Multiple forms of tubulin in the cilia and cytoplasm of Tetrahymena thermophila

Microtubules in Microorganisms: How Tubulin Isotypes Contribute to Diverse Cytoskeletal Functions

The cellular functions of the microtubule (MT) cytoskeleton range from relatively simple to amazingly complex. Assembled from tubulin, a heterodimeric protein with α- and β-tubulin subunits, microtubules are long, hollow cylindrical filaments with inherent polarity. They are intrinsically dynamic polymers that utilize GTP binding by tubulin, and subsequent hydrolysis, to drive spontaneous assembly and disassembly. Early studies indicated that cellular MTs are composed of multiple variants, or isotypes, of α- and β-tubulins, and that these multi-isotype polymers are further diversified by a range of posttranslational modifications (PTMs) to tubulin. These findings support the multi-tubulin hypothesis whereby individual, or combinations of tubulin isotypes possess unique properties needed to support diverse MT structures and/or cellular processes. Beginning 40 years ago researchers have sought to address this hypothesis, and the role of tubulin isotypes, by exploiting experimentally accessible, genetically tractable and functionally conserved model systems. Among these systems, important insights have been gained from eukaryotic microbial models. In this review, we illustrate how using microorganisms yielded among the earliest evidence that tubulin isotypes harbor distinct properties, as well as recent insights as to how they facilitate specific cellular processes. Ongoing and future research in microorganisms will likely continue to reveal basic mechanisms for how tubulin isotypes facilitate MT functions, along with valuable perspectives on how they mediate the range of conserved and diverse processes observed across eukaryotic microbes.

Purification of Tetrahymena cytoskeletal proteins

Like all eukaryotic cells, Tetrahymena thermophila contains a rich array of cytoskeletal proteins, some familiar and some novel. A detailed analysis of the structure, function, and interactions of these proteins requires procedures for purifying the individual protein components. Procedures for the purification of actin and tubulin from Tetrahymena are reviewed, followed by a description of a procedure that yields proteins from the epiplasmic layer and associated structures, including the tetrins. Finally, the challenges and opportunities for future advances are assessed.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Effect of histone deacetylase inhibitor trichostatin A (TSA) on the microtubular system of Tetrahymena

Histone deacetylases can also influence acetylation of tubulin. In the present experiments, after 60 min of 10 microM trichostatin (TSA) treatment the structure and amount of tubulin and acetylated-tubulin were studied immunocytochemically, by using confocal microscopy and flow cytometry. In TSA-treated Tetrahymena cells deep fibres were never labeled with antibody to acetylated tubulin. Flow cytometry with anti acetylated-tubulin antibody demonstrated that in the contol cell populations there were weaker and stronger labelled parts. After TSA treatment in the weaker labeled part the cell number decreased, and in the stronger labeled part increased significantly: this means that after the histone deacetylase inhibitor TSA treatment the amount of acetylated-tubulin in numerous Tetrahymena cells is significantly elevated. Labeling with anti-tubulin antibody was not changed significantly. On the basis of these results we postulate that histone deacetylase also in Tetrahymena influences the acetylation of tubulin, and this enzyme is sensitive to TSA treatments.

Distinct localization of a beta-tubulin epitope in the Tetrahymena thermophila and Paramecium caudatum cortex

Many of the highly organized microtubular arrangements in ciliates are located in the cortical area containing membrane vesicles and vacuoles. In Tetrahymena thermophila and Paramecium caudatum, immunofluorescence microscopy with the monoclonal antibody TU-06, directed against beta-tubulin, revealed distinct staining of this cortical region alone, while the cilia and other microtubular structures were unstained. The specificity of the antibody was confirmed by immunoblotting and by preabsorption of the antibody with purified tubulin. Double-label immunofluorescence with antibodies against gamma-tubulin, detyrosinated alpha-tubulin, and centrin showed that the TU-06 epitope is localized outside the basal body region. This was also confirmed by immunogold electron microscopy of thin sections. Proteolytic digestion of porcine brain beta-tubulin combined with a peptide scan of immobilized, overlapping peptides disclosed that the epitope was in the beta-tubulin region beta81-95, a region which is phylogenetically highly conserved. As known posttranslational modifications of beta-tubulin are located outside this area, the observed staining pattern cannot be interpreted as evidence of subcellular sequestration of modified tubulin. The limited distribution of the epitope could rather reflect the dependence of TU-06 epitope exposition on conformations of tubulin molecules in microtubule arrangements or on differential masking by interacting proteins.

Mutations of tubulin glycylation sites reveal cross-talk between the C termini of alpha- and beta-tubulin and affect the ciliary matrix in Tetrahymena

Two types of polymeric post-translational modifications of alpha/beta-tubulin, glycylation and glutamylation, occur widely in cilia and flagella. Their respective cellular functions are poorly understood. Mass spectrometry and immunoblotting showed that two closely related species, the ciliates Tetrahymena and Paramecium, have dramatically different compositions of tubulin post-translational modifications in structurally identical axonemes. Whereas the axonemal tubulin of Paramecium is highly glycylated and has a very low glutamylation content, the axonemal tubulin of Tetrahymena is glycylated and extensively glutamylated. In addition, only the alpha-tubulin of Tetrahymena undergoes detyrosination. Mutations of the known glycylation sites in Tetrahymena tubulin affected the level of each polymeric modification type in both the mutated and nonmutated subunits, revealing cross-talk between alpha- and beta-tubulin. Ultrastructural analyses of glycylation site mutants uncovered defects in the doublet B-subfiber of axonemes and revealed an accumulation of dense material in the ciliary matrix, reminiscent of intraflagellar transport particles seen by others in Chlamydomonas. We propose that polyglycylation and/or polyglutamylation stabilize the B-subfiber of outer doublets and regulate the intraflagellar transport.

The two alpha-tubulin isotypes in budding yeast have opposing effects on microtubule dynamics in vitro

The yeast Saccharomyces cerevisiae has two genes for alpha-tubulin, TUB1 and TUB3, and one beta-tubulin gene, TUB2. The gene product of TUB3, Tub3, represents approximately 10% of alpha-tubulin in the cell. We determined the effects of the two alpha-tubulin isotypes on microtubule dynamics in vitro. Tubulin was purified from wild-type and deletion strains lacking either Tub1 or Tub3, and parameters of microtubule dynamics were examined. Microtubules containing Tub3 as the only alpha-tubulin isotype were less dynamic than wild-type microtubules, as shown by a shrinkage rate and catastrophe frequency that were about one-third of that for wild-type microtubules. Conversely, microtubules containing Tub1 as the only alpha-tubulin isotype were more dynamic than wild-type microtubules, as shown by a shrinkage rate that was 50% higher and a catastrophe frequency that was 30% higher than those of wild-type microtubules. The results suggest that a role of Tub3 in budding yeast is to control microtubule dynamics.

Polyglycylation of tubulin is essential and affects cell motility and division in Tetrahymena thermophila

We analyzed the role of tubulin polyglycylation in Tetrahymena thermophila using in vivo mutagenesis and immunochemical analysis with modification-specific antibodies. Three and five polyglycylation sites were identified at glutamic acids near the COOH termini of alpha- and beta-tubulin, respectively. Mutants lacking all polyglycylation sites on alpha-tubulin have normal phenotype, whereas similar sites on beta-tubulin are essential. A viable mutant with three mutated sites in beta-tubulin showed reduced tubulin glycylation, slow growth and motility, and defects in cytokinesis. Cells in which all five polyglycylation sites on beta-tubulin were mutated were viable if they were cotransformed with an alpha-tubulin gene whose COOH terminus was replaced by the wild-type COOH terminus of beta-tubulin. In this double mutant, beta-tubulin lacked detectable polyglycylation, while the alpha-beta tubulin chimera was hyperglycylated compared with alpha-tubulin in wild-type cells. Thus, the essential function of polyglycylation of the COOH terminus of beta-tubulin can be transferred to alpha-tubulin, indicating it is the total amount of polyglycylation on both alpha- and beta-tubulin that is essential for survival.

Molecular mechanisms of microtubular organelle assembly in Tetrahymena

Thanks to recent technological advances, the ciliate Tetrahymena thermophila has emerged as an attractive model organism for studies on the assembly of microtubular organelles in a single cell. Tetrahymena assembles 17 types of distinct microtubules, which are localized in cilia, cell cortex, nuclei, and the endoplasm. These diverse microtubules have distinct morphologies, stabilities, and associations with specific Microtubule-Associated Proteins. For example, kinesin-111, a microtubular motor protein, is required for assembly of cilia and is preferentially targeted to microtubules of actively assembled, immature cilia. It is unlikely that the unique properties of individual microtubules are derived from the utilization of diverse tubulin genes, because Tetrahymena expresses only a single isotype of alpha- and two isotypes of 1-tubulin. However, Tetrahymena tubulins are modified secondarily by a host of posttranslational mechanisms. Each microtubule organelle type displays a unique set of secondary tubulin modifications. The results of systematic in vivo mutational analyses of modification sites indicate a divergence in significance among post-translational mechanisms affecting either alpha- or beta-tubulin. Both acetylation and polyglycylation of alpha-tubulin are not essential and their complete elimination does not change the cell's phenotype in an appreciable way. However, the multiple polyglycylation sites on 1-tubulin are essential for survival, and their partial elimination dramatically affects cell motility, growth and morphology. Thus, both high-precision targeting of molecular motors to individual organelles as well as organelle-specific tubulin modifications contribute to the creation of diverse microtubules in a single cytoplasm of Tetrahymena.

Reorganization of microtubules in the amitotically dividing macronucleus of tetrahymena

We developed a modified immunofluorescence protocol that permitted visualization of microtubules inside the macronucleus of the ciliate Tetrahymena. Although the amitotically dividing macronucleus lacks a spindle, an elaborate system of microtubules is assembled inside the macronucleus and between the macronucleus and the cortex. Microtubules could not be detected inside the interphase macronuclei. The early stage of macronuclear division was associated with the assembly of short macronuclear microtubules that localized randomly. The intramacronuclear microtubules were subsequently organized in a radial manner. During elongation of the macronucleus, the distribution of macronuclear microtubules changed from radial to parallel. During constriction of the macronucleus, dense and tangled macronuclear microtubules were detected at the region of nuclear constriction. In the cytosol, microtubules were linking the macronucleus and cell cortex. During recovery after drug-induced depolymerization, microtubules reassembled at multiple foci inside the macronucleus in close proximity to the chromatin. We propose that these microtubules play roles in chromatin partitioning, macronuclear constriction, and positioning of the macronucleus in relation to the cell cortex.

Tubulin polyglycylation: differential posttranslational modification of dynamic cytoplasmic and stable axonemal microtubules in paramecium

Polyglycylation, a posttranslational modification of tubulin, was discovered in the highly stable axonemal microtubules of Paramecium cilia where it involves the lateral linkage of up to 34 glycine units per tubulin subunit. The observation of this type of posttranslational modification mainly in axonemes raises the question as to its relationship with axonemal organization and with microtubule stability. This led us to investigate the glycylation status of cytoplasmic microtubules that correspond to the dynamic microtubules in Paramecium. Two anti-glycylated tubulin monoclonal antibodies (mAbs), TAP 952 and AXO 49, are shown here to exhibit different affinities toward mono- and polyglycylated synthetic tubulin peptides. Using immunoblotting and mass spectrometry, we show that cytoplasmic tubulin is glycylated. In contrast to the highly glycylated axonemal tubulin, which is recognized by the two mAbs, cytoplasmic tubulin reacts exclusively with TAP 952, and the alpha- and beta- tubulin subunits are modified by only 1-5 and 2-9 glycine units, respectively. Our analyses suggest that most of the cytoplasmic tubulin contains side chain lengths of 1 or 2 glycine units distributed on several glycylation sites. The subcellular partition of distinct polyglycylated tubulin isoforms between cytoplasmic and axonemal compartments implies the existence of regulatory mechanisms for glycylation. By following axonemal tubulin immunoreactivity with anti-glycylated tubulin mAbs upon incubation with a Paramecium cellular extract, the presence of a deglycylation enzyme is revealed in the cytoplasm of this organism. These observations establish that polyglycylation is reversible and indicate that, in vivo, an equilibrium between glycylating and deglycylating enzymes might be responsible for the length of the oligoglycine side chains of tubulin.

Proteolysis of tubulin isotypes within Tetrahymena axonemes

In the axonemes of Tetrahymena cilia, beta-tubulin was digested more rapidly by chymotrypsin than alpha-tubulin. On the other hand, in the solubilized state, both tubulins were digested by the protease at almost the same rate. Among alpha-tubulin isotypes within the axonemes, alpha 5-tubulin was more rapidly digested by chymotrypsin than other alpha-tubulin isotypes. The addition of ATP and vanadate (Vi) to the axonemes significantly protected the alpha 5-isotype from chymotryptic proteolysis. These tendencies could not be observed in tubulins solubilized from axonemes. Based on the case of the alpha 5-tubulin isotype, it is possible to negate the idea that all tubulin isotypes are distributed homogeneously and play similar functional roles within the axonemes.

Monoclonal and polyclonal antibodies detect a new type of post-translational modification of axonemal tubulin

Polyclonal (PAT) and monoclonal (AXO 49) antibodies against Paramecium axonemal tubulin were used as probes to reveal tubulin heterogeneity. The location, the nature and the subcellular distribution of the epitopes recognized by these antibodies were, respectively, determined by means of: (i) immunoblotting on peptide maps of Paramecium, sea urchin and quail axonemal tubulins; (ii) immunoblotting on ciliate tubulin fusion peptides generated in E. coli to discriminate antibodies directed against sequential epitopes (reactive) from post-translational ones (non reactive); and (iii) immunofluorescence on Paramecium cells, using throughout an array of antibodies directed against tubulin sequences and post-translational modifications as references. AXO 49 monoclonal antibody and PAT serum were both shown to recognize epitopes located near the carboxyl-terminal end of both subunits of Paramecium axonemal tubulin, whereas the latter recognized additional epitopes in alpha-tubulin; AXO 49 and a fraction of the PAT serum proved to be unreactive over fusion proteins; both PAT and AXO 49 labelled a restricted population of very stable microtubules in Paramecium, consisting of axonemal and cortical ones, and their reactivity was sequentially detected following microtubule assembly; finally, both antibodies stained two upward spread bands in Paramecium axonemal tubulin separated by SDS-PAGE, indicating the recognition of various alpha- and beta-tubulin isoforms displaying different apparent molecular masses. These data, taken as a whole, definitely establish that PAT and AXO 49 recognize a post-translational modification occurring in axonemal microtubules of protozoa as of metazoa. This modification appears to be distinct from the previously known ones, and all the presently available evidence indicates that it corresponds to the very recently discovered polyglycylation of Paramecium axonemal alpha- and beta-tubulin.

Acetylation of lysine 40 in alpha-tubulin is not essential in Tetrahymena thermophila

In Tetrahymena, at least 17 distinct microtubule structures are assembled from a single primary sequence type of alpha- and beta-tubulin heterodimer, precluding distinctions among microtubular systems based on tubulin primary sequence isotypes. Tetrahymena tubulins also are modified by several types of posttranslational reactions including acetylation of alpha-tubulin at lysine 40, a modification found in most eukaryotes. In Tetrahymena, axonemal alpha-tubulin and numerous other microtubules are acetylated. We completely replaced the single type of alpha-tubulin gene in the macronucleus with a version encoding arginine instead of lysine 40 and therefore cannot be acetylated at this position. No acetylated tubulin was detectable in these transformants using a monoclonal antibody specific for acetylated lysine 40. Surprisingly, mutants lacking detectable acetylated tubulin are indistinguishable from wild-type cells. Thus, acetylation of alpha-tubulin at lysine 40 is non-essential in Tetrahymena. In addition, isoelectric focusing gel analysis of axonemal tubulin from cells unable to acetylate alpha-tubulin leads us to conclude that: (a) most or all ciliary alpha-tubulin is acetylated, (b) other lysines cannot be acetylated to compensate for loss of acetylation at lysine 40, and (c) acetylated alpha-tubulin molecules in wild-type cells contain one or more additional charge-altering modifications.

Electroporation-mediated replacement of a positively and negatively selectable beta-tubulin gene in Tetrahymena thermophila

Replacement of lysine-350 by methionine in the beta-tubulin gene of Chlamydomonas confers resistance to microtubule-depolymerizing drugs and increased sensitivity to the microtubule-stabilizing drug taxol. This mutation was created in cloned BTU1, one of two coexpressed beta-tublin genes of Tetrahymena thermophila. When introduced by electroporation, the mutated gene transformed Tetrahymena exclusively by gene replacement at the homologous locus. Taxol-sensitive transformants could be retransformed with a wild-type gene and selection for taxol resistance. Analyses of phenotypic assortment and of the mRNA in transformed cells suggest that complete replacement of the BTU1 gene in the polyploid macronucleus can be obtained. These studies demonstrate the utility of this marker for studying tublin gene function and show that electroporation allows facile gene replacement in Tetrahymena.

Glutamylated tubulin probed in ciliates with the monoclonal antibody GT335

Microtubular networks are extensively developed in many ciliate species. In several of them, we investigate the occurrence of the post-translational glutamylation of tubulin [Eddé et al., 1990: Science 247:82-85; Eddé et al., 1991: J. Cell. Biochem. 46:134-142] using as a probe for such modified tubulin, the monoclonal antibody GT335 [Wolff et al., 1992: Eur. J. Cell Biol. 59:425-432]. Results obtained in Paramecium strongly suggest that both axonemal and cytoplasmic tubulin are glutamylated. As in the vertebrate brain tubulin so far tested, the GT335 epitope is located at the carboxy-terminal fragment of cytoplasmic tubulin removed by subtilisin treatment. Immunoblotting and immunofluorescence experiments reveal that, unlike tubulin acetylation, glutamylation is not restricted to cold-resistant microtubules. In addition, immunofluorescence studies performed on dividing cells show that glutamylation takes place soon after the polymerization of microtubules. Finally, glutamylated tubulin is also detected in the ciliate species Euplotes, Tetrahymena, and Paraurostyla. Together with results obtained on flagellate species, this suggests that tubulin glutamylation came out early in the course of eukaryotic evolution and has been widely exploited in various cellular strategies.

Regulation and evolution of the single alpha-tubulin gene of the ciliate Tetrahymena thermophila

The single alpha-tubulin gene of Tetrahymena thermophila was isolated from a genomic library and shown to encode a single protein. Comparisons of the rates of evolution of this gene with other alpha-tubulin sequences revealed that it belongs to a group of more evolutionarily constrained alpha-tubulin proteins in animals, plants, and protozoans versus the group of more rapidly evolving fungal and variant animal alpha-tubulins. The single alpha-tubulin of Tetrahymena must be used in a variety of microtubule structures, and we suggest that equivalently conserved alpha-tubulins in other organisms are evolutionarily constrained because they, too, are multifunctional. Reduced constraints on fungal tubulins are consistent with their simpler microtubule systems. The animal variant alpha-tubulins may also have diverged because of fewer functional requirements or they could be examples of specialized tubulins. To analyze the role of tubulin gene expression in regulation of the complex microtubule system of Tetrahymena, alpha-tubulin mRNA amounts were examined in a number of cell states. Message levels increased in growing versus starved cells and also during early stages of conjugation. These changes were correlated with increases in transcription rates. Additionally, alpha-tubulin mRNA levels oscillate in a cell cycle dependent fashion caused by changes in both transcription and decay rates. Therefore, as in other organisms, Tetrahymena adjusts alpha-tubulin message amounts via message decay. However the complex control of alpha-tubulin mRNA during the Tetrahymena life cycle involves regulation of both decay and transcription rates.

Identification of Tetrahymena ciliary surface proteins labeled with sulfosuccinimidyl 6-(biotinamido) hexanoate and Concanavalin A and fractionated with Triton X-114

Tetrahymena thermophila cells were labeled with sulfosuccinimidyl 6-(biotinamido) hexanoate, a sensitive nonradioactive probe for cell surface proteins, and Western blots of axonemes and ciliary membrane vesicles were compared to cilia fractionated with Triton X-114 (TX-114) in order to study the orientation of ciliary membrane proteins. Greater than 40 ciliary surface polypeptides, from greater than 350 kDa to less than 20 kDa, were resolved. The major surface 50-60 kDa proteins are hydrophobic and partition into the TX-114 detergent phase. Two high molecular weight proteins, one of which is biotinylated, comigrate with the heavy chains of ciliary dynein, sediment at 14S in a sucrose gradient, and partition into the TX-114 aqueous phase. Fractions containing these high molecular weight proteins as well as fractions enriched in 88-kDa and 66-kDa polypeptides contain Mg(2+)-ATPase activities. Detergent-solubilized tubulins partition into the TX-114 aqueous phase, are not biotinylated, and must not be exposed to the ciliary surface. The detergent-insoluble axoneme and membrane fraction contains a 36-kDa polypeptide and a portion of the 50-kDa polypeptides that otherwise partition into the detergent phase. These polypeptides could not be solubilized by ATP or by NaCl extraction and appear to be associated with pieces of ciliary membrane tightly linked to the axoneme. The ciliary membrane polypeptides were also tested for Concanavalin A binding and at least sixteen Con A-binding polypeptides were resolved. Of the major Con A-binding polypeptides, three are hydrophobic and partition into the TX-114 detergent phase, three partition into the TX-114 aqueous phase, and four partition exclusively in the detergent-insoluble fraction, which contains axonemes and detergent-resistant membrane vesicles.

Enhanced bioavailability of cefoxitin using palmitoyl L-carnitine. I. Enhancer activity in different intestinal regions

The conditions under which the absorption enhancer palmitoyl L-carnitine chloride (PCC) improved the bioavailability of the poorly absorbed antibiotic cefoxitin throughout the rat intestine has been studied. Cefoxitin alone was appreciably absorbed only in the duodenum (31% vs less than 7% elsewhere). PCC solutions (3 mg/rat, pH 4.0) enhanced cefoxitin bioavailability (F) by 0-, 22-, 16-, and greater than 32-fold in the duodenum, jejunum, ileum, and colon regions, respectively. The inability of PCC to improve F in the duodenum could not likely be attributed to enzymatic degradation of the enhancer, since coadministration with protease and esterase inhibitors produced similar results (F = 30%). Coadministration of PCC solution with cefoxitin in the unligated or ligated colon, increased F to 33 and 76%, respectively. Qualitatively similar results were seen with PCC suspensions (3 mg/rat, pH 6.0). Maintaining a high concentration of cefoxitin and PCC in a restricted region (i.e., by ligating a 2- to 3-cm section of the colon) afforded a two- to threefold advantage over an unligated colon section. The difference in cefoxitin bioavailability between ligated and unligated colon was probably due to sample spreading and subsequent/simultaneous dilution.

Ciliary microtubule capping structures contain a mammalian kinetochore antigen

Structures that cap the plus ends of microtubules may be involved in the regulation of their assembly and disassembly. Growing and disassembling microtubules in the mitotic apparatus are capped by kinetochores and ciliary and flagellar microtubules are capped by the central microtubule cap and distal filaments. To compare the ciliary caps with kinetochores, isolated Tetrahymena cilia were stained with CREST (Calcinosis/phenomenon esophageal dysmotility, sclerodactyly, telangiectasia) antisera known to stain kinetochores. Immunofluorescence microscopy revealed that a CREST antiserum stained the distal tips of cilia that contained capping structures but did not stain axonemes that lacked capping structures. Both Coomassie blue-stained gels and Western blots probed with CREST antiserum revealed that a 97-kD antigen copurifies with the capping structures. Affinity-purified antibodies to the 97-kD ciliary protein stained the tips of cap-containing Tetrahymena cilia and the kinetochores in HeLa, Chinese hamster ovary, and Indian muntjak cells. These results suggest that at least one polypeptide found in the kinetochore is present in ciliary microtubule capping structures and that there may be a structural and/or functional homology between these structures that cap the plus ends of microtubules.

Characteristics of basal body cartwheel reassembly

Cartwheel complexes reassembled in a fraction derived by treating isolated oral apparatuses from Tetrahymena with 1.0 M KCl for 12 h. Approximately 40% of the KCl-soluble protein reassembled into cartwheel complexes. The reassembly reaction was protein-concentration dependent, and reassembled cartwheels were stable at 3 degrees C. Sucrose gradient centrifugation resolved 3 high molecular mass protein complexes from the KCl-soluble fraction. Each of the 3 complexes has a different mass, but each contains the same 5 polypeptides, 2 of which are probably tubulins. When these complexes were removed from the KCl-soluble fraction by high speed centrifugation, cartwheel reassembly did not occur. The 5 polypeptides in the high molecular mass complexes were among several other polypeptides resolved from reassembled cartwheels by 2-dimensional gel electrophoresis. The high molecular mass complexes are probably essential for cartwheel formation. The electrophoretic data also show that several polypeptides in the KCL-soluble fraction do not appear to be incorporated into cartwheels. These polypeptides are probably non-essential for cartwheel formation.

A conditional mutant having paralyzed cilia and a block in cytokinesis is rescued by cytoplasmic exchange in Tetrahymena thermophila

Nineteen mutants that are conditional for both the ability to regain motility following deciliation and the ability to grow were isolated. The mutations causing slow growth were placed into five complementation groups. None of the mutations appears to affect energy production as all mutants remained motile at the restrictive temperature. In three complementation groups protein synthesis and the levels of mRNA encoding alpha-tubulin or actin were largely unaffected at the restrictive temperature, consistent with the hypothesis that mutations in these three groups directly affect the assembly of functional cilia and growth. Complementation group 1 was chosen for further characterization. Both phenotypes were shown to be linked, suggesting they are caused by a single mutation. Group 1 mutants regenerated cilia at the restrictive temperature, but the cilia were nonmotile. This mutation also caused a block in cytokinesis at the restrictive temperature but did not affect nuclear divisions or DNA synthesis. The block in cell division was transiently rescued by wild-type cytoplasm exchanged when mutants were paired with wild-type cells during conjugation (round 1 of genomic exclusion). Thus, at least one mutation has been isolated that affects assembly of some microtubule-based structures in Tetrahymena (cilia during regeneration) but not others (nuclei divide at 38 degrees), and the product of this gene is likely to play a role in both ciliary function and in cytokinesis.

A temperature-sensitive mutation affecting cilia regeneration, nuclear development, and the cell cycle of Tetrahymena thermophila is rescued by cytoplasmic exchange

A temperature-sensitive mutation was isolated that blocks cilia regeneration and arrests growth in Tetrahymena thermophila. Protein and RNA synthesis and ATP production appeared to be largely unaffected at the restrictive temperature, suggesting that the mutation is specific for cilia regeneration and growth. At the restrictive temperature, mutant cells arrested at a specific point in the cell cycle, after macronuclear S phase and shortly before micronuclear mitosis. Arrested cells did not undergo nuclear divisions, DNA replication, or cytokinesis, so the mutation appears to cause true cell cycle arrest. Surprisingly, the mutation does not appear to affect micronuclear mitosis directly but rather some event(s) prior to micronuclear mitosis that must be completed before cells can complete the cell cycle. The cell cycle arrest was transiently complemented by wild-type cytoplasm exchanged during conjugation with a wild-type cell. Each starved, wild-type cell apparently contained enough rescuing factor to support an average of six cell divisions. Thus, this mutation affects assembly and/or function of at least one but not all of the microtubule-based structures in T. thermophila.

A temperature-sensitive mutation affecting cilia regeneration, nuclear development, and the cell cycle of Tetrahymena thermophila is rescued by cytoplasmic exchange

A temperature-sensitive mutation was isolated that blocks cilia regeneration and arrests growth in Tetrahymena thermophila. Protein and RNA synthesis and ATP production appeared to be largely unaffected at the restrictive temperature, suggesting that the mutation is specific for cilia regeneration and growth. At the restrictive temperature, mutant cells arrested at a specific point in the cell cycle, after macronuclear S phase and shortly before micronuclear mitosis. Arrested cells did not undergo nuclear divisions, DNA replication, or cytokinesis, so the mutation appears to cause true cell cycle arrest. Surprisingly, the mutation does not appear to affect micronuclear mitosis directly but rather some event(s) prior to micronuclear mitosis that must be completed before cells can complete the cell cycle. The cell cycle arrest was transiently complemented by wild-type cytoplasm exchanged during conjugation with a wild-type cell. Each starved, wild-type cell apparently contained enough rescuing factor to support an average of six cell divisions. Thus, this mutation affects assembly and/or function of at least one but not all of the microtubule-based structures in T. thermophila.

mRNAs for alpha- and beta-tubulin and flagellar calmodulin are among those coordinately regulated when Naegleria gruberi amebae differentiate into flagellates

Three of four mRNAs that are specific to the differentiation of Naegleria gruberi amebae into flagellates (Mar, J., J. H. Lee, D. Shea, and C. J. Walsh, 1986, J. Cell Biol., 102:353-361) have been identified as coding for flagellar proteins. The products of these mRNAs, which are coordinately regulated during the differentiation, were identified by in vitro translation of hybrid-selected RNA followed by two-dimensional gel electrophoresis and antibody binding. Six cross-hybridizing clones complementary to a 1.7-kb RNA (class II) all selected mRNA that was translated into two alpha-tubulins. The principal in vitro product, alpha-1, comigrated with a cytoplasmic alpha-tubulin, while the minor product with a more acidic pI, alpha-2, comigrated with flagellar alpha-tubulin. While Naegleria flagellar alpha-tubulin was found to be acetylated based on its reaction with a monoclonal antibody specific to this form, we suggest that alpha-2 is not likely to arise due to acetylation in vitro but probably represents the product of a second alpha-tubulin gene. The class III clone, also complementary to a 1.7-kb RNA, selected beta-tubulin mRNA. In the course of this work it was found, using monoclonal antibodies to the alpha- and beta-subunits of tubulin, that Naegleria alpha-tubulin migrated faster than beta-tubulin on SDS-PAGE. The class IV clone, which hybridizes with a 0.5-kb RNA, selected an mRNA that was translated into a heat stable calcium-binding protein, flagellar calmodulin.

Subpellicular and flagellar microtubules of Trypanosoma brucei brucei contain the same alpha-tubulin isoforms

The cytoskeleton of the parasitic hemoflagellate Trypanosoma brucei brucei essentially consists of two microtubule-based structures: a subpellicular layer of singlet microtubules, which are in close contact with the cell membrane, and the flagellar axoneme. In addition, the cells contain a small pool of soluble tubulin. Two-dimensional gel electrophoretic analysis of the tubulins present in these subcellular compartments revealed two distinct electrophoretic isoforms of alpha-tubulin, termed alpha 1 and alpha 3. alpha 1-Tubulin most likely represents the primary translation product, while alpha 3-tubulin is a posttranslationally acetylated derivative of alpha 1-tubulin. In the pool of soluble cytoplasmic tubulin, alpha 1 is the predominant species, while the very stable flagellar microtubules contain almost exclusively the alpha 3-tubulin isoform. The subpellicular microtubules contain both isoforms. Neither of the two alpha-tubulin isoforms is organelle specific, but the alpha 3 isoform is predominantly located in stable microtubules.