1,737 research outputs found
The Long and Viscous Road: Uncovering Nuclear Diffusion Barriers in Closed Mitosis
During Saccharomyces cerevisiae closed mitosis, parental identity is
sustained by the asymmetric segregation of ageing factors. Such asymmetry has
been hypothesized to occur via diffusion barriers, constraining protein lateral
exchange in cellular membranes. Diffusion barriers have been extensively
studied in the plasma membrane, but their identity and organization within the
nucleus remain unknown. Here, we propose how sphingolipid domains, protein
rings, and morphological changes of the nucleus may coordinate to restrict
protein exchange between nuclear lobes. Our spatial stochastic model is based
on several lines of experimental evidence and predicts that, while a
sphingolipid domain and a protein ring could constitute the barrier during
early anaphase; a sphingolipid domain spanning the bridge between lobes during
late anaphase would be entirely sufficient. Additionally, we explore the
structural organization of plausible diffusion barriers. Our work shows how
nuclear diffusion barriers in closed mitosis may be emergent properties of
simple nanoscale biophysical interactions.Comment: 21 pages, 6 figures and supplementary material (including 8
additional figures and a Table
Time Variable Faraday Rotation Measures of 3C-273 and 3C-279
Multifrequency polarimetry with the VLBA confirms the previously reported
time-varying Faraday rotation measure (RM) in the quasar 3C-279. Variability in
the RM and electric vector position angle (EVPA) of the jet component (C4) is
seen making it an unreliable absolute EVPA calibrator. 3C-273 is also shown to
vary its RM structure on 1.5 year time-scales. Variation in the RM properties
of quasars may result from a Faraday screen which changes on time-scales of a
few years, or from the motion of jet components which sample spatial variations
in the screen. A new component emerging from the core of 3C-279 appears to be
starting to sample such a spatial variation. Future monitoring of this
component and its RM properties is suggested as a diagnostic of the narrow line
region in 3C-279. We also present a new method of EVPA calibration using the
VLA Monitoring Program.Comment: Accepted to ApJ Letters. 12 pages, 5 figure
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
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