Use of high specific activity StarFire oligonucleotide probes to visualize low-abundance pre-mRNA splicing intermediates in S. pombe

An oligonucleotide labeling system was developed that can produce radiolabeled hybridization probes with tenfold or more higher specific activity than is obtained by traditional 5'-end-labeling with polynucleotide kinase. Yet the system is as rapid and simple as kinase labeling. The reaction uses the Klenow fragment of E. coli DNA polymerase to add alpha-32P-dA residues to the 3'-end of an oligonucleotide in a primer-extension reaction. Unlike other methods of radioactive tailing (e.g., terminal transferase), a single species is produced of both known length and known specific activity. The reaction is efficient, and over 90% of probe molecules are routinely labeled. Using this method of labeling, an oligonucleotide was shown to be tenfold more sensitive in detecting target DNA sequences in a dot blot hybridization assay, compared to the same oligonucleotide labeled using polynucleotide kinase. Northern blots of Schizosaccharomyces pombe RNA were probed with an oligonucleotide specific for intron 1 of the tf2d gene, a TATA-box binding transcription factor. Kinase-labeled tf2d probe detected only unspliced RNA, while the same oligonucleotide labeled using the new method detected both unspliced tf2d RNA and rare pre-mRNA splicing intermediates.

Isolation of an essential Schizosaccharomyces pombe gene, prp31(+), that links splicing and meiosis

We carried out a screen for mutants that arrest prior to premeiotic S phase. One of the strains we isolated contains a temperature-sensitive allele mutation in the fission yeast prp31(+) gene. The prp31-E1 mutant is defective in vegetative cell growth and in meiotic progression. It is synthetically lethal with prp6 and displays a pre-mRNA splicing defect at the restrictive temperature. We cloned the wild-type gene by complementation of the temperature-sensitive mutant phenotype. Prp31p is closely related to human and budding yeast PRP31 homologs and is likely to function as a general splicing factor in both vegetative growth and sexual differentiation.

Sid4p is required to localize components of the septation initiation pathway to the spindle pole body in fission yeast

A mutation in the Schizosaccharomyces pombe sid4(+) (septation initiation defective) gene was isolated in a screen for mutants defective in cytokinesis. We have cloned sid4(+) and have found that sid4(+) encodes a previously unknown 76.4-kDa protein that localizes to the spindle pole body (SPB) throughout the cell cycle. Sid4p is required for SPB localization of key regulators of septation initiation, including the GTPase Spg1p, the protein kinase Cdc7p, and the GTPase-activating protein Byr4p. An N-terminally truncated Sid4p mutant does not localize to SPBs and when overproduced acts as a dominant-negative mutant by titrating endogenous Sid4p and Spg1p from the SPB. Conversely, the Sid4p N-terminal 153 amino acids are sufficient for SPB localization. Biochemical studies demonstrate that Sid4p interacts with itself, and yeast two-hybrid analysis shows that its self-interaction domain lies within the C-terminal half of the protein. Our data indicate that Sid4p SPB localization is a prerequisite for the execution of the Spg1p signaling cascade.

Frequency and clinical implications of fluid dynamically significant diffuse coronary artery disease manifest as graded, longitudinal, base-to-apex myocardial perfusion abnormalities by noninvasive positron emission tomography

BACKGROUND

Diffuse coronary atherosclerosis is the substrate for plaque rupture and coronary events. Therefore, in patients with mild arteriographic coronary artery disease without significant segmental dipyridamole-induced myocardial perfusion defects, we tested the hypothesis that fluid dynamically significant diffuse coronary artery narrowing is frequently manifest as a graded, longitudinal, base-to-apex myocardial perfusion abnormality by noninvasive PET.

METHODS AND RESULTS

In this study, 1001 patients with documented coronary artery disease by coronary arteriography showing any visible coronary artery narrowing underwent rest-dipyridamole PET perfusion imaging. Quantitative severity of dipyridamole-induced, circumscribed, segmental PET perfusion defects was objectively measured by automated software as the minimum quadrant average relative activity indicating localized flow limiting stenoses. Quantitative severity of the graded, longitudinal, base-to-apex myocardial perfusion gradient indicating fluid dynamic effects of diffuse coronary artery narrowing was objectively measured by automated software as the spatial slope of relative activity along the cardiac longitudinal axis.

CONCLUSIONS

In patients with mild arteriographic disease without statistically significant dipyridamole-induced segmental myocardial perfusion defects caused by flow-limiting stenoses compared with normal control subjects, there was a graded, longitudinal, base-to-apex myocardial perfusion gradient significantly different from normal control subjects (P=0. 001) that was also observed for moderate to severe dipyridamole-induced segmental perfusion defects (P=0.0001), indicating diffuse disease underlying segmental perfusion defects; 43% of patients with or without segmental perfusion defects demonstrated graded, longitudinal, base-to-apex perfusion abnormalities beyond +/-2 SD of normal control subjects, indicating diffuse coronary arterial narrowing by noninvasive PET perfusion imaging.

Pressure-derived fractional flow reserve to assess serial epicardial stenoses: theoretical basis and animal validation

Background-Fractional flow reserve (FFR) is an index of stenosis severity validated for isolated stenoses. This study develops the theoretical basis and experimentally validates equations for predicting FFR of sequential stenoses separately. Methods and Results-For 2 stenoses in series, equations were derived to predict FFR (FFR(pred)) of each stenosis separately (ie, as if the other one were removed) from arterial pressure (P(a)), pressure between the 2 stenoses (P(m)), distal coronary pressure (P(d)), and coronary occlusive pressure (P(w)). In 5 dogs with 2 stenoses of varying severity in the left circumflex coronary artery, FFR(pred) was compared with FFR(app) (ratio of the pressure just distal to that just proximal to each stenoses) and to FFR(true) (ratio of the pressures distal to proximal to each stenosis but after removal of the other one) in case of fixed distal and varying proximal stenoses (n=15) and in case of fixed proximal and varying distal stenoses (n=20). The overestimation of FFR(true) by FFR(app) was larger than that of FFR(true) by FFR(pred) (0.070+/-0.007 versus 0.029+/-0.004, P<0.01 for fixed distal stenoses, and 0.114+/-0.01 versus 0.036+/-0. 004, P<0.01 for fixed proximal stenoses). This overestimation of FFR(true) by FFR(app) was larger for fixed proximal than for fixed distal stenoses. Conclusions-The interaction between 2 stenoses is such that FFR of each lesion separately cannot be calculated by the equation for isolated stenoses (P(d)/P(a) during hyperemia) applied to each separately but can be predicted by more complete equations taking into account P(a), P(m), P(d), and P(w).

The role of the sid1p kinase and cdc14p in regulating the onset of cytokinesis in fission yeast

Coordination of mitosis and cytokinesis is crucial for ensuring proper chromosome segregation and genomic stability. In Schizosaccharomyces pombe, the sid genes (cdc7, cdc11, cdc14, spg1, sid1, sid2 and sid4) define a signaling pathway that regulates septation and cytokinesis. Here we describe the characterization of a novel protein kinase, Sid1p. Sid1p localizes asymmetrically to one spindle pole body (SPB) in anaphase. Sid1p localization is maintained during medial ring constriction and septum synthesis and disappears prior to cell separation. Additionally, we found that Cdc14p is in a complex with Sid1p. Epistasis analysis places Sid1p-Cdc14p downstream of Spg1p-Cdc7p but upstream of Sid2p. Finally, we show that cyclin proteolysis during mitosis is unaffected by inactivating the sid pathway; in fact, loss of Cdc2-cyclin activity promotes Sid1p-Cdc14p association with the SPB, possibly providing a mechanism that couples cytokinesis with mitotic exit.

A mutant of Arp2p causes partial disassembly of the Arp2/3 complex and loss of cortical actin function in fission yeast

The Arp2/3 complex is an essential component of the yeast actin cytoskeleton that localizes to cortical actin patches. We have isolated and characterized a temperature-sensitive mutant of Schizosaccharomyces pombe arp2 that displays a defect in cortical actin patch distribution. The arp2(+) gene encodes an essential actin-related protein that colocalizes with actin at the cortical actin patch. Sucrose gradient analysis of the Arp2/3 complex in the arp2-1 mutant indicated that the Arp2p and Arc18p subunits are specifically lost from the complex at restrictive temperature. These results are consistent with immunolocalization studies of the mutant that show that Arp2-1p is diffusely localized in the cytoplasm at restrictive temperature. Interestingly, Arp3p remains localized to the cortical actin patch under the same restrictive conditions, leading to the hypothesis that loss of Arp2p from the actin patch affects patch motility but does not severely compromise its architecture. Analysis of the mutant Arp2 protein demonstrated defects in ATP and Arp3p binding, suggesting a possible model for disruption of the complex.

Evidence that Myb-related CDC5 proteins are required for pre-mRNA splicing

The conserved CDC5 family of Myb-related proteins performs an essential function in cell cycle control at G(2)/M. Although c-Myb and many Myb-related proteins act as transcription factors, herein, we implicate CDC5 proteins in pre-mRNA splicing. Mammalian CDC5 colocalizes with pre-mRNA splicing factors in the nuclei of mammalian cells, associates with core components of the splicing machinery in nuclear extracts, and interacts with the spliceosome throughout the splicing reaction in vitro. Furthermore, genetic depletion of the homolog of CDC5 in Saccharomyces cerevisiae, CEF1, blocks the first step of pre-mRNA processing in vivo. These data provide evidence that eukaryotic cells require CDC5 proteins for pre-mRNA splicing.

The evolutionarily conserved Dim1 protein defines a novel branch of the thioredoxin fold superfamily

Dim1 is a small evolutionarily conserved protein essential for G2/M transition that has recently been implicated as a component of the mRNA splicing machinery. To date, the mechanism of Dim1 function remains poorly defined, in part because of the absence of informative sequence homologies between Dim1 and other functionally defined proteins or protein domains. We have used a combination of molecular modeling and NMR structural analysis to demonstrate that approximately 125 of the 142 amino acids of human Dim1 (hDim1) define a novel branch of the thioredoxin fold superfamily. Mutational analysis of Dim1 based on the predicted fold indicates that alterations in the region corresponding to the thioredoxin active site do not affect Dim1 activity. However, removal of a very short carboxy-terminal extension generates a dominant negative form of the protein [hDim1-(1-128)] that when overproduced induces cell cycle arrest in G2, via a mechanism likely to involve alteration of Dim1 association with partner molecules. In sum, this study identifies the Dim1 proteins as a novel sixth branch of the thioredoxin superfamily involved in cell cycle.

Myb-related fission yeast cdc5p is a component of a 40S snRNP-containing complex and is essential for pre-mRNA splicing

Myb-related cdc5p is required for G(2)/M progression in the yeast Schizosaccharomyces pombe. We report here that all detectable cdc5p is stably associated with a multiprotein 40S complex. Immunoaffinity purification has allowed the identification of 10 cwf (complexed with cdc5p) proteins. Two (cwf6p and cwf10p) are members of the U5 snRNP; one (cwf9p) is a core snRNP protein. cwf8p is the apparent ortholog of the Saccharomyces cerevisiae splicing factor Prp19p. cwf1(+) is allelic to the prp5(+) gene defined by the S. pombe splicing mutant, prp5-1, and there is a strong negative genetic interaction between cdc5-120 and prp5-1. Five cwfs have not been recognized previously as important for either pre-mRNA splicing or cell cycle control. Further characterization of cwf1p, cwf2p, cwf3p, and cwf4p demonstrates that they are encoded by essential genes, cosediment with cdc5p at 40S, and coimmunoprecipitate with cdc5p. We further show that cdc5p associates with the U2, U5, and U6 snRNAs and that cells lacking cdc5(+) function are defective in pre-mRNA splicing. These data raise the possibility that the cdc5p complex is an intermediate in the assembly or disassembly of an active S. pombe spliceosome.

Identification and characterization of Schizosaccharomyces pombe asp1(+), a gene that interacts with mutations in the Arp2/3 complex and actin

The Arp2/3 complex is an essential component of the actin cytoskeleton in yeast and is required for the movement of actin patches. In an attempt to identify proteins that interact with this complex in the fission yeast Schizosaccharomyces pombe, we sought high-copy suppressors of the S. pombe arp3-c1 mutant, and have identified one, which we have termed asp1(+). The asp1(+) open reading frame (ORF) predicts a highly conserved protein of 921 amino acids with a molecular mass of 106 kD that does not contain motifs of known function. Neither asp1(+) nor its apparent Saccharomyces cerevisiae ortholog, VIP1, are essential genes. However, disruption of asp1(+) leads to altered morphology and growth properties at elevated temperatures and defects in polarized growth. The asp1 disruption strain also is hypersensitive to Ca+ ions and to low pH conditions. Although Asp1p is not stably associated with the Arp2/3 complex nor localized in any discrete structure within the cytoplasm, the asp1 disruption mutant was synthetically lethal with mutations in components of the Arp2/3 complex, arp3-c1 and sop2-1, as well as with a mutation in actin, act1-48. Moreover, the vip1 disruption strain showed a negative genetic interaction with a las17Delta strain. We conclude that Asp1p/Vip1p is important for the function of the cortical actin cytoskeleton.

Phosphorylation of the myosin-II light chain does not regulate the timing of cytokinesis in fission yeast

Proper coordination of cytokinesis with chromosome separation during mitosis is crucial to ensure that each daughter cell inherits an equivalent set of chromosomes. It has been proposed that one mechanism by which this is achieved is through temporally regulated myosin regulatory light chain (RLC) phosphorylation (Satterwhite, L. L., and Pollard, T. D. (1992) Curr. Opin. Cell Biol. 4, 43-52). A variety of evidence is consistent with this model. A direct test of the importance of RLC phosphorylation in vivo has been done only in Dictyostelium and Drosophila; phosphorylation of the RLC is essential in Drosophila (Jordan, P., and Karess, R. (1997) J. Cell Biol. 139, 1805-1819) but not essential in Dictyostelium (Ostrow, B. D., Chen, P., and Chisholm, R. L. (1994) J. Cell Biol. 127, 1945-1955). The Schizosaccharomyces pombe myosin light chain Cdc4p is essential for cytokinesis, but it was unknown whether phosphorylation played a role in its regulation. Here we show that the S. pombe myosin light chain Cdc4p is phosphorylated in vivo on either serine 2 or 6 but not both. Mutation of either or both of these sites to alanine did not effect the ability of Cdc4p to bind the type II myosin Myo2p, and cells expressing only these mutated versions of Cdc4p grew and divided normally. Similarly, mutation of Ser-2, Ser-6, or both residues to aspartic acid did not affect growth or division of cells. Thus we conclude that phosphorylation of Cdc4p is not essential in vivo for the function of the protein.

The schizosaccharomyces pombe dim1(+) gene interacts with the anaphase-promoting complex or cyclosome (APC/C) component lid1(+) and is required for APC/C function

The Schizosaccharomyces pombe dim1(+) gene is required for entry into mitosis and for chromosome segregation during mitosis. To further understand dim1p function, we undertook a synthetic lethal screen with the temperature-sensitive dim1-35 mutant and isolated lid (for lethal in dim1-35) mutants. Here, we describe the temperature-sensitive lid1-6 mutant. At the restrictive temperature of 36 degrees C, lid1-6 mutant cells arrest with a "cut" phenotype similar to that of cut4 and cut9 mutants. An epitope-tagged version of lid1p is a component of a multiprotein approximately 20S complex; the presence of lid1p in this complex depends upon functional cut9(+). lid1p-myc coimmunoprecipitates with several other proteins, including cut9p and nuc2p, and the presence of cut9p in a 20S complex depends upon the activity of lid1(+). Further, lid1(+) function is required for the multiubiquitination of cut2p, an anaphase-promoting complex or cyclosome (APC/C) target. Thus, lid1p is a component of the S. pombe APC/C. In dim1 mutants, the abundances of lid1p and the APC/C complex decline significantly, and the ubiquitination of an APC/C target is abolished. These data suggest that at least one role of dim1p is to maintain or establish the steady-state level of the APC/C.

Regulating the onset of mitosis

In eukaryotes, G2/M progression is mediated by activation of mitosis promoting factor (MPF). To ensure faithful chromosome segregation, the activity of key mitotic inducers and inhibitors are coupled with chromosome replication, spindle pole duplication, morphogenesis, and DNA damage. Evidence gathered in the past two years has underscored the importance of positioning MPF and its regulators in the proper place at the proper time to ensure orderly progression through the G2/M transition. Altering the spatial organization of G2/M regulators also contributes to prevention of mitosis following DNA damage.

Evidence for F-actin-dependent and -independent mechanisms involved in assembly and stability of the medial actomyosin ring in fission yeast

Cell division in a number of eukaryotes, including the fission yeast Schizosaccharomyces pombe, is achieved through a medially placed actomyosin-based contractile ring. Although several components of the actomyosin ring have been identified, the mechanisms regulating ring assembly are still not understood. Here, we show by biochemical and mutational studies that the S.pombe actomyosin ring component Cdc4p is a light chain associated with Myo2p, a myosin II heavy chain. Localization of Myo2p to the medial ring depended on Cdc4p function, whereas localization of Cdc4p at the division site was independent of Myo2p. Interestingly, the actin-binding and motor domains of Myo2p are not required for its accumulation at the division site although the motor activity of Myo2p is essential for assembly of a normal actomyosin ring. The initial assembly of Myo2p and Cdc4p at the division site requires a functional F-actin cytoskeleton. Once established, however, F-actin is not required for the maintenance of Cdc4p and Myo2p medial rings, suggesting that the attachment of Cdc4p and Myo2p to the division site involves proteins other than actin itself.

The Arp2/3 complex: a multifunctional actin organizer

The actin-related proteins (Arps) constitute a recently characterized family of proteins, many of which function as members of multiprotein complexes. The discovery that two family members, Arp2 and Arp3, act as multifunctional organizers of actin filaments in all eukaryotes has generated much excitement. Over the past two years, newly discovered properties of the Arp2/3 complex have suggested a central role in the control of actin polymerization. First, it promotes actin assembly on the surface of the motile intracellular pathogen Listeria monocytogenes. Second, it can nucleate and cross-link actin filaments in vitro. Third, it localizes with dynamic actin-rich spots of mammalian cells suggesting a role in protrusion; it is found in cortical actin patches in the budding and fission yeasts where it may control patch movement and cortical actin function. Clearly, the complex has a central role in actin cytoskeletal function and will be the subject of much research in the coming years.

Intensive lifestyle changes for reversal of coronary heart disease

CONTEXT

The Lifestyle Heart Trial demonstrated that intensive lifestyle changes may lead to regression of coronary atherosclerosis after 1 year.

OBJECTIVES

To determine the feasibility of patients to sustain intensive lifestyle changes for a total of 5 years and the effects of these lifestyle changes (without lipid-lowering drugs) on coronary heart disease.

DESIGN

Randomized controlled trial conducted from 1986 to 1992 using a randomized invitational design.

PATIENTS

Forty-eight patients with moderate to severe coronary heart disease were randomized to an intensive lifestyle change group or to a usual-care control group, and 35 completed the 5-year follow-up quantitative coronary arteriography.

SETTING

Two tertiary care university medical centers.

INTERVENTION

Intensive lifestyle changes (10% fat whole foods vegetarian diet, aerobic exercise, stress management training, smoking cessation, group psychosocial support) for 5 years.

MAIN OUTCOME MEASURES

Adherence to intensive lifestyle changes, changes in coronary artery percent diameter stenosis, and cardiac events.

RESULTS

Experimental group patients (20 [71%] of 28 patients completed 5-year follow-up) made and maintained comprehensive lifestyle changes for 5 years, whereas control group patients (15 [75%] of 20 patients completed 5-year follow-up) made more moderate changes. In the experimental group, the average percent diameter stenosis at baseline decreased 1.75 absolute percentage points after 1 year (a 4.5% relative improvement) and by 3.1 absolute percentage points after 5 years (a 7.9% relative improvement). In contrast, the average percent diameter stenosis in the control group increased by 2.3 percentage points after 1 year (a 5.4% relative worsening) and by 11.8 percentage points after 5 years (a 27.7% relative worsening) (P=.001 between groups. Twenty-five cardiac events occurred in 28 experimental group patients vs 45 events in 20 control group patients during the 5-year follow-up (risk ratio for any event for the control group, 2.47 [95% confidence interval, 1.48-4.20]).

CONCLUSIONS

More regression of coronary atherosclerosis occurred after 5 years than after 1 year in the experimental group. In contrast, in the control group, coronary atherosclerosis continued to progress and more than twice as many cardiac events occurred.

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.

The cdr2(+) gene encodes a regulator of G2/M progression and cytokinesis in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells respond to nutrient deprivation by altering G2/M cell size control. The G2/M transition is controlled by activation of the cyclin-dependent kinase Cdc2p. Cdc2p activation is regulated both positively and negatively. cdr2(+) was identified in a screen for regulators of mitotic control during nutrient deprivation. We have cloned cdr2(+) and have found that it encodes a putative serine-threonine protein kinase that is related to Saccharomyces cerevisiae Gin4p and S. pombe Cdr1p/Nim1p. cdr2(+) is not essential for viability, but cells lacking cdr2(+) are elongated relative to wild-type cells, spending a longer period of time in G2. Because of this property, upon nitrogen deprivation cdr2(+) mutants do not arrest in G1, but rather undergo another round of S phase and arrest in G2 from which they are able to enter a state of quiescence. Genetic evidence suggests that cdr2(+) acts as a mitotic inducer, functioning through wee1(+), and is also important for the completion of cytokinesis at 36 degrees C. Defects in cytokinesis are also generated by the overproduction of Cdr2p, but these defects are independent of wee1(+), suggesting that cdr2(+) encodes a second activity involved in cytokinesis.

Coronary arteriography and lipid lowering: limitations, new concepts, and new paradigms in cardiovascular medicine

Coronary arteriography has played a central role in improving our understanding of the mechanisms of unstable coronary syndromes and the benefits of cholesterol lowering. However, coronary arteriography as currently used is outmoded and inadequate for new clinical algorithms based on vigorous lipid and other risk factor control as alternatives to invasive procedures for the primary treatment of coronary artery disease. What is needed is a way of viewing or analyzing noninvasive myocardial perfusion images and coronary arteriograms so as to identify and quantify the extent or severity of diffuse coronary atherosclerosis. Determining the relative contribution of diffuse and segmental narrowing by definitive myocardial perfusion imaging or coronary arteriography would provide the optimal basis for determining the need for revascularization procedures. In the absence of significant segmental stenoses, mild or diffuse disease identified by coronary arteriography would also provide a definitive diagnosis as the basis for lifelong cholesterol-lowering drugs and risk factor modification, even for patients with normal cholesterol levels. Thus, it is important to consider several new concepts for analyzing coronary arteriograms. More physiologically accurate invasive and noninvasive technology allows improved diagnosis and management of coronary atherosclerosis as new paradigms in cardiovascular medicine.

A phosphorylation site mutant of Schizosaccharomyces pombe cdc2p fails to promote the metaphase to anaphase transition

The protein kinase cdc2p is a key regulator of the G1-S and G2-M cell cycle transitions in the yeast Schizosaccharomyces pombe. Activation of cdc2p is regulated by its phosphorylation state and by interaction with other proteins. We have analyzed the consequences for cell cycle progression of altering the conserved threonine phosphorylation site, within the activation loop of cdc2p, to glutamic acid. This mutant, T167 E, promotes entry into mitosis, as judged by the accumulation of mitotic spindles and condensed chromosomes, despite the fact that it lacks demonstrable kinase activity both in vitro and in vivo. However, T167 E cannot promote the metaphase-anaphase transition. Since a component of the anaphase-promoting complex (APC) in S. pombe, cut9p, remains hypophosphorylated at the T167 E arrest point, the cell cycle block might be due to the inability of T167 E to activate the APC. T167 E is lethal when overexpressed, and overproduction also causes a mitotic arrest. Multicopy suppressors of the dominant negative phenotype were isolated, and identified as cdc13+ and suc1+. Overexpression of suc1+ suppresses the effects of T167 E overproduction by restoring sufficient amounts of suc1p to the cell to allow passage through mitosis.

Isolation and characterization of new fission yeast cytokinesis mutants

Schizosaccharomyces pombe is an excellent organism in which to study cytokinesis as it divides by medial fission using an F-actin contractile ring. To enhance our understanding of the cell division process, a large genetic screen was carried out in which 17 genetic loci essential for cytokinesis were identified, 5 of which are novel. Mutants identifying three genes, rng3(+), rng4(+), and rng5(+), were defective in organizing an actin contractile ring. Four mutants defective in septum deposition, septum initiation defective (sid)1, sid2, sid3, and sid4, were also identified and characterized. Genetic analyses revealed that the sid mutants display strong negative interactions with the previously described septation mutants cdc7-24, cdc11-123, and cdc14-118. The rng5(+), sid2(+), and sid3(+) genes were cloned and shown to encode Myo2p (a myosin heavy chain), a protein kinase related to budding yeast Dbf2p, and Spg1p, a GTP binding protein that is a member of the ras superfamily of GTPases, respectively. The ability of Spg1p to promote septum formation from any point in the cell cycle depends on the activity of Sid4p. In addition, we have characterized a phenotype that has not been described previously in cytokinesis mutants, namely the failure to reorganize actin patches to the medial region of the cell in preparation for septum formation.