Ca2+ treatment of detergent-extracted cytoskeletons containing microtubules, but not if microtubules have been destroyed by prior treatment with colchicine. (ii) Cold treatment of a microtubule-containing fraction also releases the same set of proteins as in criterion i above. (iii) When the mutant cells are treated as in criterion i or ii, then along with the wild type forms, the mutant forms of the above proteins are also found in the microtubule-specific fractions. The cell extracts from PodRII6 and ColR22 mutants show reduced binding of both [3H] podophyllotoxin and [3H]colchicine, indicating that the above proteins are involved in the binding of these drugs to microtubules. It is suggested that the proteins P1 and P2, which are present in Chinese hamster ovary cells in nearly equimolar amounts with (alpha + beta)-tubulins, may comprise important microtubule structural components.

Animals↗