Green fluorescent protein (GFP)-tagged actin was used to investigate the distribution and function of actin in Tetrahymena. A strain that expresses both GFP-actin and endogenous actin was developed by transformation of Tetrahymena thermophila with a ribosomal DNA-based replicative vector. Confocal microscopy of living cells and immunogold electron microscopy confirmed localization of GFP-actin to basal bodies and the contractile ring. Incorporation of the fusion protein into these and other actin-related structures correlated with severe impairment of macronuclear elongation and cytokinesis. At 30 degrees C macronuclear elongation failed to occur in 25% of the transformants despite completion of micronuclear division. At 20 degrees C macronuclear elongation failed to occur in 2% of the population. Arrest of cytokinesis coincided with failure of macronuclear elongation. Arrested cells developed into homopolar doublets with two sets of oral structures. This study indicates a requirement for actin in nuclear elongation and cytokinesis. Although GFP-actin can interfere with the functioning of actin-containing structures, the GFP-actin transformant strain can be used to monitor actin distribution and dynamics and is therefore an important new tool for further studies of Tetrahymena actin.

No abstract

The phagosome cycle was investigated in Tetrahymena thermophila, which had internalized fluorescent latex beads. Confocal microscopy of cells from a GFP-actin strain revealed actin filaments that extended 3-5 mum from the periphery of fluorescent phagosomes. In GFP-actin cells and in wild-type cells, motility of fluorescent phagosomes was directed from the oral cavity to the posterior end of the cell. Although 60% of fluorescent phagosomes in the MYO1-knockout strain were motile, movement of phagosomes was not directed toward the posterior end of the cell and was random. Forty percent of fluorescent phagosomes in knockout cells were non-motile in contrast to only 20% non-motile phagosomes in wild-type cells. The increased incidence of non-motile phagosomes in the knockout strain could reflect absence of Myo1p as a motor. Another myosin or other molecular motors could power random movement of phagosomes in the MYO1-knockout strain. In latrunculin-treated GFP-actin cells, movement of fluorescent phagosomes was random. Average velocity of random movement of fluorescent phagosomes in the knockout strain and in latrunculin-treated cells was statistically the same as the average velocity (2.0 +/- 1.9 microm/min) of phagosomes in GFP-actin cells. These findings are an indication that dynamic actin and Myo1p are required for directed motility of phagosomes.

This report presents an initial comparison of motor, neck, and tail domains of myosin genes in Tetrahymena thermophila. An unrooted phylogenetic tree drawn from alignment of predicted amino acid translations determined the relationship among

Targeted gene disruption was used to investigate the function of MYO1, an unconventional myosin gene in Tetrahymena thermophila. Phenotypic analysis of a transformed strain that lacked a functional MYO1 gene was conducted at both 20 degrees C and 35 degrees C. At either temperature the delta MYO1 strain had a smaller cytoplasm/nucleus ratio than wild type. At 20 degrees C, delta MYO1 populations had a longer doubling time than wild type, lower saturation density, and a reduced rate of food vacuole formation. However, at 35 degrees C, these characteristics were comparable to wild type. Although micronuclear division and cytokinesis appeared normal in delta MYO1 cells, failure of the macronucleus to elongate properly resulted in unequal segregation of macronuclear DNA in cells maintained at either 20 degrees C or 35 degrees C.

Degenerate primers for two regions of sequence homology in the myosin head domain were used in a polymerase chain reaction screen of Tetrahymena thermophila genomic DNA to amplify a 765 bp fragment that was cloned and sequenced. Based on the presence of conserved, myosin-specific sequences, the 765 bp PCR product was identified as a fragment of a myosin gene, the first to be discovered in ciliated protozoa and herein referred to as MYO1. An inverse polymerase chain reaction strategy was used to obtain additional sequence data that included the entire head domain of MYO1. Alignment of the predicted amino acid sequence of the MYO1 head domain with known myosin sequences identified the ATP-binding site, a phosphorylation site, and other myosin-specific consensus regions. In a northern blot analysis, a 765 bp MYO1-specific probe detected a 6.6 kb transcript under highly stringent hybridization conditions. Phylogenetic analysis revealed that the predicted protein encoded by MYO1 is not a member of any of the previously defined myosin classes and therefore represents a presumptive new myosin class.

Myo1 is one of 13 myosins in Tetrahymena thermophila. Initially, twelve of the myosins in Tetrahymena were assigned to Class XX in the myosin superfamily but recently re-assigned to a subclass within Class XIV. In a previous study, we reported that genomic knockout of MYO1 affected phagocytosis and macronuclear amitosis. These two phenotypes have appeared disparate because a possible mechanism linking phagocytosis and amitosis was unknown. In the present study, Myo1 localization was investigated in order to further link machinery for phagocytosis and amitosis. Antibodies directed against the Myo1 motor domain detected an immunospecific polypeptide at 175-180 kDa on immunoblots of wild-type proteins. The 175-180 kDa polypeptide was not detected on immunoblots of proteins from the knockout strain. For immunofluorescence microscopy, cells were allowed to internalize fluorescent beads as markers for phagosomes. In wild-type cells, anti-Myo1 and anti-actin antibodies co-localized to the periphery of phagosomes and the macronucleus. In the MYO1-knockout strain only background fluorescence was observed with anti-Myo1 antibody. Confocal x-z series through macronuclei revealed fluorescent beads within the nucleoplasm. Statistical analysis showed a significant difference between the mean distributions of fluorescent beads in the nucleoplasm of wild-type and MYO1-knockout cells. A fluorescent dye was used to label plasma membrane in living cells. Dye-labeled vacuoles trafficked to the macronucleus. Trafficking of phagosomes to the macronucleus in a myosin-dependent manner is a novel finding and a possible mechanism for targeting myosin and actin to the nucleus.

Previous studies have shown that Myo1(myosin class XIV) localizes to the cytoskeleton and is involved in amitosis of the macronucleus and trafficking of phagosomes. Myo1 contains a FERM domain that could be a site for interaction between Myo1 and the cytoskeleton. Here, we explore the function of FERM by investigating its cytoskeleton binding partners and involvement in localization of Myo1. Alignment of Myo1 FERM with a talin actin-binding sequence, a MAP-2 tubulin-binding sequence, the radixin FERM dimerization motif, and the SV40 nuclear localization sequence (NLS) revealed putative actin- and tubulin-binding sequences, a putative FERM dimerization motif, and NLS-like sequences in both the N-terminal and C-terminal regions of Myo1 FERM. Alignment of Myo1 with an ERM C-terminal motif revealed a similar sequence in the Myo1 motor domain. GFP-FERM and two truncated FERM domains were separately expressed in Tetrahymena. GFP-FERM contained the entire Myo1 FERM. Truncated Myo1 FERM domains contained either the N-terminal or the C-terminal region of FERM and one putative sequence for actin-binding, one for tubulin-binding, a putative dimerization motif, and a NLS-like sequence. Actin antibody coprecipitated GFP-fusion polypeptides and tubulin from lysate of cells expressing GFP-fusions. Cosedimentation assays performed with either whole cell extracts or anti-actin immunoprecipitation pellets revealed that F-actin (independent of ATP) and microtubules cosedimented with GFP-fusion polypeptides. GFP-FERM localized to the cytoskeleton, phagosomes, and nucleus. Truncated GFP-FERM domains localized to phagosomes but not to the cytoskeleton or nucleus.