The yeast UME6 gene product is required for transcriptional repression mediated by the CAR1 URS1 repressor binding site.

URS1 is known to be a repressor binding site in Saccharomyces cerevisiae that negatively regulates expression of many genes including CAR1 (arginase), several required for sporulation, mating type switching, inositol metabolism, and oxidative carbon metabolism. In addition to the proteins previously shown to directly bind to the URS1 site, we show here that the UME6 gene product is required for URS1 to mediate repression of gene expression in the absence of inducer. We also show that mutations in the CAR80 (CARGRI) gene are allelic to those in UME6.     

Kinase activity of fission yeast Mph1 is required for Mad2 and Mad3 to stably bind the anaphase promoting complex

Defects in chromosome segregation result in aneuploidy, which can lead to disease or cell death [1, 2]. The spindle checkpoint delays anaphase onset until all chromosomes are attached to spindle microtubules in a bipolar fashion [3, 4]. Mad2 is a key checkpoint component that undergoes conformational activation, catalyzed by a Mad1-Mad2 template enriched at unattached kinetochores [5]. Mad2 and Mad3 (BubR1) then bind and inhibit Cdc20 to form the mitotic checkpoint complex (MCC), which binds and inhibits the anaphase promoting complex (APC/C). Checkpoint kinases (Aurora, Bub1, and Mps1) are critical for checkpoint signaling, yet they have poorly defined roles and few substrates have been identified [6-8]. Here we demonstrate that a kinase-dead allele of the fission yeast MPS1 homolog (Mph1) is checkpoint defective and that levels of APC/C-associated Mad2 and Mad3 are dramatically reduced in this mutant. Thus, MCC binding to fission yeast APC/C is dependent on Mph1 kinase activity. We map and mutate several phosphorylation sites in Mad2, producing mutants that display reduced Cdc20-APC/C binding and an inability to maintain checkpoint arrest. We conclude that Mph1 kinase regulates the association of Mad2 with its binding partners and thereby mitotic arrest.     

Mad2 binding to Mad1 and Cdc20, rather than oligomerization, is required for the spindle checkpoint.

Mad2 is a key component of the spindle checkpoint, a device that controls the fidelity of chromosome segregation in mitosis. The ability of Mad2 to form oligomers in vitro has been correlated with its ability to block the cell cycle upon injection into Xenopus embryos. Here we show that Mad2 forms incompatible complexes with Mad1 and Cdc20, neither of which requires Mad2 oligomerization. A monomeric point mutant of Mad2 can sustain a cell cycle arrest of comparable strength to that of the wild-type protein. We show that the interaction of Mad2 with Mad1 is crucial for the localization of Mad2 to kinetochores, where Mad2 interacts with Cdc20. We propose a model that features the kinetochore as a 'folding factory' for the formation of a Mad2-Cdc20 complex endowed with inhibitory activity on the anaphase promoting complex.     

The interaction between thymine DNA glycosylase and nuclear receptor coactivator 3 is required for the transcriptional activation of nuclear hormone receptors

The cellular counterpart of the “soluble” guanylyl cyclase found in tissue homogenates over 30 years ago is now recognized as the physiological receptor for nitric oxide (NO). The ligand-binding site is a prosthetic haem group that, when occupied by NO, induces a conformational change in the protein that propagates to the catalytic site, triggering conversion of GTP into cGMP. This review focuses on recent research that takes this basic information forward to the beginnings of a quantitative depiction of NO signal transduction, analogous to that achieved for other major transmitters. At its foundation is an explicit enzyme-linked receptor mechanism for NO-activated guanylyl cyclase that replicates all its main properties. In cells, NO signal transduction is subject to additional, activity-dependent modifications, notably through receptor desensitization and changes in the activity of cGMP-hydrolyzing phosphodiesterases. The measurement of these parameters under varying conditions in rat platelets has made it possible to formulate a cellular model of NO-cGMP signaling. The model helps explain cellular responses to NO and their modification by therapeutic agents acting on the guanylyl cyclase or phosphodiesterase limbs of the pathway.     

Genetic interaction between the homeobox transcription factors HESX1 and SIX3 is required for normal pituitary development

Hesx1 has been shown to be essential for normal pituitary development. The homeobox gene Six3 is expressed in the developing pituitary gland during mouse development but its function in this tissue has been precluded by the fact that in the Six3-deficient embryos the pituitary gland is not induced. To gain insights into the function of Six3 during pituitary development we have generated Six3^+/−;Hesx1^Cre/+ double heterozygous mice. Strikingly, these mice show marked dwarfism, which is first detectable around weaning, and die by the 5th-6th week of age. Thyroid and gonad development is also impaired in these animals. Analysis of Six3^+/−;Hesx1^Cre/+ compound embryos indicates that hypopituitarism is the likely cause of these defects since pituitary development is severely impaired in these mutants. Similar to the Hesx1-deficient embryos, Rathke's pouch is initially expanded in Six3^+/−;Hesx1^Cre/+ compound embryos due to an increase in cell proliferation. Subsequently, the anterior pituitary gland appears bifurcated, dysmorphic and occasionally ectopically misplaced in the nasopharyngeal cavity, but cell differentiation is unaffected. Our research has revealed a role for Six3 in normal pituitary development, which has likely been conserved during evolution as SIX3 is also expressed in the pituitary gland of the human embryo.