From START to FINISH : the influence of osmotic stress on the cell cycle

Peer reviewedPublisher PD

Genome-wide prediction and functional characterization of the genetic basis of autism spectrum disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder with a strong genetic basis. Yet, only a small fraction of potentially causal genes - about 65 genes out of an estimated several hundred - are known with strong genetic evidence from sequencing studies. We developed a complementary machine-learning approach based on a human brain-specific gene network to present a genome-wide prediction of autism risk genes, including hundreds of candidates for which there is minimal or no prior genetic evidence. Our approach was validated in a large independent case-control sequencing study. Leveraging these genome-wide predictions and the brain-specific network, we demonstrated that the large set of ASD genes converges on a smaller number of key pathways and developmental stages of the brain. Finally, we identified likely pathogenic genes within frequent autism-associated copy-number variants and proposed genes and pathways that are likely mediators of ASD across multiple copy-number variants. All predictions and functional insights are available at http://asd.princeton.edu

The polarity and dynamics of microtubule assembly in the budding yeast Saccharomyces cerevisiae

Microtubule assembly in Saccharomyces cerevisiae is initiated from sites within spindle pole bodies (SPBs) in the nuclear envelope. Microtubule plus ends are thought to be organized distal to the SPBs, while minus ends are proximal. Several hypotheses for the function of microtubule motor proteins in force generation and regulation of microtubule assembly propose that assembly and disassembly occur at minus ends as well as at plus ends. Here we analyse microtubule assembly relative to the SPBs in haploid yeast cells expressing green fluorescent protein fused to α-tubulin, a microtubule subunit. Throughout the cell cycle, analysis of fluorescent speckle marks on cytoplasmic astral microtubules reveals that there is no detectable assembly or disassembly at minus ends. After laser-photobleaching, metaphase spindles recover about 63% of the bleached fluorescence, with a half-life of about 1 minute. After anaphase onset, photobleached marks in the interpolar spindle are persistent and do not move relative to the SPBs. In late anaphase, the elongated spindles disassemble at the microtubule plus ends. These results show for astral and anaphase interpolar spindle microtubules, and possibly for metaphase spindle microtubules, that microtubule assembly and disassembly occur at plus, and not minus, ends

Phosphorylation of Hsl1 by Hog1 leads to a G(2) arrest essential for cell survival at high osmolarity

Control of cell cycle progression by stress-activated protein kinases (SAPKs) is essential for cell adaptation to extracellular stimuli. Exposure of yeast to osmostress leads to activation of the Hog1 SAPK, which controls cell cycle at G(1) by the targeting of Sic1. Here, we show that survival to osmostress also requires regulation of G(2) progression. Activated Hog1 interacts and directly phosphorylates a residue within the Hsl7-docking site of the Hsl1 checkpoint kinase, which results in delocalization of Hsl7 from the septin ring and leads to Swe1 accumulation. Upon Hog1 activation, cells containing a nonphosphorylatable Hsl1 by Hog1 are unable to promote Hsl7 delocalization, fail to arrest at G(2) and become sensitive to osmostress. Together, we present a novel mechanism that regulates the Hsl1–Hsl7 complex to integrate stress signals to mediate cell cycle arrest and, demonstrate that a single MAPK coordinately modulates different cell cycle checkpoints to improve cell survival upon stress

Yeast formin Bni1p has multiple localization regions that function in polarized growth and spindle orientation

There are four distinct localization domains in formin Bni1p of budding yeast. Analysis of the functions of the domains in the actin cytoskeleton and in spindle orientation reveals unexpected complexity in the mechanism of formin localization and function

A phosphatase threshold sets the level of Cdk1 activity in early mitosis in budding yeast

Entry into mitosis is initiated by synthesis of cyclins, which bind and activate cyclin-dependent kinase 1 (Cdk1). Cyclin synthesis is gradual, yet activation of Cdk1 occurs in a stepwise manner: a low level of Cdk1 activity is initially generated that triggers early mitotic events, which is followed by full activation of Cdk1. Little is known about how stepwise activation of Cdk1 is achieved. A key regulator of Cdk1 is the Wee1 kinase, which phosphorylates and inhibits Cdk1. Wee1 and Cdk1 show mutual regulation: Cdk1 phosphorylates Wee1, which activates Wee1 to inhibit Cdk1. Further phosphorylation events inactivate Wee1. We discovered that a specific form of protein phosphatase 2A (PP2A(Cdc55)) opposes the initial phosphorylation of Wee1 by Cdk1. In vivo analysis, in vitro reconstitution, and mathematical modeling suggest that PP2A(Cdc55) sets a threshold that limits activation of Wee1, thereby allowing a low constant level of Cdk1 activity to escape Wee1 inhibition in early mitosis. These results define a new role for PP2A(Cdc55) and reveal a systems-level mechanism by which dynamically opposed kinase and phosphatase activities can modulate signal strength