Tetrahymena fimbrin localized in the division furrow bundles actin filaments in a calcium-independent manner

In cytokinesis, the contractile ring constricts the cleavage furrow. However, the formation and properties of the contractile ring are poorly understood. Fimbrin has two actin-binding domains and two EF-hand Ca(2+)-binding motifs. Ca(2+) binding to the EF-hand motifs inhibits actin-binding activity. In Tetrahymena, fimbrin is localized in the cleavage furrow during cytokinesis. In a previous study, Tetrahymena fimbrin was purified with an F-actin affinity column. However, the purified Tetrahymena fimbrin was broken in to a 60 kDa fragment of a 70 kDa full length fimbrin. In this study, we investigated the properties of recombinant Tetrahymena fimbrin. In an F-actin cosedimentation assay, Tetrahymena fimbrin bound to F-actin and bundled it in a Ca(2+)-independent manner, with a K(d) of 0.3 micro M and a stoichiometry at saturation of 1:1.4 (Tetrahymena fimbrin: actin). In the presence of 1 molecule of Tetrahymena fimbrin to 7 molecules of actin, F-actin was bundled. Immunofluorecence microscopy showed that a dotted line of Tetrahymena fimbrin along the cleavage furrow formed a ring structure. The properties and localization of Tetrahymena fimbrin suggest that it bundles actin filaments in the cleavage furrow and plays an important role in contractile ring formation during cytokinesis.     

Tetrahymena actin: localization and possible biological roles of actin in Tetrahymena cells

Though actin is ubiquitous in eukaryotes, its existence has not been clearly proven in Tetrahymena. Recently, we have succeeded in cloning and sequencing the Tetrahymena actin gene using a Dictyostelium actin probe (Hirono, M. et al. (1987) J. Mol. Biol. 194, 181-192). The primary structure of the Tetrahymena actin deduced from the nucleotide sequence of its gene is greatly divergent from those of other known actins, making it necessary to ascertain whether the predicted Tetrahymena actin is indeed an actin. In this paper, we investigated the localization of the predicted Tetrahymena actin by an immunofluorescence technique using antibody against its synthetic N-terminal peptide, in order to elucidate its possible biological roles. The results showed that immunofluorescence was localized in the division furrow of the dividing cell, and in the intranuclear filament bundles formed in cells exposed to heat shock or DMSO. In addition, the oral apparatus and the proximity of the cytoproct, which are organelles involved in endocytosis and exocytosis, respectively, also fluoresced. Thus, we conclude that the Tetrahymena actin we identified is indeed an actin and plays the same biological roles as ubiquitous actins do, although it is considerably divergent in its amino acid sequence.     

Dibucaine-induced synchronous mucocyst secretion in Tetrahymena

Synchronous secretion of all available mature mucocysts was induced in late log phase cultures of Tetrahymena thermophilia (B III) by the local anaesthetic dibucaine. No assembled fusion rosettes were seen within the plasma membrane after release until 2-3 hrs of regrowth, thus proving that the rosettes are not permanent sites within the plasma membrane but have to be reassembled each time for a new fusion event to occur. Concomitant with the reappearance of assembled fusion rosettes, the cell cytoplasm fills up with precursors of new mucocysts thus linking the two events together.     

Continuous synchronous culture of Tetrahymena pyroformis

Tetrahymena was continuously cultivated in a series of stirred tank reactors with recycle. By holding part of the reactor train at a higher temperature than the remainder a synchronizing influence was introduced into the cells' environment. The system resulted in division occurring preferentially in a small contiguous group of the reactors, this effect being observed in both the number of cells found in each reactor and also their size distribution.     

Absence of actin in the cilia of Tetrahymena pyriformis

An extensive electrophoretic analysis of the proteins of highly purified Tetrahymena pyriformis cilia has been undertaken. No component which would specifically bind rabbit muscle myosin could be identified. Furthermore, no peptides from acetone powders of these cilia could be found which co-electrophoresed with rabbit muscle actin. Lastly, the components which most closely resembled actin in molecular weight were quantitated using densitometry and found to represent less than 0.8% of the tubulin in these preparations.     

Expression of Tetrahymena actin in mammalian cells.

We previously revealed that Tetrahymena actin can copolymerize with rabbit skeletal muscle actin whereas it has a very divergent primary structure and some unusual properties. To investigate the effects of coexistence of this unusual Tetrahymena actin in mammalian cells, we here transfected Tetrahymena actin gene on an expression vector into COS-1 cells. From the results of immunofluorescence microscopy, it was found that Tetrahymena actin expressed in COS-1 cells copolymerized with intrinsic actin, and it was conspicuously localized to the tips of microfilament core bundles in microspikes. On the other hand, increase in cell number tended to cease temporarily about 24 hr after transfection with Tetrahymena actin gene, implying the inhibition of cytokinesis by Tetrahymena actin coexistence.     

Tetrahymena profilin is localized in the division furrow.

Localization of Tetrahymena profilin was examined by an immunofluorescence method. In interphase Tetrahymena cells, immunofluorescence for profilin was diffusely distributed in the cytoplasm, while in dividing cells, additional intense fluorescence was observed in the division furrow. From the result of immunofluorescence localization using cytoskeletal cell models, a significant fraction of profilin appeared to become insoluble in association with a cytoskeletal structure just beneath the division furrow during cytokinesis, although remaining profilin existed as a soluble form in the cytoplasm. Double immunofluorescence staining with anti-profilin and anti-actin antibodies revealed that the localization of profilin in the division furrow coincided with that of contractile ring microfilaments in terms of both position and timing. This is the first report describing the coexistence of profilin with actin filaments in the division furrow, implying the possible involvement of profilin in assembly and disassembly of contractile ring microfilaments in the process of cytokinesis.     

A fetal skeletal muscle actin mRNA in the mouse and its identity with cardiac actin mRNA

We compare a recombinant cDNA plasmid (pAF81) complementary to a fetal skeletal muscle actin mRNA with a plasmid (pAM91) complementary to the actin mRNA expressed in adult skeletal muscle. The two mRNAs are significantly diverged in silent nucleotide positions; they are coexpressed in fetal skeletal muscle, and in differentiating muscle cell cultures their accumulation begins coordinately. The sequence of pAF81 shows that the amino acid sequence of mouse fetal skeletal muscle actin is almost identical to that of adult bovine cardiac actin. Hybridization of pAF81 to RNA from different mouse tissues shows that fetal skeletal muscle actin mRNA is very homologous or identical to fetal and adult cardiac actin mRNA. Only one gene homologous to pAF81 is detected on blots of restricted mouse DNA. We conclude that this gene must be expressed both in fetal skeletal muscle and in fetal heart. Whereas mRNA transcribed from this gene is the major actin mRNA species in adult heart, it is present in low amounts, if at all, in adult skeletal muscle.     

Self-splicing of the Tetrahymena intron from mRNA in mammalian cells

The Tetrahymena pre-rRNA self-splicing intron is shown to function in the unnatural context of an mRNA transcribed by RNA polymerase II in mammalian cells. Mutational analysis supports the conclusion that splicing in cells occurs by the same RNA-catalyzed mechanism established for splicing in vitro. Insertion of the intron at five positions spanning the luciferase open reading frame revealed 10-fold differences in accumulation of ligated exons and in luciferase activity; thus, the intron self-splices in many exon contexts, but the context can have a significant effect on activity. In addition, even the best self-splicing constructs, which produced half as much mRNA as did an uninterrupted luciferase gene, gave approximately 100-fold less luciferase enzyme activity, revealing an unexpected discontinuity between mRNA production and translation in cells. The finding that production of accurately spliced mRNA in cells does not guarantee a corresponding level of protein production is surprising, and may have implications for the development of trans-splicing ribozymes as therapeutics.