Alterations in gene expression induced by a specific cell interaction during mating in Tetrahymena thermophila

Sexual cell cycle initiation is regulated by CDK19 and CYC9 in Tetrahymena thermophila

To investigate the mechanisms underlying initiation of the sexual cell cycle in eukaryotes, we have focused on cyclins and cyclin-dependent kinases (CDKs) in the well-studied model ciliate, Tetrahymena thermophila We identified two genes, CDK19 and CYC9, which are highly co-expressed with the mating-associated factors MTA, MTB and HAP2. Both CDK19 and CYC9 were found to be essential for mating in T. thermophila Subcellular localization experiments suggested that these proteins are located at the oral area, including the conjugation junction area, and that CDK19 or CYC9 knockout prevents mating. We found that CDK19 and CYC9 form a complex, and also identified several additional subunits, which may have regulatory or constitutive functions. RNA sequencing analyses and cytological experiments showed that mating is abnormal in both ΔCDK19 and ΔCYC9, mainly at the entry to the co-stimulation stage. These results indicate that the CDK19-CYC9 complex initiates the sexual cell cycle in T. thermophila.

Induction of blocks in nuclear divisions and overcondensation of meiotic chromosomes with cycloheximide during conjugation of Tetrahymena thermophila

During conjugation, the micronucleus of Tetrahymena thermophila undergoes five consecutive nuclear divisions: meiosis, third prezygotic division (pregamic mitosis) and two postzygotic mitoses of the synkaryon. The four products of the synkaryon differentiate into macronuclear anlagen and new micronuclei and the old macronucleus is resorbed. The protein synthesis inhibitor cycloheximide, applied during conjugation, induced several developmental blocks. Pairs shifted to the drug during early meiotic prophase (stages I-III) were arrested at prophase. Cycloheximide applied to cells at pachytene (stages IV-VI) to metaphase arrested the conjugants at the stage of modified prometaphase/metaphase with overcondensed, swollen bivalents. In contrast to other systems, in the presence of cycloheximide, separation of chromatids, decondensation of chromosomes and exit from metaphase I were inhibited in both diploid and haploid cells. Pairs shifted to the drug after metaphase I were arrested at postmeiotic interphase after completing one nuclear cycle. The same rule applied to the subsequent cycle; then cells were arrested at the stage of pronuclei, and those pairs with functional pronuclei and synkarya were arrested at the stage of two products of the first postzygotic division (pronuclei were not arrested in nuclear transfer and karyogamy). Only pairs with two products of the first postzygotic division were arrested at the same stage after the cycloheximide treatment. Pairs shifted to cycloheximide during the second postzygotic division were arrested in development of macronuclear anlagen and resorption of old macronuclei. The postmeiotic conjugants pulse-treated with cycloheximide (2 h) yielded heterokaryons retaining parental macronuclei (i.e. they exhibited macronuclear retention).

Early encounters of the repetitive kind: a prelude to cell adhesion in conjugating Tetrahymena thermophila

The relationship between direct cell contacts and subsequent cell-cell adhesion was studied in the ciliated protozoan, Tetrahymena thermophila. During sexual reproduction, adhesion into pairs begins at approximately 1 hr after mixing starved complementary mating types. However, direct contacts between cells prior to pairing are known to be required for the development of adhesion-readiness. We find here that the initial contact interactions are necessary but not sufficient to drive the cells to adhesion-readiness. Secondary interactions are needed. Two distinct experimental strategies were used. First, we examined the effects of a mutant that is unable to pair but which can stimulate two different wild-type mating type cells to pair when mixed. We showed that stimulation by the mutant is only partial; in response to mutant cells, wild-type cells ceased forming food vacuoles but did not undergo tip transformation or concanavalin A (Con A)-receptor tipping. Further, kinetic analysis shows that when mixed together, pair-formation among partially stimulated wild-type cells is slightly delayed, allowing time for these pre-pairing processes to occur. This indicates that, beyond the initial contact interaction, mutant-stimulated wild-type cells require a subsequent interaction which cannot be fulfilled by the mutants. Secondly, we found that by blocking contact interactions between wild-type mating types at various time intervals after they were mixed, additional increase in tip transformation and Con A receptor tipping was prevented. Further, both processes underwent a regression. This indicates that multiple contact interactions are required to drive the cells to adhesion readiness and to prevent developmental slip-back.

Ciliary polypeptides and glycoconjugates of wild-type and mutant Tetrahymena thermophila: starved versus nonstarved

To investigate the role of cilia in mating interactions of Tetrahymena thermophila, ciliary membrane-rich fractions were isolated from two wild-type strains, a non-discharge mucocyst mutant which possesses mating behavior similar to wild-type, and a mating mutant which is able to costimulate cells of complementary mating type but cannot enter into pair formation. In each case, proteins from the ciliary membrane-rich fractions of starved, mating-competent ("initiated") cells were compared with those from non-starved, mating-incompetent ("non-initiated") cells, by gel electrophoresis and lectin blotting. In stained gels, a 43 kDa polypeptide was reduced or absent in initiated cells but present in non-initiated cells, in all strains. In silver-stained gels, a 25 kDa polypeptide was present in all strains, both initiated and non-initiated. In blots probed with Con A-peroxidase, a 25 kDa glycoprotein was present in ciliary membrane fractions from non-initiated cells and absent in membranes of initiated cells of the two wild-type strains and the mucocyst mutant, but is present in initiated and non-initiated cells of the mating mutant (several hypotheses are presented to explain these findings). In addition, ciliary proteins of the mating mutant included at least two unique Con A-binding polypeptides. Our results support the idea that development of mating competence during starvation involves an extensive remodeling of ciliary membranes, and identify a 25 kDa glycoconjugate as having a potential role in control of pair formation during mating.

Cell-cell interactions trigger the rapid induction of a specific high mobility group-like protein during early stages of conjugation in Tetrahymena

Conjugation in Tetrahymena represents an ordered developmental pathway which represents the sexual phase of the ciliate life cycle. This pathway is initiated when starved cells of opposite mating types are mixed and are allowed to make a series of cell-cell contacts (a period termed costimulation) which lead to the formation of mating pairs. Here, we demonstrate that two previously described abundant high mobility group (HMG)-like proteins, HMG B and HMG C, whose synthesis appeared to be coordinately regulated in vegetative cells, are not required during the same stages of conjugation. The level of mRNA for both HMG B and HMG C is high during vegetative growth and during the development of new macronuclei. However, specific induction of HMG B mRNA is observed soon after cells of opposite mating types are mixed. Thus, the genes which encode HMG B and HMG C in Tetrahymena can be controlled independently or coordinately. Nuclear run-on experiments show that a significant factor underlying the rapid induction of HMG B message early in the sexual cycle is an increase in the transcriptional activity of the HMG B gene. Experiments are presented which show that this induction of HMG B message requires protein synthesis and is dependent upon the cell-cell contacts made during costimulation. Essentially all of the HMG B protein, which is newly synthesized during this period, is targeted to parental macronuclei where it serves an as yet undetermined function(s).

Concanavalin A inhibits mating type recognition in Tetrahymena

The lectin concanavalin A (Con A) inhibits adhesion of cells of complementary mating types into pairs during conjugation in the ciliate Tetrahymena thermophila. Distinct changes in protein synthesis occur in conjugating Tetrahymena, starting before cells have paired, as a result of a preliminary interaction, costimulation, involving nonadhesive, contact-mediated, specific cell-cell recognition. We report here that ConA inhibits costimulation-induced protein synthesis changes. We interpret this result as evidence that Con A inhibits cellular recognition, independent of cell-cell adhesion, in Tetrahymena.

Concanavalin A binding induces association of possible mating-type receptors with the cytoskeleton in Tetrahymena

The lectin concanavalin A (conA; 25 micrograms/ml) inhibits conjugation in the ciliate Tetrahymena, and binds to receptors localized at the junction between conjugating cells. We report here that succinyl-conA (30 micrograms/ml) has similar activity, but that two other mannosespecific lectins, lentil and pea lectins, have inhibitory activities more than tenfold lower in this system, indicating that factors other than mannose specificity are essential for biological activity. By using fluorescein-isothiocyanate (FITC)-conA, we have found that extraction of cells with the detergent Triton X-100 removes conA receptors from the extraction-resistant cytoskeleton, but that the binding of conA to its receptor before extraction associates the ligand-receptor complex with the cytoskeleton. Under the hypothesis that the conA receptor may be a mating type receptor, we have used this ligand-induced differential cytoskeletal association, in conjunction with electrophoresis and Western blotting, to identify a glycoprotein with an apparent molecular weight (MW) of 23,000 D which may be a mating type receptor. Our data are consistent with a model in which a direct interaction between the conA receptor and the cytoskeleton, rather than receptor cross-linking, is the biologically significant activity of ligand binding.

Polypeptides during early conjugation in Tetrahymena thermophila

As Tetrahymena thermophila cells differentiate from their vegetative life cycle to sexual reproduction, their polypeptide pattern undergoes a series of changes. These changes have been traced in extracellular, cellular, and subcellular compartments. The first alteration is induced by the nutritional shift-down and results in stimulation of at least one ciliary polypeptide and affects a series of polypeptides from other compartments. The second alteration is induced by mixing starved cells of complementary mating types and this stimulates the synthesis of nine ciliary polypeptides before pairs have formed and eight afterwards. At least five of these early and one of the late conjugation-related ciliary polypeptides are removed by low concentrations of EDTA, indicating that they are located on the external side of the plasma membrane. No differences were observed between polypeptides excreted during starvation and after mixing of complementary mating types. At Tris concentrations restrictive for conjugation, cilia lack the conjugation-related polypeptides. Some of these are instead found among the excreted polypeptides. Using O'Farrell gels and silver staining on isogenic cells of all possible mating types, we have been unable to correlate changes in polypeptide patterns to specific mating types.

Stage-specific changes in protein synthesis during conjugation in Tetrahymena thermophila

Conjugation in the free-living ciliate Tetrahymena thermophila is an inducible developmental system which results in a synchronized reorganization of the genetic material in both mates of a pair. The cytological events were followed by Feulgen stainings of simultaneously mating cells and protein synthesis was revealed using [35S]methionine pulse labelling and two-dimensional gel electrophoresis. At least 33 proteins, including 24 conjugation-specific proteins, with apparent molecular weights (Mr) between 61 and 200 X 10(3) are stimulated during conjugation. Two slightly acidic proteins (Mr 89 and 73 X 10(3), respectively) are stimulated shortly after mixing of mating-competent cells and mainly before tight pairs are formed. Ten proteins are stimulated during meiosis, and two of these (Mr 90 and 78 X 10(3), respectively) are particularly interesting, since they are highly stimulated and more basic (pI values around 8.5) than most other proteins detected. Twelve proteins are stimulated essentially between pairing and early macronuclear development, three are stimulated from shortly before zygote formation and during the post-zygotic divisions, and six are stimulated during late conjugation, at various parts of macronuclear development. The functions of the conjugation-stimulated proteins are discussed.

Membrane protein differences correlated with the development of mating competence in Tetrahymena thermophila

Initiation is the contact-independent phase of sexual conjugation which occurs when mature cells of Tetrahymena thermophila are shifted from growth medium to a low-salt starvation buffer. Immaturity, like high-salt starvation, restricts the ability of cells to conjugate; immature cells do not conjugate in either low- or high-salt buffers. Comparisons between sexually mature cells starved in initiation-restrictive and initiation-permissive buffers, and between immature and mature cells starved in an initiation-permissive buffer permitted the analysis of membrane protein expression correlated with mating competence. No polypeptides identified by lactoperoxidase-catalyzed iodination were found to be specific to mating-competent cells; however, several polypeptides not present in initiated cells were found to be common to the cell surfaces of immature and non-initiated cells which suggests that (1) initiation involves the removal of specific proteins from the cell surface, and (2) immaturity may be due to an inability to initiate.

An analysis of protein synthesis, membrane proteins, and concanavalin A-binding proteins during conjugation in Tetrahymena thermophila

Conjugation in the ciliate Tetrahymena thermophila has been used as a system in which to analyze biochemical events associated with the execution of a complex cell-cell interaction. Two-dimensional electrophoretic analysis of [35S]methionine-labeled whole-cell proteins revealed major changes in protein synthesis correlated with costimulation and the onset of pairing; specifically, the major induced polypeptide was one of 80 kDa. A second change in the pattern of protein synthesis was associated with the onset of meiosis; the major induced product was another, perhaps related, 80-kDa polypeptide. An effort was made to detect changes in the patterns of membrane proteins and Con A-binding proteins during conjugation; no changes were found. These results are discussed in the context of earlier hypotheses regarding the distribution of Con A receptors on the surfaces of conjugating cells.

The conjugation junction of Tetrahymena: Its structure and development

The conjugation junction of Tetrahymena has been examined by thin sections, freeze fracture preparations, and by scanning electron microscopy. The junction is formed where the anterior tips of the pairing cells attach to one another. The structure is essentially a large disk composed of two face-to-face plasma membranes separated by a gap of extracellular space measuring about 50 nm. Rows of intramembrane particles are present at the boundary between the junction and ordinary cell cortex. These particles form a ring around the junction. Subjacent to each membrane is a thick mottled layer of material. Pores form in the junction at sites of membrane fusion. Though wider than long, these structures are actually bridges of cytoplasm that connect the conjugating cells. Pores fall into certain size and shape classes, indicating that membrane fusion is highly controlled in this system. At the level of the cytoplasmic bridge the submembrane material is compact and electron-dense. Changes in the structure of the epiplasmic layer have been monitored as the normal cortex is modified during tip transformation and through formation of the mature conjugation junction. Evidence is provided that the submembrane layer plays a significant role in the regulation of pore formation. This cytoskeletal structure may also limit the extent of membrane fusion, thus controlling the size of the cytoplasmic channels.