Evolution of precipitates, in particular cruciform and cuboid particles, during simulated direct charging of thin slab cast vanadium microalloyed steels

A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route using processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80mm ( in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after 4th rolling pass and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels, which have undergone conventional processing. The four low carbon steels (~0.06wt%) which were studied contained 0.1wt%V (V-N), 0.1wt%V and 0.010wt%Ti (V-Ti), 0.1wt%V and 0.03wt%Nb (V-Nb), and 0.1wt%V, 0.03wt%Nb and 0.007wt%Ti (V-Nb-Ti). Steels V-N and V-Ti contained around 0.02wt% N, while the other two contained about 0.01wt%N. The as-cast steels were heated at three equalising temperatures of 1050C, 1100C or 1200C and held for 30-60 minutes prior to rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the precipitates. In the as-cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in Steel V-Ti, but they appeared only in the other steels after equalization. In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements which would have been expected to precipitate in ferrite as dispersion hardening particles

Budding Yeast Dma Proteins Control Septin Dynamics and the Spindle Position Checkpoint by Promoting the Recruitment of the Elm1 Kinase to the Bud Neck

The first step towards cytokinesis in budding yeast is the assembly of a septin ring at the future site of bud emergence. Integrity of this ring is crucial for cytokinesis, proper spindle positioning, and the spindle position checkpoint (SPOC). This checkpoint delays mitotic exit and cytokinesis as long as the anaphase spindle does not properly align with the division axis. SPOC signalling requires the Kin4 protein kinase and the Kin4-regulating Elm1 kinase, which also controls septin dynamics. Here, we show that the two redundant ubiquitin-ligases Dma1 and Dma2 control septin dynamics and the SPOC by promoting the efficient recruitment of Elm1 to the bud neck. Indeed, dma1 dma2 mutant cells show reduced levels of Elm1 at the bud neck and Elm1-dependent activation of Kin4. Artificial recruitment of Elm1 to the bud neck of the same cells is sufficient to re-establish a normal septin ring, proper spindle positioning, and a proficient SPOC response in dma1 dma2 cells. Altogether, our data indicate that septin dynamics and SPOC function are intimately linked and support the idea that integrity of the bud neck is crucial for SPOC signalling

Structural Comparison of Human Mammalian Ste20-Like Kinases

BACKGROUND: The serine/threonine mammalian Ste-20 like kinases (MSTs) are key regulators of apoptosis, cellular proliferation as well as polarization. Deregulation of MSTs has been associated with disease progression in prostate and colorectal cancer. The four human MSTs are regulated differently by C-terminal regions flanking the catalytic domains. PRINCIPAL FINDINGS: We have determined the crystal structure of kinase domain of MST4 in complex with an ATP-mimetic inhibitor. This is the first structure of an inactive conformation of a member of the MST kinase family. Comparison with active structures of MST3 and MST1 revealed a dimeric association of MST4 suggesting an activation loop exchanged mechanism of MST4 auto-activation. Together with a homology model of MST2 we provide a comparative analysis of the kinase domains for all four members of the human MST family. SIGNIFICANCE: The comparative analysis identified new structural features in the MST ATP binding pocket and has also defined the mechanism for autophosphorylation. Both structural features may be further explored for inhibitors design. ENHANCED VERSION: This article can also be viewed as an enhanced version in which the text of the article is integrated with interactive 3D representations and animated transitions. Please note that a web plugin is required to access this enhanced functionality. Instructions for the installation and use of the web plugin are available in Text S1

Lte1 promotes mitotic exit by controlling the localization of the spindle position checkpoint kinase Kin4

For a daughter cell to receive a complete genomic complement, it is essential that the mitotic spindle be positioned accurately within the cell. In budding yeast, a signaling system known as the spindle position checkpoint (SPOC) monitors spindle position and regulates the activity of the mitotic exit network (MEN), a GTPase signaling pathway that promotes exit from mitosis. The protein kinase Kin4 is a central component of the spindle position checkpoint. Kin4 primarily localizes to the mother cell and associates with spindle pole bodies (SPBs) located in the mother cell to inhibit MEN signaling. In contrast, the kinase does not associate with the SPB in the bud. Thus, only when a MEN bearing SPB leaves the mother cell and the spindle is accurately positioned along the mother–bud axis can MEN signaling occur and cell division proceed. Here, we describe a mechanism ensuring that Kin4 only associates with mother cell-located SPBs. The bud-localized MEN regulator Lte1, whose molecular function has long been unclear, prevents Kin4 that escapes into the bud from associating with SPBs in the daughter cell.Howard Hughes Medical InstituteNational Science Foundation (U.S.) (Predoctoral Fellowship)National Institutes of Health (U.S.) (Grant GM056800

The Stress-activated Mitogen-activated Protein Kinase Signaling Cascade Promotes Exit from Mitosis

In budding yeast, a signaling network known as the mitotic exit network (MEN) triggers exit from mitosis. We find that hypertonic stress allows MEN mutants to exit from mitosis in a manner dependent on the high osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase cascade. The HOG pathway drives exit from mitosis in MEN mutants by promoting the activation of the MEN effector, the protein phosphatase Cdc14. Activation of Cdc14 depends on the Cdc14 early anaphase release network, a group of proteins that functions in parallel to the MEN to promote Cdc14 function. Notably, exit from mitosis is promoted by the signaling branch defined by the Sho1 osmosensing system, but not by the Sln1 osmosensor of the HOG pathway. Our results suggest that the stress MAP kinase pathway mobilizes programs to promote completion of the cell cycle and entry into G(1) under unfavorable conditions

Different Levels of Bfa1/Bub2 GAP Activity Are Required to Prevent Mitotic Exit of Budding Yeast Depending on the Type of Perturbations

In budding yeast, Tem1 is a key regulator of mitotic exit. Bfa1/Bub2 stimulates Tem1 GTPase activity as a GTPase-activating protein (GAP). Lte1 possesses a guanine-nucleotide exchange factor (GEF) domain likely for Tem1. However, recent observations showed that cells may control mitotic exit without either Lte1 or Bfa1/Bub2 GAP activity, obscuring how Tem1 is regulated. Here, we assayed BFA1 mutants with varying GAP activities for Tem1, showing for the first time that Bfa1/Bub2 GAP activity inhibits Tem1 in vivo. A decrease in GAP activity allowed cells to bypass mitotic exit defects. Interestingly, different levels of GAP activity were required to prevent mitotic exit depending on the type of perturbation. Although essential, more Bfa1/Bub2 GAP activity was needed for spindle damage than for DNA damage to fully activate the checkpoint. Conversely, Bfa1/Bub2 GAP activity was insufficient to delay mitotic exit in cells with misoriented spindles. Instead, decreased interaction of Bfa1 with Kin4 was observed in BFA1 mutant cells with a defective spindle position checkpoint. These findings demonstrate that there is a GAP-independent surveillance mechanism of Bfa1/Bub2, which, together with the GTP/GDP switch of Tem1, may be required for the genomic stability of cells with misaligned spindles