Getting a transcription factor to only one nucleus following mitosis

The mitotic exit network regulates the spatiotemporal activity of Cdc42 to maintain cell size

During G1 in budding yeast, the Cdc42 GTPase establishes a polar front, along which actin is recruited to direct secretion for bud formation. Cdc42 localizes at the bud cortex and then redistributes between mother and daughter in anaphase. The molecular mechanisms that terminate Cdc42 bud-localized activity during mitosis are poorly understood. We demonstrate that the activity of the Cdc14 phosphatase, released through the mitotic exit network, is required for Cdc42 redistribution between mother and bud. Induced Cdc14 nucleolar release results in premature Cdc42 redistribution between mother and bud. Inhibition of Cdc14 causes persistence of Cdc42 bud localization, which perturbs normal cell size and spindle positioning. Bem3, a Cdc42 GAP, binds Cdc14 and is dephosphorylated at late anaphase in a Cdc14-dependent manner. We propose that Cdc14 dephosphorylates and activates Bem3 to allow Cdc42 inactivation and redistribution. Our results uncover a mechanism through which Cdc14 regulates the spatiotemporal activity of Cdc42 to maintain normal cell size at cytokinesis.

A large-scale complex haploinsufficiency-based genetic interaction screen in Candida albicans: analysis of the RAM network during morphogenesis

The morphogenetic transition between yeast and filamentous forms of the human fungal pathogen Candida albicans is regulated by a variety of signaling pathways. How these pathways interact to orchestrate morphogenesis, however, has not been as well characterized. To address this question and to identify genes that interact with the Regulation of Ace2 and Morphogenesis (RAM) pathway during filamentation, we report the first large-scale genetic interaction screen in C. albicans. Our strategy for this screen was based on the concept of complex haploinsufficiency (CHI). A heterozygous mutant of CBK1 (cbk1Δ/CBK1), a key RAM pathway protein kinase, was subjected to transposon-mediated, insertional mutagenesis. The resulting double heterozygous mutants (6,528 independent strains) were screened for decreased filamentation on Spider Medium (SM). From the 441 mutants showing altered filamentation, 139 transposon insertion sites were sequenced, yielding 41 unique CBK1-interacting genes. This gene set was enriched in transcriptional targets of Ace2 and, strikingly, the cAMP-dependent protein kinase A (PKA) pathway, suggesting an interaction between these two pathways. Further analysis indicates that the RAM and PKA pathways co-regulate a common set of genes during morphogenesis and that hyper-activation of the PKA pathway may compensate for loss of RAM pathway function. Our data also indicate that the PKA–regulated transcription factor Efg1 primarily localizes to yeast phase cells while the RAM–pathway regulated transcription factor Ace2 localizes to daughter nuclei of filamentous cells, suggesting that Efg1 and Ace2 regulate a common set of genes at separate stages of morphogenesis. Taken together, our observations indicate that CHI–based screening is a useful approach to genetic interaction analysis in C. albicans and support a model in which these two pathways regulate a common set of genes at different stages of filamentation.

Candida albicans is the most common cause of fungal infections in humans. As a diploid yeast without a classical sexual cycle, many genetic approaches developed for large-scale genetic interaction studies in the model yeast Saccharomyces cerevisiae cannot be applied to C. albicans. Genetic interaction studies have proven to be powerful genetic tools for the analysis of complex biological processes. Here, we demonstrate that libraries of C. albicans strains containing heterozygous mutations in two different genes can be generated and used to study genetic interactions in C. albicans on a large scale. Double heterozygous mutants that show more severe phenotypes than strains with single heterozygous mutations are indicative of genetic interactions through a phenomenon referred to as complex haploinsufficiency (CHI). We applied this approach to the study of the RAM (Regulation of Ace2 and Morphogenesis) signaling network during the morphogenetic transition of C. albicans from yeast to filamentous growth. Among the genes that interacted with CBK1, the key signaling kinase of the RAM pathway, were transcriptional targets of the RAM pathway and the protein kinase A pathway. Further analysis supports a model in which these two pathways co-regulate a common set of genes at different stages of filamentation.

The nucleus introduced

Now is an opportune moment to address the confluence of cell biological form and function that is the nucleus. Its arrival is especially timely because the recognition that the nucleus is extremely dynamic has now been solidly established as a paradigm shift over the past two decades, and also because we now see on the horizon numerous ways in which organization itself, including gene location and possibly self-organizing bodies, underlies nuclear functions.

Accelerated mutation accumulation in asexual lineages of a freshwater snail

Sexual reproduction is both extremely costly and widespread relative to asexual reproduction, meaning that it must also confer profound advantages in order to persist. One theorized benefit of sex is that it facilitates the clearance of harmful mutations, which would accumulate more rapidly in the absence of recombination. The extent to which ineffective purifying selection and mutation accumulation are direct consequences of asexuality and whether the accelerated buildup of harmful mutations in asexuals can occur rapidly enough to maintain sex within natural populations, however, remain as open questions. We addressed key components of these questions by estimating the rate of mutation accumulation in the mitochondrial genomes of multiple sexual and asexual representatives of Potamopyrgus antipodarum, a New Zealand snail characterized by mixed sexual/asexual populations. We found that increased mutation accumulation is associated with asexuality and occurs rapidly enough to be detected in recently derived asexual lineages of P. antipodarum. Our results demonstrate that increased mutation accumulation in asexuals can differentially affect coexisting and ecologically similar sexual and asexual lineages. The accelerated rate of mutation accumulation observed in asexual P. antipodarum provides some of the most direct evidence to date for a link between asexuality and mutation accumulation and implies that mutational buildup could be rapid enough to contribute to the short-term evolutionary mechanisms that favor sexual reproduction.

Mutations in the Saccharomyces cerevisiae kinase Cbk1p lead to a fertility defect that can be suppressed by the absence of Brr1p or Mpt5p (Puf5p), proteins involved in RNA metabolism

In Saccharomyces cerevisiae the protein kinase Cbk1p is a member of the regulation of Ace2p and cellular morphogenesis (RAM) network that is involved in cell separation after cytokinesis, cell integrity, and cell polarity. In cell separation, the RAM network promotes the daughter cell-specific localization of the transcription factor Ace2p, resulting in the asymmetric transcription of genes whose products are necessary to digest the septum joining the mother and the daughter cell. RAM and SSD1 play a role in the maintenance of cell integrity. In the presence of a wild-type SSD1 gene, deletion of any RAM component causes cell lysis. We show here that some mutations of CBK1 also lead to a reduced fertility and a reduced expression of some of the mating type-specific genes. As polarized growth is an integral part of the mating process, we have isolated suppressors of the fertility defect. Among these, mutations in BRR1 or MPT5 lead to a restoration of fertility and a more-or-less pronounced restoration of polarity; they also show genetic interactions with SSD1. Our experiments reveal a multilayered system controlling aspects of cell separation, cell integrity, mating, and polarized growth.