The fission yeast meiotic regulator Mei2p undergoes nucleocytoplasmic shuttling.

Schizosaccharomyces pombe Mei2p is an RNA-binding protein that switches the cell cycle from mitotic to meiotic. Mei2p forms a unique dot in the nucleus prior to meiosis I, aided by a non-coding RNA molecule termed meiRNA. Here we show that Mei2p intrinsically undergoes nucleocytoplasmic shuttling. Artificial acceleration of nuclear migration of Mei2p advances nuclear dot formation, but meiRNA does not appear to promote the dot formation by modulating the migration rate of Mei2p into the nucleus. Rather, this RNA is likely to facilitate the assembly of Mei2p into a dot structure and trap the protein as such in the nucleus.     

14-3-3 protein interferes with the binding of RNA to the phosphorylated form of fission yeast meiotic regulator Mei2p

The switch from mitosis to meiosis is controlled by the Pat1(Ran1) kinase-Mei2p system in Schizosaccharomyces pombe. Mei2p promotes both premeiotic DNA synthesis and meiosis I, and its RNA binding ability is essential for these two processes. Mei2p forms a dot structure in the nucleus prior to meiosis I, aided by a specific RNA species named "meiRNA". Pat1 kinase phosphorylates Mei2p on two positions and downregulates its activity. Pat1 kinase undergoes inactivation under meiotic conditions, as a result of the production of a tethering pseudosubstrate Mei3p, and accumulation of the unphosphorylated form of Mei2p commits cells to meiosis. However, the mechanism of how phosphorylation of Mei2p suppresses its activity to induce meiosis remains largely unknown. Here we show that S. pombe Rad24p, a 14-3-3 protein, functions as a negative factor for meiosis by antagonizing the function of meiRNA to promote the formation of a nuclear Mei2p dot. Rad24p binds preferentially to Mei2p phosphorylated by Pat1 kinase. It inhibits association of meiRNA to the phosphorylated form of Mei2p but not to the unphosphorylated form in vitro. We speculate that Rad24p, bound tightly to the residues phosphorylated by Pat1 kinase, may mask the RNA recognition motifs on Mei2p. This model will explain, at least partly, why phosphorylation by Pat1 kinase inhibits the meiosis-inducing activity of Mei2p.     

An SMC-domain protein in fission yeast links telomeres to the meiotic centrosome

Abnormal centrosomal structures similar to those occurring in human cancers are induced in fission yeast by overexpression of the pericentrin homolog Pcp1p. Analysis of abnormal Pcp1p-containing structures with quantitative mass spectrometry and isotope-coded affinity tags identified a coiled-coil, structural maintenance of chromosomes (SMC) domain protein. This protein, termed Ccq1p (coiled-coil protein quantitatively enriched), localizes with Taz1p to telomeres in normal vegetative cells. Fluorescence resonance energy transfer (FRET) measurements indicate that Ccq1p also interacts with centrosomal Pcp1p in mating pheromone-stimulated cells containing centrosomally clustered telomeres. We provide evidence that the Ccq1p-Pcp1p interaction, while essential for meiosis, is deleterious when forced to occur during vegetative growth. Cells lacking one ccq1 allele exhibit a loss-of-function phenotype including abnormally long cell length, chromosome segregation failure, telomeric shortening, and defective telomeric clustering during meiotic prophase. Our data indicate a mechanism underlying meiotic chromosomal bouquet formation and suggest a recruitment model for supernumerary centrosome toxicity.     

Genetic analysis of human p34CDC2 function in fission yeast

The p34^cdc2 protein kinase plays a key role in the control of the mitotic cell cycle of fission yeast, being required for both entry into S-phase and for entry into mitosis in the mitotic cell cycle, as well as for the initiation of the second meiotic nuclear division. In recent years, structural and functional homologues of p34^cdc2, as well as several of the proteins that interact with and regulate p34^cdc2 function in fission yeast, have been identified in a wide range of higher eukaryotic cell types, suggesting that the control mechanisms uncovered in this simple eukaryote are likely to be well conserved across evolution. Here we describe the construction and characterisation of a fission yeast strain in which the endogenous p34^cdc2 protein is entirely absent and is replaced by its human functional homologue p34^CDC2, We have used this strain to analyse aspects of the function of the human p34^CDC2 protein genetically. We show that the function of the human p34^CDC2 protein in fission yeast cells is dependent upon the action of the protein tyrosine phosphatase p80^cdc25 that it responds to altered levels of both the mitotic inhibitor p107²³³¹ and the p34^cdc2-binding protein p13^suc1, and is lethal in combination with the mutant B-type cyclin p56^cdc13-117. In addition, we demonstrate that the human p34^CDC2 protein is proficient for fission yeast meiosis, and examine the behaviour of two mutant p34^CDC2 proteins in fission yeast.     

Hsp90 protein in fission yeast Swo1p and UCS protein Rng3p facilitate myosin II assembly and function

The F-actin-based molecular motor myosin II is involved in a variety of cellular processes such as muscle contraction, cell motility, and cytokinesis. In recent years, a family of myosin II-specific cochaperones of the UCS family has been identified from work with yeasts, fungi, worms, and humans. Biochemical analyses have shown that a complex of Hsp90 and the Caenorhabditis elegans UCS domain protein UNC-45 prevent myosin head aggregation, thereby allowing it to assume a proper structure. Here we demonstrate that a temperature-sensitive mutant of the fission yeast Hsp90 (Swo1p), swo1-w1, is defective in actomyosin ring assembly at the restrictive temperature. Two alleles of swo1, swo1-w1 and swo1-26, showed synthetic lethality with a specific mutant allele of the fission yeast type II myosin head, myo2-E1, but not with two other mutant alleles of myo2 or with mutations affecting 14 other genes important for cytokinesis. swo1-w1 also showed a strong genetic interaction with rng3-65, a gene encoding a mutation in the fission yeast UCS domain protein Rng3p, which has previously been shown to be important for myosin II assembly. A similar deleterious effect was found when myo2-E1, swo1-w1, and rng3-65 were pharmacologically treated with geldanamycin to partially inhibit Hsp90 function. Interestingly, Swo1p-green fluorescent protein is detected at the improperly assembled actomyosin rings in myo2-E1 but not in a wild-type strain. Yeast two-hybrid and coimmunoprecipitation analyses verified interactions between Rng3p and the myosin head domain as well as interactions between Rng3p and Swo1p. Our analyses of Myo2p, Swo1p, and the UCS domain protein Rng3p establish that Swo1p and Rng3p collaborate in vivo to modulate myosin II function.     

The WD-repeat protein pfs2p bridges two essential factors within the yeast pre-mRNA 3'-end-processing complex

In the yeast Saccharomyces cerevisiae, pre-mRNA 3'-end processing requires six factors: cleavage factor IA (CF IA), cleavage factor IB (CF IB), cleavage factor II (CF II), polyadenylation factor I (PF I), poly(A) polymerase (Pap1p) and poly(A)-binding protein I (Pab1p). We report the characterization of Pfs2p, a WD-repeat protein previously identified in a multiprotein complex carrying PF I-Pap1p activity. The 3'-end-processing defects of pfs2 mutant strains and the results of immunodepletion and immunoinactivation experiments indicate an essential function for Pfs2p in cleavage and polyadenylation. With a one-step affinity purification method that exploits protein A-tagged Pfs2p, we showed that this protein is part of a CF II-PF I complex. Pull-down experiments with GST fusion proteins revealed direct interactions of Pfs2p with subunits of CF II-PF I and CF IA. These results show that Pfs2p plays an essential role in 3'-end formation by bridging different processing factors and thereby promoting the assembly of the processing complex.     

The meiosis-specific Sid2p-related protein Slk1p regulates forespore membrane assembly in fission yeast

Cytokinesis in all organisms involves the creation of membranous barriers that demarcate individual daughter cells. In fission yeast, a signaling module termed the septation initiation network (SIN) plays an essential role in the assembly of new membranes and cell wall during cytokinesis. In this study, we have characterized Slk1p, a protein-kinase related to the SIN component Sid2p. Slk1p is expressed specifically during meiosis and localizes to the spindle pole bodies (SPBs) during meiosis I and II in a SIN-dependent manner. Slk1p also localizes to the forespore membrane during sporulation. Cells lacking Slk1p display defects associated with sporulation, leading frequently to the formation of asci with smaller and/or fewer spores. The ability of slk1Δ cells to sporulate, albeit inefficiently, is fully abolished upon compromise of function of Sid2p, suggesting that Slk1p and Sid2p play overlapping roles in sporulation. Interestingly, increased expression of the syntaxin Psy1p rescues the sporulation defect of sid2-250 slk1Δ. Thus, it is likely that Slk1p and Sid2p play a role in forespore membrane assembly by facilitating recruitment of components of the secretory apparatus, such as Psy1p, to allow membrane expansion. These studies thereby provide a novel link between the SIN and vesicle trafficking during cytokinesis.     

The 14-3-3 protein rad24p modulates function of the cdc14p family phosphatase clp1p/flp1p in fission yeast

Schizosaccharomyces pombe cells divide through the use of an actomyosin-based contractile ring. In response to perturbation of the actomyosin ring, S. pombe cells delay in a "cytokinesis-competent" state characterized by continuous repair and maintenance of the actomyosin ring and a G2 delay. This checkpoint mechanism requires the function of the Cdc14p-family phosphatase Clp1p/Flp1p and the septation initiation network (SIN). In response to cytokinetic defects, Clp1p, normally nucleolar in interphase, is retained in the cytoplasm until completion of cell division in a SIN-dependent manner. Here, we show that a phosphorylated form of Clp1p binds the 14-3-3 protein Rad24p and is retained in the cytoplasm in a Rad24p-dependent manner in response to cytokinesis defects. This physical interaction depends on the function of the SIN component, Sid2p. In the absence of Rad24p, cells are unable to maintain SIN signaling and lose viability upon mild cytokinetic stress. The requirement of Rad24p in this checkpoint is bypassed by ectopic activation of the SIN. Furthermore, SIN-dependent nuclear exclusion of Clp1p is dependent on Rad24p function. We conclude that Rad24p-mediated cytoplasmic retention of Clp1p/Flp1p is important for cell viability upon stress to the division apparatus.     

Anillin-related protein Mid1p coordinates the assembly of the cytokinetic contractile ring in fission yeast

In fission yeast cells cortical nodes containing the protein Blt1p and several kinases appear early in G2, mature into cytokinetic nodes by adding anillin Mid1p, myosin-II, formin Cdc12p, and other proteins, and condense into a contractile ring by movements that depend on actin and myosin-II. Previous studies concluded that cells without Mid1p lack cytokinetic nodes and assemble rings unreliably from myosin-II strands but left open questions. Why do strands form outside the equatorial region? Why is ring assembly unreliable without Mid1p? We found in Δmid1 cells that Cdc12p accumulates in cytokinetic nodes scattered in the cortex and produces actin filaments that associate with myosin-II, Rng2p, and Cdc15p to form strands located between the nodes. Strands incorporate nodes, and in ∼67% of cells, strands slowly close into rings that constrict without the normal ∼25-min maturation period. Ring assembly is unreliable and slow without Mid1p because the scattered Cdc12p nodes generate strands spread widely beyond the equator, and growing strands depend on random encounters to merge with other strands into a ring. We conclude that orderly assembly of the contractile ring in wild-type cells depends on Mid1p to recruit myosin-II, Rng2p, and Cdc15p to nodes and to place cytokinetic nodes around the cell equator.     

Dissociation of meiotic and mitotic roles of the fission yeast cdc2 gene

Summary The fission yeastcdc2 gene is pleiotropic, functioning both in the cell division cycle and in meiosis. Here we show thatcdc2 is allelic totws1, a previously isolated meiotic gene. Dissociation of meiotic and mitotic roles of the gene is also demonstrated by finding mutant alleles specifically altered in only one of the two processes.     

Genetic analysis of human p34CDC2 function in fission yeast

The p34^cdc2 protein kinase plays a key role in the control of the mitotic cell cycle of fission yeast, being required for both entry into S-phase and for entry into mitosis in the mitotic cell cycle, as well as for the initiation of the second meiotic nuclear division. In recent years, structural and functional homologues of p34^cdc2, as well as several of the proteins that interact with and regulate p34^cdc2 function in fission yeast, have been identified in a wide range of higher eukaryotic cell types, suggesting that the control mechanisms uncovered in this simple eukaryote are likely to be well conserved across evolution. Here we describe the construction and characterisation of a fission yeast strain in which the endogenous p34^cdc2 protein is entirely absent and is replaced by its human functional homologue p34^CDC2, We have used this strain to analyse aspects of the function of the human p34^CDC2 protein genetically. We show that the function of the human p34^CDC2 protein in fission yeast cells is dependent upon the action of the protein tyrosine phosphatase p80^cdc25 that it responds to altered levels of both the mitotic inhibitor p107²³³¹ and the p34^cdc2-binding protein p13^suc1, and is lethal in combination with the mutant B-type cyclin p56^cdc13-117. In addition, we demonstrate that the human p34^CDC2 protein is proficient for fission yeast meiosis, and examine the behaviour of two mutant p34^CDC2 proteins in fission yeast.     

Genetic requirements and meiotic function of phosphorylation of the yeast axial element protein Red1

The synaptonemal complex (SC) is a meiosis-specific tripartite structure that forms between two homologous chromosomes; it consists of a central region and two parallel lateral elements. Lateral elements also are called axial elements prior to synapsis. In Saccharomyces cerevisiae, Red1, Hop1, and Mek1 are structural components of axial/lateral elements. The red1/mek1/hop1 mutants all exhibit reduced levels of interhomolog recombination and produce no viable spores. Red1 is a phosphoprotein. Several earlier reports proposed that phosphorylated Red1 plays important roles in meiosis, including in signaling meiotic DNA damage or in preventing exit from the pachytene chromosomes. We report here that the phosphorylation of Red1 is carried out in CDC28-dependent and CDC28-independent manners. In contrast to previous results, we found Red1 phosphorylation to be independent of meiotic DNA recombination, the Mec1/Tel1 DNA damage checkpoint kinases, and the Mek1 kinase. To functionally validate the phosphorylation of Red1, we mapped the phosphorylation sites on this protein. A red1(14A) mutant showing no detectable Red1 phosphorylation did not exhibit decreased sporulation efficiency, defects in viable spore production, or defects in meiotic DNA damage checkpoints. Thus, our results suggest that the phosphorylation of Red1 is not essential for its functions in meiosis.     

ATP-binding motifs play key roles in Krp1p, kinesin-related protein 1, function for bi-polar growth control in fission yeast

Kinesin is a microtubule-based motor protein with various functions related to the cell growth and division. It has been reported that Krp1p, kinesin-related protein 1, which belongs to the kinesin heavy chain superfamily, localizes on microtubules and may play an important role in cytokinesis. However, the function of Krp1p has not been fully elucidated. In this study, we overexpressed an intact form and three different mutant forms of Krp1p in fission yeast constructed by site-directed mutagenesis in two ATP-binding motifs or by truncation of the leucine zipper-like motif (LZiP). We observed hyper-extended microtubules and the aberrant nuclear shape in Krp1p-overexpressed fission yeast. As a functional consequence, a point mutation of ATP-binding domain 1 (G89E) in Krp1p reversed the effect of Krp1p overexpression in fission yeast, whereas the specific mutation in ATP-binding domain 2 (G238E) resulted in the altered cell polarity. Additionally, truncation of the leucine zipper-like domain (LZiP) at the C-terminal of Krp1p showed a normal nuclear division. Taken together, we suggest that krp1p is involved in regulation of cell-polarized growth through ATP-binding motifs in fission yeast.     

p13suc1 acts in the fission yeast cell division cycle as a component of the p34cdc2 protein kinase.

cdc2+ encodes a protein kinase that is required during both G1 and G2 phases of the cell division cycle in fission yeast. suc1+ is an essential gene that was originally identified as a plasmid-borne sequence that could rescue certain temperature-sensitive cdc2 mutants. To investigate the role of the suc1+ gene product in the cell cycle p13suc1 has been expressed in Escherichia coli and purified. An immunoaffinity purified anti-p13suc1 polyclonal serum has been prepared and used to identify p13suc1 in fission yeast. The abundance of this protein did not alter either during the cell cycle or during entry into stationary phase. p13suc1 was found in yeast lysates in a complex with the cdc2+ gene product. Approximately 5% of cellular p34cdc2 was associated with p13suc1, and this fraction of p34cdc2 was active as a protein kinase. The stability of the complex was disrupted in yeast strains carrying temperature-sensitive alleles of cdc2 that are suppressible by overexpression of suc1+. The level of association between p13suc1 and p34cdc2 was not affected by cell cycle arrest in adverse nutritional conditions. p13suc1 is not a substrate of the p34cdc2 protein kinase. We propose instead that it acts as a regulatory component of p34cdc2 that facilitates interaction with other proteins.     

Characterization of the roles of Blt1p in fission yeast cytokinesis

Spatial and temporal regulation of cytokinesis is essential for cell division, yet the mechanisms that control the formation and constriction of the contractile ring are incompletely understood. In the fission yeast Schizosaccharomyces pombe proteins that contribute to the cytokinetic contractile ring accumulate during interphase in nodes—precursor structures around the equatorial cortex. During mitosis, additional proteins join these nodes, which condense to form the contractile ring. The cytokinesis protein Blt1p is unique in being present continuously in nodes from early interphase through to the contractile ring until cell separation. Blt1p was shown to stabilize interphase nodes, but its functions later in mitosis were unclear. We use analytical ultracentrifugation to show that purified Blt1p is a tetramer. We find that Blt1p interacts physically with Sid2p and Mob1p, a protein kinase complex of the septation initiation network, and confirm known interactions with F-BAR protein Cdc15p. Contractile rings assemble normally in blt1∆ cells, but the initiation of ring constriction and completion of cell division are delayed. We find three defects that likely contribute to this delay. Without Blt1p, contractile rings recruited and retained less Sid2p/Mob1p and Clp1p phosphatase, and β-glucan synthase Bgs1p accumulated slowly at the cleavage site.