Accumulation of Mad2–Cdc20 complex during spindle checkpoint activation requires binding of open and closed conformers of Mad2 in Saccharomyces cerevisiae

The spindle assembly checkpoint (SAC) coordinates mitotic progression with sister chromatid alignment. In mitosis, the checkpoint machinery accumulates at kinetochores, which are scaffolds devoted to microtubule capture. The checkpoint protein Mad2 (mitotic arrest deficient 2) adopts two conformations: open (O-Mad2) and closed (C-Mad2). C-Mad2 forms when Mad2 binds its checkpoint target Cdc20 or its kinetochore receptor Mad1. When unbound to these ligands, Mad2 folds as O-Mad2. In HeLa cells, an essential interaction between C- and O-Mad2 conformers allows Mad1-bound C-Mad2 to recruit cytosolic O-Mad2 to kinetochores. In this study, we show that the interaction of the O and C conformers of Mad2 is conserved in Saccharomyces cerevisiae. MAD2 mutant alleles impaired in this interaction fail to restore the SAC in a mad2 deletion strain. The corresponding mutant proteins bind Mad1 normally, but their ability to bind Cdc20 is dramatically impaired in vivo. Our biochemical and genetic evidence shows that the interaction of O- and C-Mad2 is essential for the SAC and is conserved in evolution

Karyotypic Determinants of Chromosome Instability in Aneuploid Budding Yeast

Recent studies in cancer cells and budding yeast demonstrated that aneuploidy, the state of having abnormal chromosome numbers, correlates with elevated chromosome instability (CIN), i.e. the propensity of gaining and losing chromosomes at a high frequency. Here we have investigated ploidy- and chromosome-specific determinants underlying aneuploidy-induced CIN by observing karyotype dynamics in fully isogenic aneuploid yeast strains with ploidies between 1N and 2N obtained through a random meiotic process. The aneuploid strains exhibited various levels of whole-chromosome instability (i.e. chromosome gains and losses). CIN correlates with cellular ploidy in an unexpected way: cells with a chromosomal content close to the haploid state are significantly more stable than cells displaying an apparent ploidy between 1.5 and 2N. We propose that the capacity for accurate chromosome segregation by the mitotic system does not scale continuously with an increasing number of chromosomes, but may occur via discrete steps each time a full set of chromosomes is added to the genome. On top of such general ploidy-related effect, CIN is also associated with the presence of specific aneuploid chromosomes as well as dosage imbalance between specific chromosome pairs. Our findings potentially help reconcile the divide between gene-centric versus genome-centric theories in cancer evolution

Meeting Report: Experimental and Evolutionary Approaches to Yeast and Other Organisms 2018

The 2018 European Molecular Biology Laboratory (EMBL) Experimental and Evolutionary Approaches to Yeast and Other Organisms conference brought together researchers addressing fundamental questions in microbial evolutionary systems biology. Topics spanning evolution in the wild and the lab to molecular mechanisms of adaptation and ecological interactions between species were covered

The mutator phenotype : adapting microbial evolution to cancer biology

The mutator phenotype hypothesis was postulated almost 40 years ago to reconcile the observation that while cancer cells display widespread mutational burden, acquisition of mutations in non-transformed cells is a rare event. Moreover, it also suggested that cancer evolution could be fostered by increased genome instability. Given the evolutionary conservation throughout the tree of life and the genetic tractability of model organisms, yeast and bacterial species pioneered studies to dissect the functions of genes required for genome maintenance (caretaker genes) or for cell growth control (gatekeeper genes). In this review, we first provide an overview of what we learned from model organisms about the roles of these genes and the genome instability that arises as a consequence of their dysregulation. We then discuss our current understanding of how mutator phenotypes shape the evolution of bacteria and yeast species. We end by bringing clinical evidence that lessons learned from single-cell organisms can be applied to tumor evolution.NRF (Natl Research Foundation, S’pore)Published versio

Polarized Cell Growth: Double Grip by CDK1

Precise coupling of cell growth and cell-cycle progression is crucial for achieving cell homeostasis. A recent study sheds light on two distinct roles of cyclin-dependent kinase 1 (CDK1) in promoting polarized cell growth in budding yeast

Association of chromosome copy number imbalance with CIN.

<p>(A) Enrichment of strains belonging to the combined MU and HU classes of CIN relative to the S class (left) or to the HU class of CIN relative to the combined S and MU classes (right) among aneuploid strains displaying a particular chromosome copy number imbalance, calculated for each non-redundant pair of chromosomes. The enrichment is color-coded based on p-values calculated by means of Hypergeometric tests. Darker colors indicate more significant enrichment of strains belonging to the S class of CIN (left) or HU class of CIN (right) among strains with a non-1 copy number ratio between a given pair of chromosomes. (B) Diagram illustrating the experimental design for the assessment of the relationship between CIN and <i>MAD2</i>:<i>MAD1</i> ratio in 56 freshly generated aneuploid strains from isogenic triploid sporulation. (C–D) Frequency of aneuploid strains with stable or unstable ploidy grouped by their <i>MAD2</i>:<i>MAD1</i> ratio. Ploidy-stable strains (black histograms) were identified on the basis of their low level of ploidy variation among single colonies analyzed by FACS; ploidy-unstable strains (white histograms) were identified on the basis of high ploidy variation among single colonies. <i>MAD2</i>:<i>MAD1</i> ratios were determined by qPCR and are indicated on the x-axis. P-values at the top of (C–D) graphs refer to statistical association between the stability category and the <i>MAD2</i>:<i>MAD1</i> ratio category by means of a Fisher's exact test. Aneuploid strains were divided into all four possible <i>MAD2</i>:<i>MAD1</i> ratio classes (C) or based on <i>MAD2</i>:<i>MAD1</i> ratio equal or not equal to 0.5 (D).</p

Frequency of aneuploid chromosomes in the different CIN classes.

<p>The graph shows the frequency at which each of the 16 chromosomes was found in aneuploidy (either monosomic in a diploid background or disomic in a haploid background) in the analyzed aneuploid strains. The frequency histogram is stratified across the three different CIN classes. Red: HU strains; blue: MU strains; green: S strains. P-value refers to the global association between the 16 chromosomes and the three CIN classes according to a Fisher's exact test.</p