Scanning Electron Microscopy of Cytoskeletal Elements in the Oral Apparatus ofTetrahymena1

Multiple tubulin forms in ciliated protozoan Tetrahymena and Paramecium species

Tetrahymena and Paramecium species are widely used representatives of the phylum Ciliata. Ciliates are particularly suitable model organisms for studying the functional heterogeneity of tubulins, since they provide a wide range of different microtubular structures in a single cell. Sequencing projects of the genomes of members of these two genera are in progress. Nearly all members of the tubulin superfamily (alpha-, beta-, gamma-, delta-, epsilon-, eta-, theta-, iota-, and kappa-tubulins) have been identified in Paramecium tetraurelia. In Tetrahymena spp., the functional consequences of different posttranslational tubulin modifications (acetylation, tyrosination and detyrosination, phosphorylation, glutamylation, and glycylation) have been studied by different approaches. These model organisms provide the opportunity to determine the function of tubulins found in ciliates, as well as in humans, but absent in some other model organisms. They also give us an opportunity to explore the mechanisms underlying microtubule diversity. Here we review current knowledge concerning the diversity of microtubular structures, tubulin genes, and posttranslational modifications in Tetrahymena and Paramecium species.

Basal body duplication and maintenance require one member of the Tetrahymena thermophila centrin gene family

Centrins, small calcium binding EF-hand proteins, function in the duplication of a variety of microtubule organizing centers. These include centrioles in humans, basal bodies in green algae, and spindle pole bodies in yeast. The ciliate Tetrahymena thermophila contains at least four centrin genes as determined by sequence homology, and these have distinct localization and expression patterns. CEN1's role at the basal body was examined more closely. The Cen1 protein localizes primarily to two locations: one is the site at the base of the basal body where duplication is initiated. The other is the transition zone between the basal body and axoneme. CEN1 is an essential gene, the deletion of which results in the loss of basal bodies, which is likely due to defects in both basal body duplication and basal body maintenance. Analysis of the three other centrins indicates that two of them function at microtubule-rich structures unique to ciliates, whereas the fourth is not expressed under conditions examined in this study, although when artificially expressed it localizes to basal bodies. This study provides evidence that in addition to its previously known function in the duplication of basal bodies, centrin is also important for the integrity of these organelles.

The dynamics of filamentous structures in the apical band, oral crescent, fission line and the postoral meridional filament in Tetrahymena thermophila revealed by monoclonal antibody 12G9

The ciliate Tetrahymena thermophila possesses a multitude of cytoskeletal structures whose differentiation is related to the basal bodies - the main mediators of the cortical pattern. This investigation deals with immunolocalization using light and electron microscopy of filaments labeled by the monoclonal antibody 12G9, which in other ciliates identifies filaments involved in transmission of cellular polarities and marks cell meridians with the highest morphogenetic potential. In Tetrahymena interphase cells, mAb 12G9 localizes to the sites of basal bodies and to the striated ciliary rootlets, to the apical band of filaments and to the fine fibrillar oral crescent. We followed the sequence of development of these structures during divisional morphogenesis. The labeling of the maternal oral crescent disappears in pre-metaphase cells and reappears during anaphase, concomitantly with differentiation of the new structure in the posterior daughter cell. In the posterior daughter cell, the new apical band originates as small clusters of filaments located at the base of the anterior basal bodies of the apical basal body couplets during early anaphase. The differentiation of the band is completed in the final stages of cytokinesis and in the young post-dividing cell. The maternal band is reorganized earlier, simultaneously with the oral structure. The mAb 12G9 identifies two transient structures present only in dividing cells. One is a medial structure demarcating the two daughter cells during metaphase and anaphase, and defining the new anterior border of the posterior daughter cell. The other is a post-oral meridional filament marking the stomatogenic meridian in postmetaphase cells. Comparative analysis of immunolocalization of transient filaments labeled with mAb12G9 in Tetrahymena and other ciliates indicates that this antibody identifies a protein bound to filamentous structures, which might play a role in relying polarities of cortical domains and could be a part of a mechanism which governs the positioning of cortical organelles in ciliates.

Calmodulin and Ca2+/calmodulin-binding proteins are involved in Tetrahymena thermophila phagocytosis

The ciliated protist, Tetrahymena thermophila, possesses one oral apparatus for phagocytosis, one of the most important cell functions, in the anterior cell cortex. The apparatus comprises four membrane structures which consist of ciliated and unciliated basal bodies, a cytostome where food is collected by oral ciliary motility, and a cytopharynx where food vacuoles are formed. The food vacuole is thought to be transported into the cytoplasm by a deep fiber which connects with the oral apparatus. Although a large number of studies have been done on the structure of the oral apparatus, the molecular mechanisms of phagocytosis in Tetrahymena thermophila are not well understood. In this study, using indirect immunofluorescence, we demonstrated that the deep fiber consisted of actin, CaM, and Ca2+/CaM-binding proteins, p85 and EF-1alpha, which are closely involved in cytokinesis. Moreover, we showed that CaM, p85, and EF-1alpha are colocalized in the cytostome and the cytopharynx of the oral apparatus. Next, we examined whether Ca2+/CaM signal regulates Tetrahymena thermophila phagocytosis, using Ca2+/CaM inhibitors chlorpromazine, trifluoperazine, N-(6-aminohexyl)-1-naphthalenesulfonamide, and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide HCI. In Tetrahymena, it is known that Ca2+/CaM signal is closely involved in ciliary motility and cytokinesis. The results showed that one of the inhibitors, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide HCl, inhibited the food vacuole formation rather than the ciliary motility, while the other three inhibitors effectively prevented the ciliary motility. Considering the colocalization of CaM, p85, and EF-1alpha to the cytopharynx, these results suggest that the Ca2+/CaM signal plays a pivotal role in Tetrahymena thermophila food vacuole formation.

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.

Deciliation induces phosphorylation of a 90-kDa cortical protein in Tetrahymena thermophila

We have used the anti-phosphoprotein antibody MPM-2 to examine changes in phosphorylation of cortical proteins during cilia regeneration in Tetrahymena thermophila. Although numerous cortical proteins are phosphorylated in both nondeciliated and deciliated cells, deciliation induces a dramatic increase in the phosphorylation of a 90-kDa cortical protein. The 90-kDa protein remained phosphorylated during cilia regeneration and then gradually became dephosphorylated. The 90-kDa protein was phosphorylated and dephosphorylated normally in Tetrahymena mutants that assemble short cilia, suggesting that achievement of full length is not the signal that triggers dephosphorylation of the 90-kDa protein. When initiation of cilia assembly is blocked, the 90-kDa protein becomes phosphorylated and remains phosphorylated for an extended period of time, suggesting that initiation of cilia elongation triggers eventual dephosphorylation of the 90-kDa protein, regardless of how long the cilia actually become.

Oral assembly in left-handed Tetrahymena thermophila

We have investigated oral development in a non-genetically derived left-handed (LH) form of Tetrahymena thermophila, in which the large-scale asymmetry of arrangement of cortical structures is reversed whereas the local asymmetry of ciliary architecture remains normal. Approximately 1/2 of the oral apparatuses (OAs) of LH cells develop in the form of superficial mirror-images of OAs of RH cells. In most of these OAs, membranelles are assembled from the cells' anterior to posterior. Nonetheless, the posterior ends of these membranelles undergo the basal body displacements that lead to a "sculptured" appearance, so that the membranelles of LH OAs become organized as rotational permutations of membranelles of normal RH OAs. Many of these membranelles re-orient to a normal orientation near the end of oral development. Membranelles and undulating membranes (UMs) may develop independently of each other, and formation of postciliary microtubules of UMs is separate from that of ribbed wall microtubules. In some cases, the entire OA develops and remains as a 180 degrees rotational permutation of the normal, resembling the inverted OAs of mirror-image doublets and LH cells of Glaucoma scintillans described by Suhama. We present a model for these complex developmental outcomes. These developmental patterns resemble those described previously and less completely for "secondary" OAs of cells with mirror-image global patterns, including janus cells. The present study demonstrates that such alterations in oral development are not a direct outcome of genotypic changes.

Oral filament proteins and their regulation in Tetrahymena pyriformis

Two proteins from the Triton X-100-insoluble fraction of Tetrahymena pyriformis have been isolated and shown by immunological methods to be major components of a pervasive system of filaments localized within the oral apparatus. These proteins, OF-1 and OF-2, have apparent molecular weights (MWapp) in polyacrylamide gels of 87,000 and 80,000 D, respectively. Peptide maps obtained and the absence of immunological cross-reactivity suggest that these proteins are not closely related to each other. Indirect immunofluorescence studies on dividing cells have shown that the oral filament system forms late in the cell cycle. The filaments appeared first after the basal bodies in the oral primordium had organized into groups and the fission furrow had begun to form. Dedifferentiation of the oral filament system in the anterior (old) oral apparatus was also observed at this point in the cell cycle. Following this, the oral filament systems in both old and new oral apparatuses completed development synchronously. Proteins showing antigenic similarity to OF-1 were found in a number of other cell types. Tests with heterologous antisera failed to demonstrate a relationship between vertebrate cytoskeletal proteins and the oral filament proteins of Tetrahymena.

The 5S and 5.8S ribosomal RNA sequences of Tetrahymena thermophila and T. pyriformis

The nucleotide sequences of the 5S rRNAs of Tetrahymena thermophila and two strains of T. pyriformis have been determined to be identical. The 5.8S rRNA sequences have also been determined; these sequences correct several errors in an earlier report. The 5.8S rRNAs of the two species differ at a single position. The sequencing results indicate that the species are of recent common ancestry. Molecular evidence that has been interpreted in the past as suggestive of an ancient divergence has been reviewed and found to be consistent with a T. pyriformis complex radiation beginning approximately 30-40 million years ago.

Oral ultrastructure and oral development of the misaligned undulating membrane mutant of Tetrahymena thermophila

The misaligned undulating membrane (mum) mutant of Tetrahymena thermophila is a non-conditional, single gene recessive mutation. The major effect of the mum mutation is the production of multiple undulating membrane (UM) fragments in the oral apparatus (OA). The ultrastructure of the UM fragments of mum OAs is identical to that of the single UM of wild-type OAs. Analysis of OA development at midbody using a combination of light microscopy of protargol-stained cells and SEM of demembranated whole cells showed that the phenotypic effect of the mum mutation first becomes evident during mid to late stage 4 and is fully manifested in early stage 5. The effect of the mutation involves a proliferation of excess basal bodies in the UM field. Subsequent events in the development of the mum OA from mid to late stage 5 are identical to those in wild-type OAs. This study suggests that the mum mutation establishes conditions that allow the production of multiple UMs and thus reveals that the UM field is competent for the complete and coordinated development of several adjacent UMs. This level of regional control is not clearly evident when a single UM is present. The comparison of development of wild-type and mum OAs required an extensive reanalysis of stages 4 and 5 of normal oral development. On the basis of current and previous observations, we propose a new and more subdivided staging system for oral development in Tetrahymena.