Mid2p stabilizes septin rings during cytokinesis in fission yeast

Septins are filament-forming proteins with a conserved role in cytokinesis. In the fission yeast Schizosaccharomyces pombe, septin rings appear to be involved primarily in cell-cell separation, a late stage in cytokinesis. Here, we identified a protein Mid2p on the basis of its sequence similarity to S. pombe Mid1p, Saccharomyces cerevisiae Bud4p, and Candida albicans Int1p. Like septin mutants, mid2delta mutants had delays in cell-cell separation. mid2delta mutants were defective in septin organization but not contractile ring closure or septum formation. In wild-type cells, septins assembled first during mitosis in a single ring and during septation developed into double rings that did not contract. In mid2delta cells, septins initially assembled in a single ring but during septation appeared in the cleavage furrow, forming a washer or disc structure. FRAP studies showed that septins are stable in wild-type cells but exchange 30-fold more rapidly in mid2delta cells. Mid2p colocalized with septins and required septins for its localization. A COOH-terminal pleckstrin homology domain of Mid2p was required for its localization and function. No genetic interactions were found between mid2 and the related gene mid1. Thus, these studies identify a new factor responsible for the proper stability and function of septins during cytokinesis.     

Casein kinase II is required for the spindle assembly checkpoint by regulating Mad2p in fission yeast

The spindle checkpoint is a surveillance mechanism that ensures the fidelity of chromosome segregation in mitosis. Here we show that fission yeast casein kinase II (CK2) is required for this checkpoint function. In the CK2 mutants mitosis occurs in the presence of a spindle defect, and the spindle checkpoint protein Mad2p fails to localize to unattached kinetochores. The CK2 mutants are sensitive to the microtubule depolymerising drug thiabendazole, which is counteracted by ectopic expression of mad2⁺. The level of Mad2p is low in the CK2 mutants. These results suggest that CK2 has a role in the spindle checkpoint by regulating Mad2p.     

Parameters that specify the timing of cytokinesis.

One model for the timing of cytokinesis is based on findings that p34(cdc2) can phosphorylate myosin regulatory light chain (LC20) on inhibitory sites (serines 1 and 2) in vitro (Satterwhite, L.L., M.H. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 118:595-605), and this inhibition is proposed to delay cytokinesis until p34(cdc2) activity falls at anaphase. We have characterized previously several kinase activities associated with the isolated cortical cytoskeleton of dividing sea urchin embryos (Walker, G.R., C.B. Shuster, and D.R. Burgess. 1997. J. Cell Sci. 110:1373-1386). Among these kinases and substrates is p34(cdc2) and LC20. In comparison with whole cell activity, cortical H1 kinase activity is delayed, with maximum levels in cortices prepared from late anaphase/telophase embryos. To determine whether cortical-associated p34(cdc2) influences cortical myosin II activity during cytokinesis, we labeled eggs in vivo with [(32)P]orthophosphate, prepared cortices, and mapped LC20 phosphorylation through the first cell division. We found no evidence of serine 1,2 phosphorylation at any time during mitosis on LC20 from cortically associated myosin. Instead, we observed a sharp rise in serine 19 phosphorylation during anaphase and telophase, consistent with an activating phosphorylation by myosin light chain kinase. However, serine 1,2 phosphorylation was detected on light chains from detergent-soluble myosin II. Furthermore, cells arrested in mitosis by microinjection of nondegradable cyclin B could be induced to form cleavage furrows if the spindle poles were physically placed in close proximity to the cortex. These results suggest that factors independent of myosin II inactivation, such as the delivery of the cleavage stimulus to the cortex, determine the timing of cytokinesis.     

Phosphorylation of the amino-terminus of the AGC kinase Gad8 prevents its interaction with TORC2

Cell proliferation, metabolism, migration and survival are coordinated through the tight control of two target of rapamycin (TOR) kinase complexes: TORC1 and TORC2. Here, we show that a novel phosphorylation of fission yeast Gad8 (AGC kinase) on the evolutionarily conserved threonine 6 (Thr6) prevents the physical association between Gad8 and TORC2. Accordingly, this block to protein interactions by Gad8 Thr6 phosphorylation decreases TORC2-controlled activation of Gad8. Likewise, phosphorylation of Gad8 Thr6, possibly by PKC, prevents the association of Gad8 with TORC2 thereby increasing TORC2 activity, because it reduces Gad8-mediated feedback inhibition of TORC2. Consistently, the introduction of a Gad8 T6D mutant, that mimics phosphorylation, increased TORC2 activity. Increased PKC^Pck2 expression prevented Gad8–TORC2 binding and so reduced the TORC2-mediated phosphorylation of Gad8 serine 546 that activates Gad8. Interestingly, independent of the Ser546 phosphorylation status, Gad8 Thr6 phosphorylation is important for remodelling the actin cytoskeleton and survival upon potassium ion and heat stresses. In contrast, Ser546 phosphorylation is required for the control of G1 arrest, mating, cell length at division and vascular size. Finally, these findings reveal a novel mode of TORC2 activation that is essential for cell survival following stress.     

Two-hybrid search for proteins that interact with Sad1 and Kms1, two membrane-bound components of the spindle pole body in fission yeast

In interphase cells of fission yeast, the spindle pole body (SPB) is thought to be connected with chromosomal centromeres by an as yet unknown mechanism that spans the nuclear membrane. To elucidate this mechanism, we performed two-hybrid screens for proteins that interact with Kms1 and Sad1, which are constitutive membrane-bound components of the SPB that interact with each other. Seven and 26 genes were identified whose products potentially interact with Kms1 and Sad1, respectively. With the exception of Dlc1 (a homolog of the 14-kDa dynein light chain), all of the Kms1 interactors also interacted with Sad1. Among the genes identified were the previously known genes rhp9 ⁺/ crb2 ⁺, cut6 ⁺, ags1 ⁺/ mok1 ⁺, gst3 ⁺, kms2 ⁺, and sid4 ⁺. The products of kms2 ⁺ and sid4 ⁺ localize to the SPB. The novel genes were characterized by constructing disruption mutations and by localization of the gene products. Two of them, putative homologues of budding yeast UFE1 (which encodes a t-SNARE) and SFH1 (an essential component of a chromatin-remodeling complex), were essential for viability. Two further genes, which were only conditionally essential, genetically interact with sad1 ⁺ . One of these was named sif1 ⁺ (for Sad1-interacting factor) and is required for proper septum formation at high temperature. Cells in which this gene was overexpressed displayed a wee -like phenotype. The product of the other gene, apm1 ⁺, is very similar to the medium chain of an adaptor protein complex in clathrin-coated vesicles. Apm1 appears to be required for SPB separation and spindle formation, and tended to accumulate at the SPB when it was overproduced. It was functionally distinct from its homologues Apm2 and Apm4. Other novel genes identified in this study included one for a nucleoporin and genes encoding novel membrane-bound proteins that were genetically related to Sad1. We found that none of the newly identified genes tested were necessary for centromere/telomere clustering.Communicated by C. P. HollenbergThe first three authors contributed equally to this work     

An anillin homologue,Mid2p,acts during fission yeast cytokinesis to organize the septin ring and promote cell separation

Anillin is a conserved protein required for cell division (Field, C.M., and B.M. Alberts. 1995. J. Cell Biol. 131:165-178; Oegema, K., M.S. Savoian, T.J. Mitchison, and C.M. Field. 2000. J. Cell Biol. 150:539-552). One fission yeast homologue of anillin, Mid1p, is necessary for the proper placement of the division site within the cell (Chang, F., A. Woollard, and P. Nurse. 1996. J. Cell Sci. 109(Pt 1):131-142; Sohrmann, M., C. Fankhauser, C. Brodbeck, and V. Simanis. 1996. Genes Dev. 10:2707-2719). Here, we identify and characterize a second fission yeast anillin homologue, Mid2p, which is not orthologous with Mid1p. Mid2p localizes as a single ring in the middle of the cell after anaphase in a septin- and actin-dependent manner and splits into two rings during septation. Mid2p colocalizes with septins, and mid2 Delta cells display disorganized, diffuse septin rings and a cell separation defect similar to septin deletion strains. mid2 gene expression and protein levels fluctuate during the cell cycle in a sep1- and Skp1/Cdc53/F-box (SCF)-dependent manner, respectively, implying that Mid2p activity must be carefully regulated. Overproduction of Mid2p depolarizes cell growth and affects the organization of both the septin and actin cytoskeletons. In the presence of a nondegradable Mid2p fragment, the septin ring is stabilized and cell cycle progression is delayed. These results suggest that Mid2p influences septin ring organization at the site of cell division and its turnover might normally be required to permit septin ring disassembly.     

The yeast protein kinase Mps1p is required for assembly of the integral spindle pole body component Spc42p

Saccharomyces cerevisiae MPS1 encodes an essential protein kinase that has roles in spindle pole body (SPB) duplication and the spindle checkpoint. Previously characterized MPS1 mutants fail in both functions, leading to aberrant DNA segregation with lethal consequences. Here, we report the identification of a unique conditional allele, mps1-8, that is defective in SPB duplication but not the spindle checkpoint. The mutations in mps1-8 are in the noncatalytic region of MPS1, and analysis of the mutant protein indicates that Mps1-8p has wild-type kinase activity in vitro. A screen for dosage suppressors of the mps1-8 conditional growth phenotype identified the gene encoding the integral SPB component SPC42. Additional analysis revealed that mps1-8 exhibits synthetic growth defects when combined with certain mutant alleles of SPC42. An epitope-tagged version of Mps1p (Mps1p-myc) localizes to SPBs and kinetochores by immunofluorescence microscopy and immuno-EM analysis. This is consistent with the physical interaction we detect between Mps1p and Spc42p by coimmunoprecipitation. Spc42p is a substrate for Mps1p phosphorylation in vitro, and Spc42p phosphorylation is dependent on Mps1p in vivo. Finally, Spc42p assembly is abnormal in a mps1-1 mutant strain. We conclude that Mps1p regulates assembly of the integral SPB component Spc42p during SPB duplication.     

MAD3 encodes a novel component of the spindle checkpoint which interacts with Bub3p, Cdc20p, and Mad2p

We show that MAD3 encodes a novel 58-kD nuclear protein which is not essential for viability, but is an integral component of the spindle checkpoint in budding yeast. Sequence analysis reveals two regions of Mad3p that are 46 and 47% identical to sequences in the NH(2)-terminal region of the budding yeast Bub1 protein kinase. Bub1p is known to bind Bub3p (Roberts et al. 1994) and we use two-hybrid assays and coimmunoprecipitation experiments to show that Mad3p can also bind to Bub3p. In addition, we find that Mad3p interacts with Mad2p and the cell cycle regulator Cdc20p. We show that the two regions of homology between Mad3p and Bub1p are crucial for these interactions and identify loss of function mutations within each domain of Mad3p. We discuss roles for Mad3p and its interactions with other spindle checkpoint proteins and with Cdc20p, the target of the checkpoint.     

The cytokinesis gene KEULE encodes a Sec1 protein that binds the syntaxin KNOLLE

KEULE is required for cytokinesis in Arabidopsis thaliana. We have positionally cloned the KEULE gene and shown that it encodes a Sec1 protein. KEULE is expressed throughout the plant, yet appears enriched in dividing tissues. Cytokinesis-defective mutant sectors were observed in all somatic tissues upon transformation of wild-type plants with a KEULE-green fluorescent protein gene fusion, suggesting that KEULE is required not only during embryogenesis, but at all stages of the plant's life cycle. KEULE is characteristic of a Sec1 protein in that it appears to exist in two forms: soluble or peripherally associated with membranes. More importantly, KEULE binds the cytokinesis-specific syntaxin KNOLLE. Sec1 proteins are key regulators of vesicle trafficking, capable of integrating a large number of intra- and/or intercellular signals. As a cytokinesis-related Sec1 protein, KEULE appears to represent a novel link between cell cycle progression and the membrane fusion apparatus.     

Expression of a dominant negative allele of cdc2 prevents activation of the endogenous p34cdc2 kinase

Summary The cdc2 gene of the fission yeast Schizosaccharomyces pombe encodes a 34 kDa phosphoprotein with serine/threonine protein kinase activity that acts as the key component in regulation of the eukaryotic cell cycle. We used a repressible promoter fused to the cdc2 cDNA to isolate conditionally dominant negative mutants of cdc2. One of these mutants, DL5, is described in this paper. Overexpression of the mutant protein in a wild-type cdc2 background is lethal and confers cell cycle arrest with a typical cdc phenotype. Sequencing of the mutant cdc2 gene revealed a single amino acid substitution in a region highly conserved in cdc2-like proteins. The mutant protein exhibits no protein kinase activity, but is able to bind a component(s) required for an active protein kinase complex and thereby prevents binding of this component(s) to the co-existing wild-type cdc2 protein. We also demonstrate that S. pombe p34^cdc2 contains no phosphoserine.     

Mps3p is a novel component of the yeast spindle pole body that interacts with the yeast centrin homologue Cdc31p

Accurate duplication of the Saccharomyces cerevisiae spindle pole body (SPB) is required for formation of a bipolar mitotic spindle. We identified mutants in SPB assembly by screening a temperature-sensitive collection of yeast for defects in SPB incorporation of a fluorescently marked integral SPB component, Spc42p. One SPB assembly mutant contained a mutation in a previously uncharacterized open reading frame that we call MPS3 (for monopolar spindle). mps3-1 mutants arrest in mitosis with monopolar spindles at the nonpermissive temperature, suggesting a defect in SPB duplication. Execution point experiments revealed that MPS3 function is required for the first step of SPB duplication in G1. Like cells containing mutations in two other genes required for this step of SPB duplication (CDC31 and KAR1), mps3-1 mutants arrest with a single unduplicated SPB that lacks an associated half-bridge. MPS3 encodes an essential integral membrane protein that localizes to the SPB half-bridge. Genetic interactions between MPS3 and CDC31 and binding of Cdc31p to Mps3p in vitro, as well as the fact that Cdc31p localization to the SPB is partially dependent on Mps3p function, suggest that one function for Mps3p during SPB duplication is to recruit Cdc31p, the yeast centrin homologue, to the half-bridge.     

Role of Polo Kinase and Mid1p in Determining the Site of Cell Division in Fission Yeast

The fission yeast Schizosaccharomyces pombe divides symmetrically using a medial F-actin– based contractile ring to produce equal-sized daughter cells. Mutants defective in two previously described genes, mid1 and pom1, frequently divide asymmetrically. Here we present the identification of three new temperature-sensitive mutants defective in localization of the division plane. All three mutants have mutations in the polo kinase gene, plo1, and show defects very similar to those of mid1 mutants in both the placement and organization of the medial ring. In both cases, ring formation is frequently initiated near the cell poles, indicating that Mid1p and Plo1p function in recruiting medial ring components to the cell center. It has been reported previously that during mitosis Mid1p becomes hyperphosphorylated and relocates from the nucleus to a medial ring. Here we show that Mid1p first forms a diffuse cortical band during spindle formation and then coalesces into a ring before anaphase. Plo1p is required for Mid1p to exit the nucleus and form a ring, and Pom1p is required for proper placement of the Mid1p ring. Upon overexpression of Plo1p, Mid1p exits the nucleus prematurely and displays a reduced mobility on gels similar to that of the hyperphosphorylated form observed previously in mitotic cells. Genetic and two-hybrid analyses suggest that Plo1p and Mid1p act in a common pathway distinct from that involving Pom1p. Plo1p localizes to the spindle pole bodies and spindles of mitotic cells and also to the medial ring at the time of its formation. Taken together, the data indicate that Plo1p plays a role in the positioning of division sites by regulating Mid1p. Given its previously known functions in mitosis and the timing of cytokinesis, Plo1p is thus implicated as a key molecule in the spatial and temporal coordination of cytokinesis with mitosis.     

Feedback Regulation of SIN by Etd1 and Rho1 in Fission Yeast

In fission yeast, the septation initiation network (SIN) is thought to promote cytokinesis by downstream activation of Rho1, a conserved GTPase that controls cell growth and division. Here we show that Etd1 and PP2A-Pab1, antagonistic regulators of SIN, are Rho1 regulators. Our genetic and biochemical studies indicate that a C-terminal region of Etd1 may activate Rho1 by directly binding it, whereas an N-terminal domain confers its ability to localize at the growing tips and the division site where Rho1 functions. In opposition to Etd1, our results indicate that PP2A-Pab1 inhibits Rho1. The SIN cascade is upstream-regulated by the Spg1 GTPase. In the absence of Etd1, activity of Spg1 drops down prematurely, thereby inactivating SIN. Interestingly, we find that ectopic activation of Rho1 restores Spg1 activity in Etd1-depleted cells. By using a cytokinesis block strategy, we show that Rho1 is essential to feedback-activate Spg1 during actomyosin ring constriction. Therefore, activation of Spg1 by Rho1, which in turn is regulated by Etd1, uncovers a novel feedback loop mechanism that ensures SIN activity while cytokinesis is progressing.     

The PCH family protein,Cdc15p,recruits two F-actin nucleation pathways to coordinate cytokinetic actin ring formation in Schizosaccharomyces pombe

Cytokinetic actin ring (CAR) formation in Schizosaccharomyces pombe requires two independent actin nucleation pathways, one dependent on the Arp2/3 complex and another involving the formin Cdc12p. Here we investigate the role of the S. pombe Cdc15 homology family protein, Cdc15p, in CAR assembly and find that it interacts with proteins from both of these nucleation pathways. Cdc15p binds directly to the Arp2/3 complex activator Myo1p, which likely explains why actin patches and the Arp2/3 complex fail to be medially recruited during mitosis in cdc15 mutants. Cdc15p also binds directly to Cdc12p. Cdc15p and Cdc12p not only display mutual dependence for CAR localization, but also exist together in a ring-nucleating structure before CAR formation. The disruption of these interactions in cdc15 null cells is likely to be the reason for their complete lack of CARs. We propose a model in which Cdc15p plays a critical role in recruiting and coordinating the pathways essential for the assembly of medially located F-actin filaments and construction of the CAR.     

Sporadic Distribution of Prion-Forming Ability of Sup35p from Yeasts and Fungi

Sup35p of Saccharomyces cerevisiae can form the [PSI+] prion, an infectious amyloid in which the protein is largely inactive. The part of Sup35p that forms the amyloid is the region normally involved in control of mRNA turnover. The formation of [PSI+] by Sup35p’s from other yeasts has been interpreted to imply that the prion-forming ability of Sup35p is conserved in evolution, and thus of survival/fitness/evolutionary value to these organisms. We surveyed a larger number of yeast and fungal species by the same criteria as used previously and find that the Sup35p from many species cannot form prions. [PSI+] could be formed by the Sup35p from Candida albicans, Candida maltosa, Debaromyces hansenii, and Kluyveromyces lactis, but orders of magnitude less often than the S. cerevisiae Sup35p converts to the prion form. The Sup35s from Schizosaccharomyces pombe and Ashbya gossypii clearly do not form [PSI+]. We were also unable to detect [PSI+] formation by the Sup35ps from Aspergillus nidulans, Aspergillus fumigatus, Magnaporthe grisea, Ustilago maydis, or Cryptococcus neoformans. Each of two C. albicans SUP35 alleles can form [PSI+], but transmission from one to the other is partially blocked. These results suggest that the prion-forming ability of Sup35p is not a conserved trait, but is an occasional deleterious side effect of a protein domain conserved for another function.     

Drosophila APC2 is a cytoskeletally-associated protein that regulates wingless signaling in the embryonic epidermis.

The tumor suppressor adenomatous polyposis coli (APC) negatively regulates Wingless (Wg)/Wnt signal transduction by helping target the Wnt effector beta-catenin or its Drosophila homologue Armadillo (Arm) for destruction. In cultured mammalian cells, APC localizes to the cell cortex near the ends of microtubules. Drosophila APC (dAPC) negatively regulates Arm signaling, but only in a limited set of tissues. We describe a second fly APC, dAPC2, which binds Arm and is expressed in a broad spectrum of tissues. dAPC2's subcellular localization revealed colocalization with actin in many but not all cellular contexts, and also suggested a possible interaction with astral microtubules. For example, dAPC2 has a striking asymmetric distribution in neuroblasts, and dAPC2 colocalizes with assembling actin filaments at the base of developing larval denticles. We identified a dAPC2 mutation, revealing that dAPC2 is a negative regulator of Wg signaling in the embryonic epidermis. This allele acts genetically downstream of wg, and upstream of arm, dTCF, and, surprisingly, dishevelled. We discuss the implications of our results for Wg signaling, and suggest a role for dAPC2 as a mediator of Wg effects on the cytoskeleton. We also speculate on more general roles that APCs may play in cytoskeletal dynamics.     

Single-Nucleotide-Specific Targeting of the Tf1 Retrotransposon Promoted by the DNA-Binding Protein Sap1 of Schizosaccharomyces pombe

Transposable elements (TEs) constitute a substantial fraction of the eukaryotic genome and, as a result, have a complex relationship with their host that is both adversarial and dependent. To minimize damage to cellular genes, TEs possess mechanisms that target integration to sequences of low importance. However, the retrotransposon Tf1 of Schizosaccharomyces pombe integrates with a surprising bias for promoter sequences of stress-response genes. The clustering of integration in specific promoters suggests that Tf1 possesses a targeting mechanism that is important for evolutionary adaptation to changes in environment. We report here that Sap1, an essential DNA-binding protein, plays an important role in Tf1 integration. A mutation in Sap1 resulted in a 10-fold drop in Tf1 transposition, and measures of transposon intermediates support the argument that the defect occurred in the process of integration. Published ChIP-Seq data on Sap1 binding combined with high-density maps of Tf1 integration that measure independent insertions at single-nucleotide positions show that 73.4% of all integration occurs at genomic sequences bound by Sap1. This represents high selectivity because Sap1 binds just 6.8% of the genome. A genome-wide analysis of promoter sequences revealed that Sap1 binding and amounts of integration correlate strongly. More important, an alignment of the DNA-binding motif of Sap1 revealed integration clustered on both sides of the motif and showed high levels specifically at positions +19 and −9. These data indicate that Sap1 contributes to the efficiency and position of Tf1 integration.     

Fission Yeast Bub1 Is a Mitotic Centromere Protein Essential for the Spindle Checkpoint and the Preservation of Correct Ploidy through Mitosis

The spindle checkpoint ensures proper chromosome segregation by delaying anaphase until all chromosomes are correctly attached to the mitotic spindle. We investigated the role of the fission yeast bub1 gene in spindle checkpoint function and in unperturbed mitoses. We find that bub1 ⁺ is essential for the fission yeast spindle checkpoint response to spindle damage and to defects in centromere function. Activation of the checkpoint results in the recruitment of Bub1 to centromeres and a delay in the completion of mitosis. We show that Bub1 also has a crucial role in normal, unperturbed mitoses. Loss of bub1 function causes chromosomes to lag on the anaphase spindle and an increased frequency of chromosome loss. Such genomic instability is even more dramatic in Δbub1 diploids, leading to massive chromosome missegregation events and loss of the diploid state, demonstrating that bub1 ⁺ function is essential to maintain correct ploidy through mitosis. As in larger eukaryotes, Bub1 is recruited to kinetochores during the early stages of mitosis. However, unlike its vertebrate counterpart, a pool of Bub1 remains centromere-associated at metaphase and even until telophase. We discuss the possibility of a role for the Bub1 kinase after the metaphase–anaphase transition.     

Isolation of a novel rmn1 gene genetically linked to spnab2 with respect to mRNA export in fission yeast

In fission yeast, Schizosaccharomyces pombe, the spnab2 gene encodes an ortholog of the budding yeast nuclear abundant poly(A)⁺ RNA-binding protein 2 (Nab2) that is an essential protein required for both mRNA biogenesis and nuclear export of mRNA to the cytoplasm. We have previously isolated three mutants (SLnab1–3) that showed synthetic lethality under the repressed condition of spnab2 expression. In this study, we isolated a novel rmn1 gene as a multicopy suppressor that complemented the defects in growth and mRNA export of SLnab1 mutant cells. The rmn1 gene contained three introns and encoded a 589 amino-acid protein with the RNA recognition motif (RRM) in the central region. The Δrmn1 null mutant was viable but showed a s light mRNA export defect. However, its over-expression caused a deleterious effect on growth accompanied by intense accumulation of poly(A)⁺ RNA in the nucleus. The combination of Δrmn1 with Δspnab2 or Δspmex67 also inhibited growth. In addition, Rmn1p was associated with Rae1p in vivo. These results suggest that rmn1 is a novel gene that is functionally linked to spnab2.     

Tay1 protein, a novel telomere binding factor from Yarrowia lipolytica

Inspection of the complete genome of the yeast Yarrowia lipolytica for the presence of genes encoding homologues of known telomere-binding proteins surprisingly revealed no counterparts of typical yeast Myb domain-containing telomeric factors including Rap1 or Taz1. Instead, we identified a gene, YALIOD10923g, encoding a protein containing two Myb domains, exhibiting a high degree of similarity to the Myb domain of human telomeric proteins TRF1 and TRF2 and homologous to an essential fission yeast protein Mug152 whose expression is elevated during meiosis. The protein, which we named Tay1p (telomere-associated in Yarrowia lipolytica 1), was purified for biochemical studies. Using a model Y. lipolytica telomere, we demonstrate that the protein preferentially binds to Y. lipolytica telomeric tracts. Tay1p binds along the telomeric tract as dimers and larger oligomers, and it is able to remodel the telomeric DNA into both looped structures and synaptic complexes of two model telomere DNAs. The ability of Tay1p to induce dimerization of telomeres in vitro goes in line with its oligomeric nature, where each oligomer can employ several Myb domains to form intermolecular telomere clusters. We also provide experimental evidence that Tay1p may be associated with Y. lipolytica telomeres in vivo. Together with its homologues from Schizosaccharomyces pombe and several basidiomycetous fungi (Sánchez-Alonso, P., and Guzman, P. (2008) Fungal Genet. Biol. 45, S54-S62), Tay1p constitutes a novel family of putative telomeric factors whose analysis may be instrumental in understanding the function and evolution of double-stranded DNA telomeric proteins.