The Trypanosoma brucei Cyclin, CYC2, Is Required for Cell Cycle Progression through G1 Phase and for Maintenance of Procyclic Form Cell Morphology

Hammarton, Tansy C.; Engstler, Markus; Mottram, Jeremy C.

doi:10.1074/jbc.m401276200

Cited by 57 publications

(69 citation statements)

References 39 publications

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“…tbTRF RNAi arrested bloodstream and procyclic cells at different stages of the cell cycle, suggesting that the checkpoint mechanisms differ in these two life cycle stages. Consistent with this possibility, different cyclins are used in bloodstream and procyclic stages to control cell-cycle progress (28,69). Interestingly, the overexpression of hTRF1 in procyclic T. brucei also resulted in S-phase arrest, though the phenotypes are not exactly the same as with tbTRF RNAi (52).…”

Section: Discussionmentioning

confidence: 73%

Trypanosome Telomeres Are Protected by a Homologue of Mammalian TRF2

Espinal

Cross

2005

Molecular and Cellular Biology

186

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Putative TTAGGG repeat-binding factor (TRF) homologues in the genomes of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major were identified. They have significant sequence similarity to higher eukaryotic TRFs in their C-terminal DNA-binding myb domains but only weak similarity in their N-terminal domains. T. brucei TRF (tbTRF) is essential and was shown to bind to duplex TTAGGG repeats. The RNA interferencemediated knockdown of tbTRF arrested bloodstream cells at G 2 /M and procyclic cells partly at S phase. Functionally, tbTRF resembles mammalian TRF2 more than TRF1, as knockdown diminished telomere single-stranded G-overhang signals. This suggests that tbTRF, like vertebrate TRF2, is essential for telomere end protection, and this also supports the hypothesis that TRF rather than Rap1 is the more ancient DNA-binding component of the telomere protein complex. Identification of the first T. brucei telomere DNAbinding protein and characterization of its function provide a new route to explore the roles of telomeres in pathogenesis of this organism. This work also establishes T. brucei as an attractive model for telomere biology.Telomeres are specialized protein-DNA complexes at the ends of eukaryotic chromosomes. Telomere DNA generally consists of simple, repetitive, TG-rich sequences that are maintained by telomerase (5) and end with a single-stranded G-rich overhang (78). A specialized telomere structure, the T loop, formed by the invasion of telomere G overhangs into the telomeric double-stranded region, in mammals, hypotrichous ciliates, and Trypanosoma brucei has been identified (26,51,53). It is hypothesized that hiding the telomere single-stranded G overhangs in a T-loop structure helps to protect the telomere ends.Proteins that bind to duplex or single-stranded telomere DNA are integral components of the telomere complex and play critical roles in both telomere length regulation and end protection (36). In mammalian cells, two paralogues, TT AGGG repeat-binding factor 1 (TRF1) and TRF2, bind duplex TTAGGG repeats (4,9,12). Both TRF1 and TRF2 have a C-terminal myb motif for DNA binding (3, 9) and an upstream TRF homology (TRFH) domain for homodimerization (24). However, the N termini are quite different: TRF1 is acidic, while TRF2 is basic. Mammalian TRF1 negatively regulates telomere length through a telomerase-dependent pathway (61, 75), whereas TRF2 is involved in both telomere length regulation (38, 61) and telomere end protection (17). Removal of TRF2 from telomeres by overexpression of a dominantnegative mutant of TRF2 resulted in an at least 30%

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Section: Discussionmentioning

confidence: 73%

Trypanosome Telomeres Are Protected by a Homologue of Mammalian TRF2

Espinal

Cross

2005

Molecular and Cellular Biology

186

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“…Previous publications have suggested that nuclear DNA replication or mitosis does not have to be completed in order for cytokinesis to occur (15,28). Silencing of UMSBP results in cell-cycle arrest and impedes mitosis.…”

Section: Discussionmentioning

confidence: 99%

Mitochondrial origin-binding protein UMSBP mediates DNA replication and segregation in trypanosomes

Milman

Motyka

Englund

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Kinetoplast DNA (kDNA) is the remarkable mitochondrial genome of trypanosomatids. Its major components are several thousands of topologically linked DNA minicircles, whose replication origins are bound by the universal minicircle sequence-binding protein (UMSBP). The cellular function of UMSBP has been studied in Trypanosoma brucei by using RNAi analysis. Silencing of the trypanosomal UMSBP genes resulted in remarkable effects on the trypanosome cell cycle. It significantly inhibited the initiation of minicircle replication, blocked nuclear DNA division, and impaired the segregation of the kDNA network and the flagellar basal body, resulting in growth arrest. These observations, revealing the function of UMSBP in kDNA replication initiation and segregation as well as in mitochondrial and nuclear division, imply a potential role for UMSBP in linking kDNA replication and segregation to the nuclear S-phase control during the trypanosome cell cycle.kDNA replication initiation ͉ kDNA segregation ͉ kinetoplast DNA ͉ trypanosomes cell cycle control ͉ universal minicircle sequence-binding protein K inetoplast DNA (kDNA) is a unique extrachromosomal DNA, found in the single mitochondrion of trypanosomatids. It consists, in the different species, of a few dozen maxicircles (20-40 kb each) and a few thousand minicircles (0.5-10 kb each), that are interlocked topologically into a DNA network (1, 2). Minicircles in most trypanosomatid species contain two short sequences that are associated with replication initiation: a dodecamer, designated the universal minicircle sequence (UMS), and a hexamer. These sequences have been mapped to the replication origins of the minicircle's light (L) and heavy (H) strands, respectively. kDNA replication occurs during S phase of the cell cycle, approximately in parallel with nuclear DNA replication (3). Minicircles are released from the network into the kinetoflagellar zone, located in the mitochondrial matrix, between the kDNA network and the flagellar basal body. Each minicircle replicates as an individual replicon, forming gapped and nicked progeny molecules. Minicircle replication intermediates then migrate onto two antipodal sites, flanking the kDNA disk, in which primer-removal, repair of the gaps between Okazaki fragments and reattachment of the progeny minicircles to the network occurs. The final gap-filling and sealing of the topologically linked minicircles take place before the network division (recently reviewed in refs. 1 and 2).We have previously reported the presence in Crithidia fasciculata of a UMS-binding protein (UMSBP). The protein, which contains five tandemly arranged CCHC-type zinc-finger motifs, has been purified from cell extracts, and its encoding gene and genomic locus were cloned and analyzed (4-7). Genes encoding orthologous proteins have been identified in other trypanosomatid species [ref. 8 and supporting information (SI) Table 1]. UMSBP binds specifically the two conserved sequences, located at the minicircle replication origins: the UMS dodecamer and an H-st...

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“…Seven of the 10 cyclins resemble the budding yeast PHO80 cyclin, which is involved in signaling phosphate starvation, whereas the other three cyclins are homologous to the mitotic B-type cyclins found in animals and plants (41). RNA interference (RNAi)-mediated silencing of some of these cyclins demonstrated that CYC2 plays a major role in promoting G 1 /S transition (43,70) and that CYC4 likely plays a minor role in G 1 /S transition (70). RNAi of CRK1 or CRK2 also arrests the trypanosome cells at the G 1 phase, suggesting their roles in the G 1 /S transition (109,110).…”

Section: Regulation Of the G 1 /S Transitionmentioning

confidence: 99%

Regulation of the Cell Division Cycle in Trypanosoma brucei

Li¹

2012

Eukaryot Cell

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The cell division cycle is tightly regulated by the activation and inactivation of a series of proteins that control the replication and segregation of organelles to the daughter cells. During the past decade, we have witnessed significant advances in our understanding of the cell cycle in Trypanosoma brucei and how the cycle is regulated by various regulatory proteins. However, many other regulators, especially those unique to trypanosomes, remain to be identified, and we are just beginning to delineate the signaling pathways that drive the transitions through different cell cycle stages, such as the G 1 /S transition, G 2 /M transition, and mitosis-cytokinesis transition. Trypanosomes appear to employ both evolutionarily conserved and trypanosome-specific molecules to regulate the various stages of its cell cycle, including DNA replication initiation, spindle assembly, chromosome segregation, and cytokinesis initiation and completion. Strikingly, trypanosomes lack some crucial regulators that are well conserved across evolution, such as Cdc6 and Cdt1, which are involved in DNA replication licensing, the spindle motor kinesin-5, which is required for spindle assembly, the central spindlin complex, which has been implicated in cytokinesis initiation, and the actomyosin contractile ring, which is located at the cleavage furrow. Conversely, trypanosomes possess certain regulators, such as cyclins, cyclin-dependent kinases, and mitotic centromere-associated kinesins, that are greatly expanded and likely play diverse cellular functions. Overall, trypanosomes apparently have integrated unique regulators into the evolutionarily conserved pathways to compensate for the absence of those conserved molecules and, additionally, have evolved certain cell cycle regulatory pathways that are either different from its human host or distinct between its own life cycle forms.

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The Trypanosoma brucei Cyclin, CYC2, Is Required for Cell Cycle Progression through G1 Phase and for Maintenance of Procyclic Form Cell Morphology

Cited by 57 publications

References 39 publications

Trypanosome Telomeres Are Protected by a Homologue of Mammalian TRF2

Trypanosome Telomeres Are Protected by a Homologue of Mammalian TRF2

Mitochondrial origin-binding protein UMSBP mediates DNA replication and segregation in trypanosomes

Regulation of the Cell Division Cycle in Trypanosoma brucei

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