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PubMed2006

Ppc89 links multiple proteins, including the septation initiation network, to the core of the fission yeast spindle-pole body.

The spindle-pole body (SPB), the yeast analog of the centrosome, serves as the major microtubule (MT) organizing center in the yeast cell. In addition to this central function, the SPB organizes and concentrates proteins required for proper coordination between the nuclear-division cycle and cytokinesis. For example, the Schizosaccharomyces pombe septation-initiation network (SIN), which is responsible for initiating actomyosin ring constriction and septation, is assembled at the SPB through its two scaffolding components, Sid4 and Cdc11. In an effort to identify novel SIN interactors, we purified Cdc11 and identified by mass spectrometry a previously uncharacterized protein associated with it, Ppc89. Ppc89 localizes constitutively to the SPB and interacts directly with Sid4. A fusion between the N-terminal 300 amino acids of Sid4 and a SPB targeting domain of Ppc89 supplies the essential function of Sid4 in anchoring the SIN. ppc89Delta cells are inviable and exhibit defects in SPB integrity, and hence in spindle formation, chromosome segregation, and SIN localization. Ppc89 overproduction is lethal, resulting primarily in a G2 arrest accompanied by massive enlargement of the SPB and increased SPB MT nucleation. These results suggest a fundamental role for Ppc89 in organization of the S. pombe SPB.

Fluorescence Resonance Energy Transfer↗

PubMed2000

Role of the spindle pole body of yeast in mediating assembly of the prospore membrane during meiosis.

Spindle pole bodies (SPBs) are the centrosome equivalents in yeast, required for microtubule organization. In yeast, the SPB further serves as the attachment sites of the prospore membrane during meiosis. Here we report the identification of two new meiosis-specific components of the SPB, Mpc54p and Mpc70p, and the first protein specific for the prospore membrane, Don1p. Mpc54p and Mpc70p are not present in mitotic SPBs, and during meiosis II they are components of a meiosis-specific structural alteration of the outer plaque of the SPB. Both proteins are dispensable for the meiotic divisions but are essentially required for the formation of the prospore membrane. In the mpc54 and mpc70 mutants, the Don1p-containing precursors of the prospore membrane can still be found in the cytoplasm and associated with the SPB. Unexpectedly, however, the assembly of the precursors to a continuous membrane system is affected. Thus, the meiotic SPB is directly involved in the formation of a specialized membrane system, the membrane of the prospore.

Amino Acid Sequence↗

PubMed2001

Modes of spindle pole body inheritance and segregation of the Bfa1p-Bub2p checkpoint protein complex.

Yeast spindle pole bodies (SPBs) duplicate once per cell cycle by a conservative mechanism resulting in a pre-existing 'old' and a newly formed SPB. The two SPBs of yeast cells are functionally distinct. It is only the SPB that migrates into the daughter cell, the bud, which carries the Bfa1p-Bub2p GTPase-activating protein (GAP) complex, a component of the spindle positioning checkpoint. We investigated whether the functional difference of the two SPBs correlates with the time of their assembly. We describe that in unperturbed cells the 'old' SPB always migrates into the bud. However, Bfa1p localization is not determined by SPB inheritance. It is the differential interaction of cytoplasmic microtubules with the mother and bud cortex that directs the Bfa1p-Bub2p GAP to the bud-ward-localized SPB. In response to defects of cytoplasmic microtubules to interact with the cell cortex, the Bfa1p-Bub2p complex binds to both SPBs. This may provide a mechanism to delay cell cycle progression when cytoplasmic microtubules fail to orient the spindle. Thus, SPBs are able to sense cytoplasmic microtubule properties and regulate the Bfa1p-Bub2p GAP accordingly.

Antineoplastic Agents↗

PubMed2003

The XMAP215 homologue Stu2 at yeast spindle pole bodies regulates microtubule dynamics and anchorage.

The yeast protein Stu2 belongs to the XMAP215 family of conserved microtubule-binding proteins which regulate microtubule plus end dynamics. XMAP215-related proteins also bind to centrosomes and spindle pole bodies (SPBs) through proteins like the mammalian transforming acidic coiled coil protein TACC or the yeast Spc72. We show that yeast Spc72 has two distinct domains involved in microtubule organization. The essential 100 N-terminal amino acids of Spc72 interact directly with the gamma-tubulin complex, and an adjacent non-essential domain of Spc72 mediates binding to Stu2. Through these domains, Spc72 brings Stu2 and the gamma-tubulin complex together into a single complex. Manipulation of Spc72-Stu2 interaction at SPBs compromises the anchorage of astral microtubules at the SPB and surprisingly also influences the dynamics of microtubule plus ends. Permanently tethering Stu2 to SPBs by fusing it to a version of Spc72 that lacks the Stu2-binding site in part complements these defects in a manner which is dependent upon the microtubule-binding domain of Stu2. Thus, the SPB-associated Spc72-Stu2 complex plays a key role in regulating microtubule properties.

Genotype↗

PubMed2002

Fission yeast mutants affecting telomere clustering and meiosis-specific spindle pole body integrity.

In meiotic prophase of many eukaryotic organisms, telomeres attach to the nuclear envelope and form a polarized configuration called the bouquet. Bouquet formation is hypothesized to facilitate homologous chromosome pairing. In fission yeast, bouquet formation and telomere clustering occurs in karyogamy and persists throughout the horsetail stage. Here we report the isolation and characterization of six mutants from our screen for meiotic mutants. These mutants show defective telomere clustering as demonstrated by mislocalization of Swi6::GFP, a heterochromatin-binding protein, and Taz1p::GFP, a telomere-specific protein. These mutants define four complementation groups and are named dot1 to dot4-defective organization of telomeres. dot3 and dot4 are allelic to mat1-Mm and mei4, respectively. Immunolocalization of Sad1, a protein associated with the spindle pole body (SPB), in dot mutants showed an elevated frequency of multiple Sad1-nuclei signals relative to wild type. Many of these Sad1 foci were colocalized with Taz1::GFP. Impaired SPB structure and function were further demonstrated by failure of spore wall formation in dot1, by multiple Pcp1::GFP signals (an SPB component) in dot2, and by abnormal microtubule organizations during meiosis in dot mutants. The coincidence of impaired SPB functions with defective telomere clustering suggests a link between the SPB and the telomere cluster.

Cell Cycle Proteins↗

PubMed2002

Ady3p links spindle pole body function to spore wall synthesis in Saccharomyces cerevisiae.

Spore formation in Saccharomyces cerevisiae requires the de novo synthesis of prospore membranes and spore walls. Ady3p has been identified as an interaction partner for Mpc70p/Spo21p, a meiosis-specific component of the outer plaque of the spindle pole body (SPB) that is required for prospore membrane formation, and for Don1p, which forms a ring-like structure at the leading edge of the prospore membrane during meiosis II. ADY3 expression has been shown to be induced in midsporulation. We report here that Ady3p interacts with additional components of the outer and central plaques of the SPB in the two-hybrid assay. Cells that lack ADY3 display a decrease in sporulation efficiency, and most ady3Delta/ady3Delta asci that do form contain fewer than four spores. The sporulation defect in ady3Delta/ady3Delta cells is due to a failure to synthesize spore wall polymers. Ady3p forms ring-like structures around meiosis II spindles that colocalize with those formed by Don1p, and Don1p rings are absent during meiosis II in ady3Delta/ady3Delta cells. In mpc70Delta/mpc70Delta cells, Ady3p remains associated with SPBs during meiosis II. Our results suggest that Ady3p mediates assembly of the Don1p-containing structure at the leading edge of the prospore membrane via interaction with components of the SPB and that this structure is involved in spore wall formation.

Cell Wall↗

PubMed2002

subito encodes a kinesin-like protein required for meiotic spindle pole formation in Drosophila melanogaster.

The female meiotic spindle lacks a centrosome or microtubule-organizing center in many organisms. During cell division, these spindles are organized by the chromosomes and microtubule-associated proteins. Previous studies in Drosophila melanogaster implicated at least one kinesin motor protein, NCD, in tapering the microtubules into a bipolar spindle. We have identified a second Drosophila kinesin-like protein, SUB, that is required for meiotic spindle function. At meiosis I in males and females, sub mutations affect only the segregation of homologous chromosomes. In female meiosis, sub mutations have a similar phenotype to ncd; even though chromosomes are joined by chiasmata they fail to segregate at meiosis I. Cytological analyses have revealed that sub is required for bipolar spindle formation. In sub mutations, we observed spindles that were unipolar, multipolar, or frayed with no defined poles. On the basis of these phenotypes and the observation that sub mutations genetically interact with ncd, we propose that SUB is one member of a group of microtubule-associated proteins required for bipolar spindle assembly in the absence of the centrosomes. sub is also required for the early embryonic divisions but is otherwise dispensable for most mitotic divisions.

Amino Acid Sequence↗

PubMed2002

Mutant membrane protein of the budding yeast spindle pole body is targeted to the endoplasmic reticulum degradation pathway.

Mutation of either the yeast MPS2 or the NDC1 gene leads to identical spindle pole body (SPB) duplication defects: The newly formed SPB is improperly inserted into the nuclear envelope (NE), preventing the cell from forming a bipolar mitotic spindle. We have previously shown that both MPS2 and NDC1 encode integral membrane proteins localized at the SPB. Here we show that CUE1, previously known to have a role in coupling ubiquitin conjugation to ER degradation, is an unusual dosage suppressor of mutations in MPS2 and NDC1. Cue1p has been shown to recruit the soluble ubiquitin-conjugating enzyme, Ubc7p, to the cytoplasmic face of the ER membrane where it can ubiquitinate its substrates and target them for degradation by the proteasome. Both mps2-1 and ndc1-1 are also suppressed by disruption of UBC7 or its partner, UBC6. The Mps2-1p mutant protein level is markedly reduced compared to wild-type Mps2p, and deletion of CUE1 restores the level of Mps2-1p to nearly wild-type levels. Our data indicate that Mps2p may be targeted for degradation by the ER quality control pathway.

Carrier Proteins↗

PubMed2002

Mutational analysis reveals a role for the C terminus of the proteasome subunit Rpt4p in spindle pole body duplication in Saccharomyces cerevisiae.

The ubiquitin/proteasome pathway plays a key role in regulating cell cycle progression. Previously, we reported that a conditional mutation in the Saccharomyces cerevisiae gene RPT4/PCS1, which encodes one of six ATPases in the proteasome 19S cap complex/regulatory particle (RP), causes failure of spindle pole body (SPB) duplication. To improve our understanding of Rpt4p, we created 58 new mutations, 53 of which convert clustered, charged residues to alanine. Virtually all mutations that affect the N-terminal region, which contains a putative nuclear localization signal and coiled-coil motif, result in a wild-type phenotype. Nine mutations that affect the central ATPase domain and the C-terminal region confer recessive lethality. The two conditional mutations identified, rpt4-145 and rpt4-150, affect the C terminus. After shift to high temperature, these mutations generally cause cells to progress slowly through the first cell cycle and to arrest in the second cycle with large buds, a G2 content of DNA, and monopolar spindles, although this phenotype can vary depending on the medium. Additionally, we describe a genetic interaction between RPT4 and the naturally polymorphic gene SSD1, which in wild-type form modifies the rpt4-145 phenotype such that cells arrest in G2 of the first cycle with complete bipolar spindles.

Adenosine Triphosphatases↗

PubMed2004

Spastin interacts with the centrosomal protein NA14, and is enriched in the spindle pole, the midbody and the distal axon.

Hereditary spastic paraplegia (HSP) is characterized by the specific retrograde degeneration of the longest axons in the central nervous system, the corticospinal tracts. The gene most frequently involved in autosomal dominant cases of this disease, SPG4, encodes spastin, an ATPase belonging to the AAA family. AAA proteins are thought to exert their function by the energy-dependent rearrangement of protein complexes. The composite function of these proteins is directed by their binding to regulatory factors and adaptor proteins that target their activity into specific pathways in vivo. We previously found that overexpressed spastin interacts dynamically with microtubules and displays microtubule-severing activity. Here, we demonstrate that spastin is enriched in cell regions containing dynamic microtubules. During cell division spastin is found in the spindle pole, the central spindle and the midbody, whereas in immortalized motoneurons it is enriched in the distal axon and the branching points. Furthermore, spastin interacts with the centrosomal protein NA14, and co-fractionates with gamma-tubulin, a centrosomal marker. Deletion of the region required for binding to NA14 disrupts spastin interaction with microtubules, suggesting that NA14 may be an important adaptor to target spastin activity at the centrosome. These data strongly argue that spastin plays a role in cytoskeletal rearrangements and dynamics, and provide an attractive explanation for the degeneration of motor axons in HSP.

Adenosine Triphosphatases↗

PubMed2005

Decision of spindle poles and division plane by double preprophase bands in a BY-2 cell line expressing GFP-tubulin.

The preprophase band (PPB) of microtubules is thought to be involved in deciding the future division site. In this study, we investigated the effects of double PPBs on spindle formation and the directional decision of cytokinesis by using transgenic BY-2 cells expressing green fluorescent protein (GFP)-tubulin. At prophase, most of the cells with double PPBs formed multipolar spindles, whereas all cells with single PPBs formed normal bipolar spindles, clearly implicating the PPB in deciding the spindle poles. At metaphase, however, both cell types possessed the bipolar spindles, indicating the existence of correctional mechanism(s) at prometaphase. From prometaphase to metaphase, the spindles in double PPB cells altered their directions to become oblique to the cell-elongating axis, and these orientations were maintained in the phragmoplast and resulted in the oblique division planes. These oblique cell plates decreased when actin microfilaments were disrupted, and double actin-depleted zones (ADZs) appeared where the double PPBs had existed. These results suggest that the information necessary for proper cytokinesis may be transferred from the PPBs to the ADZs, even in the case of the double PPBs.

Actin Cytoskeleton↗

PubMed1980

Axial flap from the inferior pole of the breast.

The dermal-adipose segment of the inferior pole of the breast is usually resected during mammaplasty. However, this segment, based only on a medial or lateral pedicle, may be used as an axial flap to provide a moderate amount of tissue in reconstruction of the anterior thoracic regions.

Breast↗

PubMed2003

Interference with vision by TMS over the occipital pole: a fourth period.

We investigated the effect of single-pulse transcranial magnetic stimulation (TMS) over the occipital pole on a forced-choice visual letter-identification task. Magnetic stimuli were applied on the midline but with the initial current directed pseudorandomly toward either left or right hemisphere; visual stimuli were presented randomly in either left or right hemifield; magnetic-visual stimulus onset asynchrony varied randomly between 12 values: -500 ms and from -50 ms to +50 ms in 10 ms steps. The data revealed the existence of a hitherto unknown fourth task-interfering TMS effect that was maximal at -10 ms and specific for magnetic stimulus polarity and visual stimulus location. This -10 ms effect cannot be explained by reflex blinking (as the -50 ms effect can) and direct disruption of letter-induced activity (as the +20 ms and +100 ms effects can), but it could be explained by direct disruption of pre-letter activity or indirect disruption of letter-induced activity.

Adult↗

PubMed1983

Urinary excretion of activatable kallikrein in one-kidney pole-ligated hypertensive rats.

Kallikrein activatable by trypsin was found in the urine of normal rats, corresponding to about 33% of the total kallikrein excretion. This fraction was inhibited by aprotinin and kallikrein antiserum. There was no spontaneous activation at 20 degrees C for 24 h or at 37 degrees C for 15 h which indicates that the amount detectable in the urine may represent the levels of activatable kallikrein in the renal tubule. Sephacryl S-200 chromatography of the urine disclosed the presence of two forms of kallikrein, active and activatable, with apparent molecular weights of 30 000 and 34 500 respectively. These findings allow us to assume that the activatable fraction would correspond to prokallikrein, as described by others. In hypertensive one-kidney pole-ligated rats, the total urinary kallikrein did not differ from that excreted by solely uninephrectomized rats, though the active kallikrein showed a significant drop (P less than 0.001). These results suggest that in hypertensive rats there is an alteration in the activation mechanisms of kallikrein.

Animals↗

PubMed1995

Ureterocele associated with ureteral duplication and a nonfunctioning upper pole segment: management by partial nephroureterectomy alone.

We reviewed the records of 87 patients with a ureterocele associated with duplication and a nonfunctioning upper pole segment. All patients involved in this study were treated by partial nephroureterectomy and observation. Additional surgery to correct vesicoureteral reflux became necessary in 54 cases (62%). A careful retrospective chart and radiographic review identified that the need for additional surgery was directly related to the number of renal moieties that had a ureterocele or vesicoureteral reflux present. In particular, when a ureterocele alone was present 21 of 21 patients (100%) did not require additional surgery. When low grade (less than 3/5) reflux was present into 1 ureter 8 of 15 patients (60%) did not require surgery. The presence of high grade reflux into 1 moiety or vesicoureteral reflux into more than 1 moiety, regardless of the grade of reflux, almost inevitably resulted in the need for further surgery with only 2 of 50 patients (4%) with these abnormalities cured by partial nephroureterectomy alone. Our data suggest that this latter group of patients may be treated more efficaciously by complete genitourinary reconstruction rather than partial nephroureterectomy alone.

Adolescent↗

PubMed1984

Concentration of a major outer membrane protein at the cell poles in Escherichia coli.

Autoradiography of cell envelope ghosts obtained from a strain of Escherichia coli which lacks two major outer membrane proteins has been used to demonstrate the polar concentration of another major outer membrane protein, ompA protein. The beta-lactam antibiotic cephalexin prevents the insertion of newly synthesized ompA protein into the poles but removal of the antibiotic allows the randomly dispersed protein to migrate to the polar and possibly the septal areas of the cell. Labelling of whole cells with bacteriophage K3 has confirmed a polar concentration of ompA protein.

Autoradiography↗

PubMed2003

Physical and functional interactions between polo kinase and the spindle pole component Cut12 regulate mitotic commitment in S. pombe.

Commitment to mitosis is regulated by a protein kinase complex called MPF. MPF is inhibited by Wee1-related kinases and activated by Cdc25 phosphatase. MPF activation further boosts Cdc25 and represses Wee1. This feedback control probably involves polo kinase. A dominant cut12.s11 mutation in the Schizosaccharomyces pombe spindle pole body (SPB) component Cut12 both suppresses the conditional lethal mitotic commitment defect of cdc25.22 and promotes premature association of the S. pombe polo kinase, Plo1, with the SPB. We now show that Cut12 associated with Plo1 in two hybrid and immunoprecipitation assays. Plo1 function was required for recognition of the mitotic SPB by the phospho-specific antibody MPM-2. In vivo MPM-2 staining and in vitro kinase assays established that the loss-of-function mutation, cut12.1, reduced mitotic activation of Plo1, whereas the gain-of-function mutation, cut12.s11, promoted higher levels of Plo1 activity than were normally seen in interphase. cut12.s11 could not promote mitotic commitment of cdc25.22 cells when Plo1 function was compromised. Expression of a constitutively active plo1 allele suppressed the mitotic commitment defect of cdc25.22. These data suggest that cut12.s11 suppresses cdc25.22 by promoting Plo1 activity. Furthermore, the delayed mitotic commitment of plo1.ts2 cells suggests that Plo1 is an integral part of the core controls that modulate MPF activation in S. pombe.

Antibodies, Monoclonal↗

PubMed2004

Recruitment of NIMA kinase shows that maturation of the S. pombe spindle-pole body occurs over consecutive cell cycles and reveals a role for NIMA in modulating SIN activity.

Mitotic exit in Saccharomyces cerevisiae and septation in Schizosaccharomyces pombe are regulated by a conserved signaling network called the mitotic exit and septum initiation networks (SIN), respectively. The network is active on one of the two anaphase B spindle-pole bodies (SPBs). Whereas the inherent asymmetry of growth by budding accounts for elements of the asymmetry in S. cerevisiae, it has been unclear how, or why, the pathway is asymmetric in S. pombe. We show that elements of SPB duplication in S. pombe are conservative, and that the SIN is active on the new SPB. SIN association with the new SPB persists after transient depolymerization of microtubules. The localization of the NIMA-related kinase, Fin1, reveals further complexity in SPB inheritance. Fin1 associates with the SPB bearing the older components in all cells and with the "new" SPB in half of the population. Fin1 only binds the new SPB when this new SPB has arisen from the duplication of an SPB that is two or more cycles old. Thus, each of the four SPBs generated over two consecutive cell cycles are different, because they have distinct fates in the next cell cycle. Fin1 binds the SPB once the SIN is active and the association requires the SIN inhibitors Byr4 and Cdc16. Fin1 physically associates with Byr4. Compromising Fin1 function leads to SIN activation on both anaphase B SPBs and promotes septation, indicating that Fin1 restrains SIN activity on the old SPB.

Cell Cycle↗