802 research outputs found
Ultrastructural plasma membrane asymmetries in tension and curvature promote yeast cell fusion.
Cell-cell fusion is central for sexual reproduction, and generally involves gametes of different shapes and sizes. In walled fission yeast Schizosaccharomyces pombe, the fusion of h+ and h- isogametes requires the fusion focus, an actin structure that concentrates glucanase-containing vesicles for cell wall digestion. Here, we present a quantitative correlative light and electron microscopy (CLEM) tomographic dataset of the fusion site, which reveals the fusion focus ultrastructure. Unexpectedly, gametes show marked asymmetries: a taut, convex plasma membrane of h- cells progressively protrudes into a more slack, wavy plasma membrane of h+ cells. Asymmetries are relaxed upon fusion, with observations of ramified fusion pores. h+ cells have a higher exo-/endocytosis ratio than h- cells, and local reduction in exocytosis strongly diminishes membrane waviness. Reciprocally, turgor pressure reduction specifically in h- cells impedes their protrusions into h+ cells and delays cell fusion. We hypothesize that asymmetric membrane conformations, due to differential turgor pressure and exocytosis/endocytosis ratios between mating types, favor cell-cell fusion
Condensation of the fusion focus by the intrinsically disordered region of the formin Fus1 is essential for cell-cell fusion.
Secretory vesicle clusters transported on actin filaments by myosin V motors for local secretion underlie various cellular processes, such as neurotransmitter release at neuronal synapses, <sup>1</sup> hyphal steering in filamentous fungi, <sup>2</sup> <sup>,</sup> <sup>3</sup> and local cell wall digestion preceding the fusion of yeast gametes. <sup>4</sup> During fission yeast Schizosaccharomyces pombe gamete fusion, the actin fusion focus assembled by the formin Fus1 concentrates secretory vesicles carrying cell wall digestive enzymes. <sup>5-7</sup> The position and coalescence of the vesicle focus are controlled by local signaling and actin-binding proteins to prevent inappropriate cell wall digestion that would cause lysis, <sup>6</sup> <sup>,</sup> <sup>8-10</sup> but the mechanisms of focusing have been elusive. Here, we show that the regulatory N terminus of Fus1 contains an intrinsically disordered region (IDR) that mediates Fus1 condensation in vivo and forms dense assemblies that exclude ribosomes. Fus1 lacking its IDR fails to concentrate in a tight focus and causes cell lysis during attempted cell fusion. Remarkably, the replacement of Fus1 IDR with a heterologous low-complexity region that forms molecular condensates fully restores Fus1 focusing and function. By contrast, the replacement of Fus1 IDR with a domain that forms more stable oligomers restores focusing but poorly supports cell fusion, suggesting that condensation is tuned to yield a selectively permeable structure. We propose that condensation of actin structures by an IDR may be a general mechanism for actin network organization and the selective local concentration of secretory vesicles
Onion morphology and microstructure of polyhedral serpentine
We describe the shape and internal structure of polyhedral spheroids found in serpentinized peridotites which correspond to a new serpentine microstructure. Serpentine spheroids resemble geodesic domes made of c. 160 to 180 triangular facets. At facet edges, the nested layers bend by c. 14° along their three crystallographic directions, resulting in an onion-like structure with lateral continuity of the layers. The stacking of the serpentine layers within sectors is controlled by interlayer bonding. These polyhedral onions are made of a novel type of spherical nanostructure for layered materials
Correlation length of X-ray brightest Abell clusters
We compute the cluster auto-correlation function of an X-ray
flux limited sample of Abell clusters (XBACs, \cite{ebe}). For the total XBACs
sample we find a power-law fit with Mpc
hand consistent with the results of Abell
clusters. We also analyze for subsamples defined by different
X-ray luminosity thresholds where we find a weak tendency of larger values of
with increasing X-ray luminosity although with a low statistical
significance. In the different subsamples analyzed we find Mpc
h and . Our analysis suggests that cluster X-ray
luminosities may be used for a reliable confrontation of cluster spatial
distribution properties in models and observations.Comment: Accepted for publication in Astrophysical Journa
Chronicles of Oklahoma
Notes and Documents section for Volume 30, Number 3, Fall 1952. It includes documents about the first telephone line in Oklahoma, the history of Little Robe Township in Ellis County, an description of a book published by the '89ers, some notes on the organization of the intertribal Council of the Five Tribes, and documents paying tribute to members of the historical community who passed
Nonlocal aspects of -symmetries and ODEs reduction
A reduction method of ODEs not possessing Lie point symmetries makes use of
the so called -symmetries (C. Muriel and J. L. Romero, \emph{IMA J.
Appl. Math.} \textbf{66}, 111-125, 2001). The notion of covering for an ODE
is used here to recover -symmetries of as
nonlocal symmetries. In this framework, by embedding into a
suitable system determined by the function ,
any -symmetry of can be recovered by a local symmetry of
. As a consequence, the reduction method of Muriel and
Romero follows from the standard method of reduction by differential invariants
applied to .Comment: 13 page
The luminosity function of field galaxies
Schmidt's method for construction of luminosity function of galaxies is
generalized by taking into account the dependence of density of galaxies from
the distance in the near Universe. The logarithmical luminosity function (LLF)
of field galaxies depending on morphological type is constructed. We show that
the LLF for all galaxies, and also separately for elliptical and lenticular
galaxies can be presented by Schechter function in narrow area of absolute
magnitudes. The LLF of spiral galaxies was presented by Schechter function for
enough wide area of absolute magnitudes: . Spiral galaxies differ slightly by
parameter . At transition from early spirals to the late spirals parameter in
Schechter function is reduced. The reduction of mean luminosity of galaxies is
observed at transition from elliptical galaxies to lenticular galaxies, to
early spiral galaxies, and further, to late spiral galaxies, in a bright end, .
The completeness and the average density of samples of galaxies of different
morphological types are estimated. In the range the mean number density of all
galaxies is equal 0.127 Mpc-3.Comment: 14 page, 8 figures, to appear in Astrophysic
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