SAS-4 is recruited to a dynamic structure in newly forming centrioles that is stabilized by the gamma-tubulin-mediated addition of centriolar microtubules.

Centrioles are surrounded by pericentriolar material (PCM), which is proposed to promote new centriole assembly by concentrating gamma-tubulin. Here, we quantitatively monitor new centriole assembly in living Caenorhabditis elegans embryos, focusing on the conserved components SAS-4 and SAS-6. We show that SAS-4 and SAS-6 are coordinately recruited to the site of new centriole assembly and reach their maximum levels during S phase. Centriolar SAS-6 is subsequently reduced by a mechanism intrinsic to the early assembly pathway that does not require progression into mitosis. Centriolar SAS-4 remains in dynamic equilibrium with the cytoplasmic pool until late prophase, when it is stably incorporated in a step that requires gamma-tubulin and microtubule assembly. These results indicate that gamma-tubulin in the PCM stabilizes the nascent daughter centriole by promoting microtubule addition to its outer wall. Such a mechanism may help restrict new centriole assembly to the vicinity of preexisting parent centrioles that recruit PCM

DAF-18/PTEN signals through AAK-1/AMPK to inhibit MPK-1/MAPK in feedback control of germline stem cell proliferation

Under replete growth conditions, abundant nutrient uptake leads to the systemic activation of insulin/IGF-1 signalling (IIS) and the promotion of stem cell growth/proliferation. Activated IIS can stimulate the ERK/MAPK pathway, the activation of which also supports optimal stem cell proliferation in various systems. Stem cell proliferation rates can further be locally refined to meet the resident tissue’s need for differentiated progeny. We have recently shown that the accumulation of mature oocytes in the C. elegans germ line, through DAF-18/PTEN, inhibits adult germline stem cell (GSC) proliferation, despite high systemic IIS activation. We show here that this feedback occurs through a novel cryptic signalling pathway that requires PAR-4/LKB1, AAK-1/AMPK and PAR-5/14-3-3 to inhibit the activity of MPK-1/MAPK, antagonize IIS, and inhibit both GSC proliferation and the production of additional oocytes. Interestingly, our results imply that DAF-18/PTEN, through PAR-4/LKB1, can activate AAK-1/AMPK in the absence of apparent energy stress. As all components are conserved, similar signalling cascades may regulate stem cell activities in other organisms and be widely implicated in cancer

Cell Size: Chromosomes Get Slapped by a Midzone Ruler

SummarySpatial and temporal coordination of mitotic events has been generally attributed to the coincidental outcome of increasing cyclin-dependent kinase activity. A recent study reports that mitotic events and structures previously considered to be independently controlled are capable of trans-regulation to ensure genomic integrity

Nucleosomal composition at the centromere: a numbers game

The Centromere is a unique chromosomal locus where the kinetochore is formed to mediate faithful chromosome partitioning, thus maintaining ploidy during cell division. Centromere identity is inherited via an epigenetic mechanism involving a histone H3 variant, called centromere protein A (CENP-A) which replaces H3 in centromeric chromatin. In spite of extensive efforts in field of centromere biology during the past decade, controversy persists over the structural nature of the CENP-A-containing epigenetic mark, both at nucleosomal and chromatin levels. Here, we review recent findings and hypotheses regarding the structure of CENP-A-containing complexes

SAS-4 is recruited to a dynamic structure in newly forming centrioles that is stabilized by the γ-tubulin–mediated addition of centriolar microtubules

Centrioles are surrounded by pericentriolar material (PCM), which is proposed to promote new centriole assembly by concentrating γ-tubulin. Here, we quantitatively monitor new centriole assembly in living Caenorhabditis elegans embryos, focusing on the conserved components SAS-4 and SAS-6. We show that SAS-4 and SAS-6 are coordinately recruited to the site of new centriole assembly and reach their maximum levels during S phase. Centriolar SAS-6 is subsequently reduced by a mechanism intrinsic to the early assembly pathway that does not require progression into mitosis. Centriolar SAS-4 remains in dynamic equilibrium with the cytoplasmic pool until late prophase, when it is stably incorporated in a step that requires γ-tubulin and microtubule assembly. These results indicate that γ-tubulin in the PCM stabilizes the nascent daughter centriole by promoting microtubule addition to its outer wall. Such a mechanism may help restrict new centriole assembly to the vicinity of preexisting parent centrioles that recruit PCM

A theoretical model of cytokinesis implicates feedback between membrane curvature and cytoskeletal organization in asymmetric cytokinetic furrowing

During cytokinesis, the cell undergoes a dramatic shape change as it divides into two daughter cells. Cell shape changes in cytokinesis are driven by a cortical ring rich in actin filaments and nonmuscle myosin II. The ring closes via actomyosin contraction coupled with actin depolymerization. Of interest, ring closure and hence the furrow ingression are nonconcentric (asymmetric) within the division plane across Metazoa. This nonconcentricity can occur and persist even without preexisting asymmetric cues, such as spindle placement or cellular adhesions. Cell-autonomous asymmetry is not explained by current models. We combined quantitative high-resolution live-cell microscopy with theoretical modeling to explore the mechanistic basis for asymmetric cytokinesis in the Caenorhabditis elegans zygote, with the goal of uncovering basic principles of ring closure. Our theoretical model suggests that feedback among membrane curvature, cytoskeletal alignment, and contractility is responsible for asymmetric cytokinetic furrowing. It also accurately predicts experimental perturbations of conserved ring proteins. The model further suggests that curvature-mediated filament alignment speeds up furrow closure while promoting energy efficiency. Collectively our work underscores the importance of membrane–cytoskeletal anchoring and suggests conserved molecular mechanisms for this activity

Cross-linkers both drive and brake cytoskeletal remodeling and furrowing in cytokinesis.

Cell shape changes such as cytokinesis are driven by the actomyosin contractile cytoskeleton. The molecular rearrangements that bring about contractility in nonmuscle cells are currently debated. Specifically, both filament sliding by myosin motors, as well as cytoskeletal cross-linking by myosins and nonmotor cross-linkers, are thought to promote contractility. Here we examined how the abundance of motor and nonmotor cross-linkers affects the speed of cytokinetic furrowing. We built a minimal model to simulate contractile dynamics in the Caenorhabditis elegans zygote cytokinetic ring. This model predicted that intermediate levels of nonmotor cross-linkers are ideal for contractility; in vivo, intermediate levels of the scaffold protein anillin allowed maximal contraction speed. Our model also demonstrated a nonlinear relationship between the abundance of motor ensembles and contraction speed. In vivo, thorough depletion of nonmuscle myosin II delayed furrow initiation, slowed F-actin alignment, and reduced maximum contraction speed, but partial depletion allowed faster-than-expected kinetics. Thus, cytokinetic ring closure is promoted by moderate levels of both motor and nonmotor cross-linkers but attenuated by an over-abundance of motor and nonmotor cross-linkers. Together, our findings extend the growing appreciation for the roles of cross-linkers in cytokinesis and reveal that they not only drive but also brake cytoskeletal remodeling

Luminous K-band Selected Quasars from UKIDSS

The largest K-band flux-limited sample of luminous quasars to date has been constructed from the UKIDSS Large Area Survey Early Data Release, covering an effective area of 12.8 deg^2. Exploiting the K-band excess of all quasars with respect to foreground stars, including quasars experiencing dust reddening and objects with non-standard SEDs, a list of targets suitable for spectroscopic follow-up observations with the AAOmega multi-object spectrograph is constructed, resulting in more than 200 confirmed AGN. KX-selection successfully identifies as quasar candidates objects that are excluded from the SDSS quasar selection algorithm due to their colours being consistent with the stellar locus in optical colour space (with the space density of the excluded objects agreeing well with results from existing completeness analyses). Nearly half of the KX-selected quasars with K<17.0 at z<3 are too faint in the i-band to have been targeted by the SDSS quasar selection algorithm, revealing a large population of quasars with red i-K colours. The majority of these objects have significant amounts of host galaxy light contributing to their K-band magnitudes, consistent with previous predictions. The remaining objects are morphologically stellar and have colours consistent with quasars experiencing SMC-type reddening with 0.10<E(B-V)<0.25. The i-K colour distribution indicates that <10 per cent of the quasar population is missing from this K-band selected sample due to dust reddening, and comparisons with simulations strongly favour an obscured fraction of <20 per cent. (Abridged)Comment: 22 pages, 18 figures, accepted for publication in MNRAS. Resolution of Figure 2 has been reduced for astro-ph submission. The complete Table 5 can be found at http://www.aip.de/People/nmaddox/KX/catalogue.tx

Functional genomics identifies a Myb domain–containing protein family required for assembly of CENP-A chromatin

Nucleosomes containing the centromere-specific histone H3 variant centromere protein A (CENP-A) create the chromatin foundation for kinetochore assembly. To understand the mechanisms that selectively target CENP-A to centromeres, we took a functional genomics approach in the nematode Caenorhabditis elegans, in which failure to load CENP-A results in a signature kinetochore-null (KNL) phenotype. We identified a single protein, KNL-2, that is specifically required for CENP-A incorporation into chromatin. KNL-2 and CENP-A localize to centromeres throughout the cell cycle in an interdependent manner and coordinately direct chromosome condensation, kinetochore assembly, and chromosome segregation. The isolation of KNL-2–associated chromatin coenriched CENP-A, indicating their close proximity on DNA. KNL-2 defines a new conserved family of Myb DNA-binding domain–containing proteins. The human homologue of KNL-2 is also specifically required for CENP-A loading and kinetochore assembly but is only transiently present at centromeres after mitotic exit. These results implicate a new protein class in the assembly of centromeric chromatin and suggest that holocentric and monocentric chromosomes share a common mechanism for CENP-A loading

Regional variation of microtubule flux reveals microtubule organization in the metaphase meiotic spindle

Continuous poleward movement of tubulin is a hallmark of metaphase spindle dynamics in higher eukaryotic cells and is essential for stable spindle architecture and reliable chromosome segregation. We use quantitative fluorescent speckle microscopy to map with high resolution the spatial organization of microtubule flux in Xenopus laevis egg extract meiotic spindles. We find that the flux velocity decreases near spindle poles by ∼20%. The regional variation is independent of functional kinetochores and centrosomes and is suppressed by inhibition of dynein/dynactin, kinesin-5, or both. Statistical analysis reveals that tubulin flows in two distinct velocity modes. We propose an association of these modes with two architecturally distinct yet spatially overlapping and dynamically cross-linked arrays of microtubules: focused polar microtubule arrays of a uniform polarity and slower flux velocities are interconnected by a dense barrel-like microtubule array of antiparallel polarities and faster flux velocities