Biological significance of nuclear localization of mitogen-activated protein kinase Pmk1 in fission yeast.

Mitogen-activated protein kinase (MAPK) signaling pathways play a fundamental role in the response of eukaryotic cells to environmental changes. Also, much evidence shows that the stimulus-dependent nuclear targeting of this class of regulatory kinases is crucial for adequate regulation of distinct cellular events. In the fission yeast Schizosaccharomyces pombe, the cell integrity MAPK pathway, whose central element is the MAPK Pmk1, regulates multiple processes such as cell wall integrity, vacuole fusion, cytokinesis, and ionic homeostasis. In non-stressed cells Pmk1 is constitutively localized in both cytoplasm and nucleus, and its localization pattern appears unaffected by its activation status or in response to stress, thus questioning the biological significance of the presence of this MAPK into the nucleus. We have addressed this issue by characterizing mutants expressing Pmk1 versions excluded from the cell nucleus and anchored to the plasma membrane in different genetic backgrounds. Although nuclear Pmk1 partially regulates cell wall integrity at a transcriptional level, membrane-tethered Pmk1 performs many of the biological functions assigned to wild type MAPK like regulation of chloride homeostasis, vacuole fusion, and cellular separation. However, we found that down-regulation of nuclear Pmk1 by MAPK phosphatases induced by the stress activated protein kinase pathway is important for the fine modulation of extranuclear Pmk1 activity. These results highlight the importance of the control of MAPK activity at subcellular level.

Complete genome sequence of Bifidobacterium breve CECT 7263, a strain isolated from human milk.

Bifidobacterium breve is an actinobacterium frequently isolated from colonic microbiota of breastfeeding babies. Here, we report the complete and annotated genome sequence of a B. breve strain isolated from human milk, B. breve CECT 7263. The genome sequence will provide new insights into the biology of this potential probiotic organism and will allow the characterization of genes related to beneficial properties.

Rga2 is a Rho2 GAP that regulates morphogenesis and cell integrity in S. pombe.

Schizosaccharomyces pombe Rho2 GTPase regulates alpha-D-glucan synthesis and acts upstream of Pck2 to activate the MAP kinase pathway for cell integrity. However, little is known about its regulation. Here we describe Rga2 as a Rho2 GTPase-activating protein (GAP) that regulates cell morphology. rga2+ gene is not essential for growth but its deletion causes longer and thinner cells whereas rga2+ overexpression causes shorter and broader cells. rga2+ overexpression also causes abnormal accumulation of Calcofluor-stained material and cell lysis, suggesting that it also participates in cell wall integrity. Rga2 localizes to growth tips and septum region. The N-terminal region of the protein is required for its correct localization whereas the PH domain is necessary exclusively for Rga2 localization to the division area. Also, Rga2 localization depends on polarity markers and on actin polymerization. Rga2 interacts with Rho2 and possesses in vitro and in vivo GAP activity for this GTPase. Accordingly, rga2Delta cells contain more alpha-D-glucan and therefore partially suppress the thermosensitivity of mok1-664 cells, which have a defective alpha-D-glucan synthase. Additionally, genetic interactions and biochemical analysis suggest that Rga2 regulates Rho2-Pck2 interaction and might participate in the regulation of the MAPK cell integrity pathway.

Rga6 is a Fission Yeast Rho GAP Involved in Cdc42 Regulation of Polarized Growth.

Active Cdc42 is essential for the establishment of polarized growth. This GTPase is negatively regulated by the GTPase-activating proteins (GAPs), which are important for the spatial specificity of Cdc42 function. Rga4 is the only GAP described as negative regulator of fission yeast Cdc42. We report here that Rga6 is another fission yeast Cdc42 GAP which shares some functions with Rga4. Cells lacking Rga6 are viable but slightly shorter and broader than wild type, and cells lacking Rga6 and Rga4 simultaneously are rounded. In these cells, active Cdc42 is observed all around the membrane. These additive effects indicate that both GAPs collaborate in the spatial regulation of active Cdc42. Rga6 localizes to the plasma membrane forming clusters different from those formed by Rga4. A polybasic region at the Rga6 C-terminus is responsible for its membrane localization. Rga6-GFP fluorescence decreases considerably at the growing tips, and this decrease is dependent on the actin cables. Notably, in the absence of Rga6 the amplitude of active Cdc42 oscillations at the tips decreases, and less GTP-Cdc42 accumulates at the new end of the cells. We propose here that Rga6 collaborates with Rga4 to spatially restrict active Cdc42 at the cell tips and maintain cell dimensions.

Pob1 participates in the Cdc42 regulation of fission yeast actin cytoskeleton.

Rho GTPases regulate the actin cytoskeleton in all eukaryotes. Fission yeast Cdc42 is involved in actin cable assembly and formin For3 regulation. We isolated cdc42-879 as a thermosensitive strain with actin cable and For3 localization defects. In a multicopy suppressor screening, we identified pob1(+) as suppressor of cdc42-879 thermosensitivity. Pob1 overexpression also partially restores actin cables and localization of For3 in the mutant strain. Pob1 interacts with Cdc42 and this GTPase regulates Pob1 localization and/or stability. The C-terminal pleckstrin homology (PH) domain of Pob1 is required for Cdc42 binding. Pob1 also binds to For3 through its N-terminal sterile alpha motif (SAM) domain and contributes to the formin localization at the cell tips. The previously described pob1-664 mutant strain (Mol. Biol. Cell. 10, 2745-2757, 1999), which carries a mutation in the PH domain, as well as pob1 mutant strains in which Pob1 lacks the N-terminal region (pob1DeltaN) or the SAM domain (pob1DeltaSAM), have cytoskeletal defects similar to that of cdc42-879 cells. Expression of constitutively active For3DAD* partially restores actin organization in cdc42-879, pob1-664, pob1DeltaN, and pob1DeltaSAM. Therefore, we propose that Pob1 is required for For3 localization to the tips and facilitates Cdc42-mediated relief of For3 autoinhibition to stimulate actin cable formation.