SIN and the art of splitting the fission yeast cell.

The septation initiation network (SIN) triggers the onset of cytokinesis in the fission yeast Schizosaccharomyces pombe by promoting contraction of the medially placed F-actin ring. SIN signaling is regulated by the polo-like kinase plo1p and by cdc2p, the initiator of mitosis, and its activation is co-ordinated with other events in mitosis to ensure that cytokinesis does not begin until chromosomes have been separated. Though the SIN controls the contractile ring, the signal originates from the poles of the mitotic spindle. Recent studies suggest that the spindle pole body may act as a dynamic assembly site for active SIN signaling complexes. In the budding yeast Saccharomyces cerevisiae the counterpart of the SIN, called the MEN, mediates both mitotic exit and cytokinesis, in part through regulating activation of the phosphoprotein phosphatase Cdc14p. Flp1p, the S. pombe ortholog of Cdc14p, is not essential for mitotic exit, but may contribute to an orderly mitosis-G1 transition by regulating the destruction of the mitotic inducer cdc25p

Phosphorylation of the Cdc42 exchange factor Cdc24 by the PAK-like kinase Cla4 may regulate polarized growth in yeast.

Rho-type GTPases control many cytoskeletal rearrangements, but their regulation remains poorly understood. Here, we show that in S. cerevisiae, activation of the CDK Cdc28-Cln2 at bud emergence triggers relocalization of Cdc24, the GEF for Cdc42, from the nucleus to the polarization site, where it is stably maintained by binding to the adaptor Bem1. Locally activated Cdc42 then polarizes the cytoskeleton in a manner dependent on its effectors Bni1 and the PAK-like kinase Cla4. In addition, Cla4 induces phosphorylation of Cdc24, leading to its dissociation from Bem1 at bud tips, thereby ending polarized bud growth in vivo. Our results thus suggest a dynamic temporal and spatial regulation of the Cdc42 module: Cdc28-Cln triggers actin polarization by activating Cdc42, which in turn restricts its own activation via a negative feedback loop acting on its GEF Cdc24

Gef1p, a New Guanine Nucleotide Exchange Factor for Cdc42p, Regulates Polarity in Schizosaccharomyces pombe

Schizosaccharomyces pombe cdc42(+) regulates cell morphology and polarization of the actin cytoskeleton. Scd1p/Ral1p is the only described guanine nucleotide exchange factor (GEF) for Cdc42p in S. pombe. We have identified a new GEF, named Gef1p, specifically regulating Cdc42p. Gef1p binds to inactive Cdc42p but not to other Rho GTPases in two-hybrid assays. Overexpression of gef1(+) increases specifically the GTP-bound Cdc42p, and Gef1p is capable of stimulating guanine nucleotide exchange of Cdc42p in vitro. Overexpression of gef1(+) causes changes in cell morphology similar to those caused by overexpression of the constitutively active cdc42G12V allele. Gef1p localizes to the septum. gef1(+) deletion is viable but causes a mild cell elongation and defects in bipolar growth and septum formation, suggesting a role for Gef1p in the control of cell polarity and cytokinesis. The double mutant gef1Δ scd1Δ is not viable, indicating that they share an essential function as Cdc42p activators. However, both deletion and overexpression of either gef1(+) or scd1(+) causes different morphological phenotypes, which suggest different functions. Genetic evidence revealed a link between Gef1p and the signaling pathway of Shk1/Orb2p and Orb6p. In contrast, no genetic interaction between Gef1p and Shk2p-Mkh1p pathway was observed

Synthetic Lethal Analysis Implicates Ste20p, a p21-activated Protein Kinase, in Polarisome Activation

The p21-activated kinases Ste20p and Cla4p carry out undefined functions that are essential for viability during budding in Saccharomyces cerevisiae. To gain insight into the roles of Ste20p, we have used a synthetic lethal mutant screen to identify additional genes that are required in the absence of Cla4p. Altogether, we identified 65 genes, including genes with roles in cell polarity, mitosis, and cell wall maintenance. Herein, we focus on a set that defines a function carried out by Bni1p and several of its interacting proteins. We found that Bni1p and a group of proteins that complex with Bni1p (Bud6p, Spa2p, and Pea2p) are essential in a cla4Δ mutant background. Bni1p, Bud6p, Spa2, and Pea2p are members of a group of polarity determining proteins referred to as the polarisome. Loss of polarisome proteins from a cla4Δ strain causes cells to form elongated buds that have mislocalized septin rings. In contrast, other proteins that interact with or functionally associate with Bni1p and have roles in nuclear migration and cytokinesis, including Num1p and Hof1p, are not essential in the absence of Cla4p. Finally, we have found that Bni1p is phosphorylated in vivo, and a substantial portion of this phosphorylation is dependent on STE20. Together, these results suggest that one function of Ste20p may be to activate the polarisome complex by phosphorylation of Bni1p

Genomics of tolerance to abiotic stress in the Triticeae

Genomics platforms offer unprecedented opportunities to identify, select and in some cases clone the genes and the quantitative trait loci (QTLs) that govern the tolerance of Triticeae to abiotic stresses and, consequently, grain yield. Transcriptome profiling and the other \u201comics\u201d platforms provide further information to unravel gene functions and validate the role of candidate genes. This review provides a synopsis of the main results on the studies that have investigated the genomics of Triticeae crops under conditions of abiotic constraints. With their rich biodiversity and high functional plasticity in response to environmental stresses, Triticeae crops provide an ideal ground for taking full advantage of the opportunities offered by genomics approaches. Ultimately, the practical impact of the knowledge and materials generated through genomics-based approaches will depend on their integration and exploitation within the extant breeding programs