107 research outputs found
Evolution-guided functional analyses reveal diverse antiviral specificities encoded by IFIT1 genes in mammals.
IFIT (interferon-induced with tetratricopeptide repeats) proteins are critical mediators of mammalian innate antiviral immunity. Mouse IFIT1 selectively inhibits viruses that lack 2'O-methylation of their mRNA 5' caps. Surprisingly, human IFIT1 does not share this antiviral specificity. Here, we resolve this discrepancy by demonstrating that human and mouse IFIT1 have evolved distinct functions using a combination of evolutionary, genetic and virological analyses. First, we show that human IFIT1 and mouse IFIT1 (renamed IFIT1B) are not orthologs, but are paralogs that diverged >100 mya. Second, using a yeast genetic assay, we show that IFIT1 and IFIT1B proteins differ in their ability to be suppressed by a cap 2'O-methyltransferase. Finally, we demonstrate that IFIT1 and IFIT1B have divergent antiviral specificities, including the discovery that only IFIT1 proteins inhibit a virus encoding a cap 2'O-methyltransferase. These functional data, combined with widespread turnover of mammalian IFIT genes, reveal dramatic species-specific differences in IFIT-mediated antiviral repertoires
Binding and Relocalization of PKR 1 by Murine Cytomegalovirus
Many viruses have evolved mechanisms to evade the repression of translation mediated by protein kinase R (PKR). In the case of murine cytomegalovirus (MCMV), the protein products of two essential genes, m142 and m143, bind to dsRNA and block phosphorylation of PKR and eukaryotic initiation factor 2. A distinctive feature of MCMV is that two proteins are required to block PKR activation whereas other viral dsRNA-binding proteins that prevent PKR activation contain all the necessary functions in a single protein. In order to better understand the mechanism by which MCMV evades the PKR response, we investigated the associations of pm142 and pm143 with each other and with PKR. Both pm142 and pm143 interact with PKR in infected and transfected cells. However, the ~200 kDa pm142:pm143 complex that forms in these cells does not contain substantial amounts of PKR, suggesting that the interactions between pm142:pm143 and PKR are unstable or transient. The stable, soluble pm142:pm143 complex appears to be a heterotetramer consisting of two molecules of pm142 associated with each other and each one binding to and stabilizing a monomer of pm143. MCMV infection also causes relocalization of PKR into the nucleus and to an insoluble cytoplasmic compartment. These results suggest a model in which the pm142:pm143 multimer interacts with PKR and causes its sequestration in cellular compartments where it is unable to shut off translation and repress viral replication
ULAS J141623.94+134836.3: a Blue T Dwarf Companion to a Blue L Dwarf
We confirm the substellar nature of ULAS J141623.94+134836.3, a common proper
motion companion to the blue L dwarf SDSS J141624.08+134826.7 identified by
Burningham et al. and Scholz. Low-resolution 0.8-2.4 micron spectroscopy
obtained with IRTF/SpeX shows strong H2O and CH4 absorption bands, consistent
with a T7.5 spectral type, and we see possible indications of NH3 absorption in
the 1.0-1.3 micron region. More importantly, the spectrum of ULAS J1416+1348
shows a broadened Y-band peak and highly suppressed K-band flux, both
indicative of high surface gravity and/or subsolar metallicity. These traits
are verified through spectral model fits, from which we derive atmospheric
parameters Teff = 650+/-60 K, log g = 5.2+/-0.4 cgs, [M/H] <= -0.3 and Kzz =
10^4 cm^2/s, the temperature being significantly warmer than that estimated by
Burningham et al. These fits also indicate a model-dependent spectroscopic
distance of 10.6(+3.0,-2.8) pc for ULAS J1416+1348, formally consistent with
the 7.9+/-1.7 pc astrometric distance for SDSS J1416+1348 from Scholz. The
common peculiarities of these two co-spatial, co-moving sources suggest that
their unusual blue colors - and those of other blue L and T dwarfs in general -
arise from age or metallicity, rather than cloud properties alone.Comment: 21 pages, 3 figures (manuscript format); submitted to ApJ,
constructive comments welcom
Discovery of An Unusually Blue L Dwarf Within 10 pc of the Sun
We report the discovery of an unusually blue L5 dwarf within 10 pc of the Sun
from a search of Sloan Digital Sky Survey (SDSS) spectra. A spectrophotometric
distance estimate of 8.0+/-1.6 pc places SDSS J141624.08+134826.7 among the six
closest known L dwarfs. SDSS 1416+13 was overlooked in infrared color-based
searches because of its unusually blue J-K_S color, which also identifies it as
the nearest member of the blue L dwarf subclass. We present additional infrared
and optical spectroscopy from the IRTF/SpeX and Magellan/MagE spectrographs and
determine UVW motions that indicate thin disk kinematics. The inclusion of SDSS
1416+13 in the 20 pc sample of L dwarfs increases the number of L5 dwarfs by
20% suggesting that the L dwarf luminosity function may be far from complete.Comment: 15 pages, 3 figures, accepted for publication in AJ; updated version
includes corrected radial velocit
WISEP J180026.60+013453.1: A Nearby Late L Dwarf Near the Galactic Plane
We report a nearby L7.5 dwarf discovered using the Preliminary Data Release
of the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All-Sky
Survey (2MASS). WISEP J180026.60+013453.1 has a motion of 0.42 arcsec/yr and an
estimated distance of 8.8 \pm 1.0 pc. With this distance, it currently ranks as
the sixth closest known L dwarf, although a trigonometric parallax is needed to
confirm this distance. It was previously overlooked because it lies near the
Galactic Plane (b=12). As a relatively bright and nearby late L dwarf with
normal near-infrared colors, W1800+0134 will serve as a benchmark for studies
of cloud-related phenomena in cool substellar atmospheres.Comment: 12 pages, 2 figure, accepted to the Astronomical Journal (AJ
Atmospheric circulation of hot Jupiters: Coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b
We present global, three-dimensional numerical simulations of HD 189733b and
HD 209458b that couple the atmospheric dynamics to a realistic representation
of non-gray cloud-free radiative transfer. The model, which we call the
Substellar and Planetary Atmospheric Radiation and Circulation (SPARC) model,
adopts the MITgcm for the dynamics and uses the radiative model of McKay,
Marley, Fortney, and collaborators for the radiation. Like earlier work with
simplified forcing, our simulations develop a broad eastward equatorial jet,
mean westward flow at higher latitudes, and substantial flow over the poles at
low pressure. For HD 189733b, our simulations without TiO and VO opacity can
explain the broad features of the observed 8 and 24-micron light curves,
including the modest day-night flux variation and the fact that the planet/star
flux ratio peaks before the secondary eclipse. Our simulations also provide
reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16,
and 24 microns and the groundbased upper limit at 2.2 microns. However, we
substantially underpredict the 3.6-micron secondary-eclipse depth, suggesting
that our simulations are too cold in the 0.1-1 bar region. Predicted temporal
variability in secondary-eclipse depths is ~1% at Spitzer bandpasses,
consistent with recent observational upper limits at 8 microns. We also show
that nonsynchronous rotation can significantly alter the jet structure. For HD
209458b, we include TiO and VO opacity; these simulations develop a hot (>2000
K) dayside stratosphere. Despite this stratosphere, we do not reproduce current
Spitzer photometry of this planet. Light curves in Spitzer bandpasses show
modest phase variation and satisfy the observational upper limit on day-night
phase variation at 8 microns. (abridged)Comment: 20 pages (emulate-apj format), 21 figures, final version now
published in ApJ. Includes expanded discussion of radiative-transfer methods
and two new figure
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. IV. Discovery of an M9 + L6 BInary Separated by Over 100 AU
We report the discovery of a faint L6 \pm 1 companion to the previously known
M9 dwarf, 2MASS J01303563-4445411, based on our near-infrared imaging and
spectroscopic observations with the 3m Infrared Telescope Facility SpeX
imager/spectrometer. The visual binary is separated by 3. 28 \pm 0. 05 on the
sky at a spectrophotometric distance of 40 \pm 14 pc. The projected physical
separation is 130 \pm 50 AU, making it one of the widest VLM field multiples
containing a brown dwarf companion. 2MASS J0130-4445 is only one of ten wide
VLM pairs and only one of six in the field. The secondary is considerably
fainter ({\Delta}K ~ 2.35 mag) and redder ({\Delta} (J - Ks) ~ 0.81 dex),
consistent with component near-infrared types of M9.0 \pm 0.5 and L6 \pm 1
based on our resolved spectroscopy. The component types suggest a secondary
mass well within the hydrogen-burning limit and an age-dependent mass ratio of
0.6-0.9. The system's space motion and spectroscopic indicators suggest an age
of 2-4 Gyr while the model-dependent masses and binding energies suggest that
this system is unlikely to have formed via dynamical ejection. The age,
composition, and separation of the 2MASS J01303563-4445411 system make it
useful for tests of VLM formation theories and of condensate cloud formation in
L dwarfs.Comment: Accepted by the AJ (8 pages, emulateapj format
Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C
Condensate clouds are a salient feature of L dwarf atmospheres, but have been
assumed to play little role in shaping the spectra of the coldest T-type brown
dwarfs. Here we report evidence of condensate opacity in the near-infrared
spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port
Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data
verify the low-temperature nature of this source, indicating a T8 spectral
classification, log Lbol/Lsun = -5.62+/-0.03, Teff = 650+/-25 K, and a mass at
or below the deuterium burning limit. The data also reveal enhanced emission at
K-band associated with youth (low surface gravity) and supersolar metallicity,
reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] =
+0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS
J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to
cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C we
confirm a low surface gravity and supersolar metallicity, while the temperature
differs depending on the presence (635 [+25,-35] K) or absence (760 [+70,-45]
K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar
temperatures (595 [+25,-45] K), but distinct surface gravities (log g = 4.0-4.5
cgs versus 5.0-5.5 cgs) and metallicities ([M/H] ~ +0.2 versus -0.2). In all
three cases, cloudy models provide better fits to the spectral data,
significantly so for Ross 458C. These results indicate that clouds are an
important opacity source in the spectra of young cold T dwarfs, and should be
considered when characterizing the spectra of planetary-mass objects in young
clusters and directly-imaged exoplanets. The characteristics of Ross 458C
suggest it could itself be regarded as a planet, albeit one whose cosmogony
does not conform with current planet formation theories.Comment: Accepted for publication to ApJ: 18 pages, 11 figures in emulateapj
forma
A Disk Around the Planetary-Mass Companion GSC 06214-00210 b: Clues About the Formation of Gas Giants on Wide Orbits
We present Keck/OSIRIS 1.1-1.8 um adaptive optics integral field spectroscopy
of the planetary-mass companion to GSC 06214-00210, a member of the ~5 Myr
Upper Scorpius OB association. We infer a spectral type of L0+/-1, and our
spectrum exhibits multiple signs of youth. The most notable feature is
exceptionally strong PaBeta emission (EW=-11.4 +/- 0.3 A) which signals the
presence of a circumplanetary accretion disk. The luminosity of GSC 06214-00210
b combined with its age yields a model-dependent mass of 14 +/- 2 MJup, making
it the lowest-mass companion to show evidence of a disk. With a projected
separation of 320 AU, the formation of GSC 06214-00210 b and other very
low-mass companions on similarly wide orbits is unclear. One proposed mechanism
is formation at close separations followed by planet-planet scattering to much
larger orbits. Since that scenario involves a close encounter with another
massive body, which is probably destructive to circumplanetary disks, it is
unlikely that GSC 06214-00210 b underwent a scattering event in the past. This
implies that planet-planet scattering is not solely responsible for the
population of gas giants on wide orbits. More generally, the identification of
disks around young planetary companions on wide orbits offers a novel method to
constrain the formation pathway of these objects, which is otherwise
notoriously difficult to do for individual systems. We also refine the spectral
type of the primary from M1 to K7 and detect a mild (2-sigma) excess at 22 um
using WISE photometry.Comment: 25 pages, 13 figures; Accepted by Ap
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