In eukaryotic cells, structures called heterochromatin play critical roles in nuclear processes ranging from gene repression to chromosome segregation. Biochemical and in vivo studies over the past ...several decades have implied that the diverse functions of heterochromatin rely on the ability of these structures to spread across large regions of the genome, to compact the underlying DNA, and to recruit different types of activities. Recent observations have suggested that heterochromatin may possess liquid droplet-like properties. Here, we discuss how these observations provide a new perspective on the mechanisms for the assembly, regulation, and functions of heterochromatin.
What is the relationship between film viewers' eye movements and their film comprehension? Typical Hollywood movies induce strong attentional synchrony-most viewers look at the same things at the ...same time. Thus, we asked whether film viewers' eye movements would differ based on their understanding-the mental model hypothesis-or whether any such differences would be overwhelmed by viewers' attentional synchrony-the tyranny of film hypothesis. To investigate this question, we manipulated the presence/absence of prior film context and measured resulting differences in film comprehension and eye movements. Viewers watched a 12-second James Bond movie clip, ending just as a critical predictive inference should be drawn that Bond's nemesis, "Jaws," would fall from the sky onto a circus tent. The No-context condition saw only the 12-second clip, but the Context condition also saw the preceding 2.5 minutes of the movie before seeing the critical 12-second portion. Importantly, the Context condition viewers were more likely to draw the critical inference and were more likely to perceive coherence across the entire 6 shot sequence (as shown by event segmentation), indicating greater comprehension. Viewers' eye movements showed strong attentional synchrony in both conditions as compared to a chance level baseline, but smaller differences between conditions. Specifically, the Context condition viewers showed slightly, but significantly, greater attentional synchrony and lower cognitive load (as shown by fixation probability) during the critical first circus tent shot. Thus, overall, the results were more consistent with the tyranny of film hypothesis than the mental model hypothesis. These results suggest the need for a theory that encompasses processes from the perception to the comprehension of film.
Gene silencing by heterochromatin is proposed to occur in part as a result of the ability of heterochromatin protein 1 (HP1) proteins to spread across large regions of the genome, compact the ...underlying chromatin and recruit diverse ligands. Here we identify a new property of the human HP1α protein: the ability to form phase-separated droplets. While unmodified HP1α is soluble, either phosphorylation of its N-terminal extension or DNA binding promotes the formation of phase-separated droplets. Phosphorylation-driven phase separation can be promoted or reversed by specific HP1α ligands. Known components of heterochromatin such as nucleosomes and DNA preferentially partition into the HP1α droplets, but molecules such as the transcription factor TFIIB show no preference. Using a single-molecule DNA curtain assay, we find that both unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, although with different characteristics. We show by direct protein delivery into mammalian cells that an HP1α mutant incapable of phase separation in vitro forms smaller and fewer nuclear puncta than phosphorylated HP1α. These findings suggest that heterochromatin-mediated gene silencing may occur in part through sequestration of compacted chromatin in phase-separated HP1 droplets, which are dissolved or formed by specific ligands on the basis of nuclear context.
Highlights • HP1 proteins play diverse roles in vivo. • Functional versatility can arise from small sequence differences between HP1 paralogs. • Functional versatility may arise from conformational ...flexibility of an HP1 protein.
The conserved transcriptional regulator heat shock factor 1 (Hsf1) is a key sensor of proteotoxic and other stress in the eukaryotic cytosol. We surveyed Hsf1 activity in a genome-wide ...loss-of-function library in Saccaromyces cerevisiae as well as ∼78,000 double mutants and found Hsf1 activity to be modulated by highly diverse stresses. These included disruption of a ribosome-bound complex we named the Ribosome Quality Control Complex (RQC) comprising the Ltn1 E3 ubiquitin ligase, two highly conserved but poorly characterized proteins (Tae2 and Rqc1), and Cdc48 and its cofactors. Electron microscopy and biochemical analyses revealed that the RQC forms a stable complex with 60S ribosomal subunits containing stalled polypeptides and triggers their degradation. A negative feedback loop regulates the RQC, and Hsf1 senses an RQC-mediated translation-stress signal distinctly from other stresses. Our work reveals the range of stresses Hsf1 monitors and elucidates a conserved cotranslational protein quality control mechanism.
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► Comprehensive characterization of the stresses sensed by Hsf1 ► Characterization of a complex that targets ribosomes stalled at translation ► An autoregulatory loop regulates activity of the complex ► Discovery of a translation-stress signaling pathway from the ribosome to Hsf1
A ribosome-bound complex designated RQC associates with 60S ribosomal subunits containing stalled polypeptides to trigger their degradation.
Intracellular transport is typically bidirectional, consisting of a series of back and forth movements. Kinesin-1 and cytoplasmic dynein require each other for bidirectional transport of ...intracellular cargo along microtubules; i.e., inhibition or depletion of kinesin-1 abolishes dynein-driven cargo transport and vice versa. Using Drosophila melanogaster S2 cells, we demonstrate that replacement of endogenous kinesin-1 or dynein with an unrelated, peroxisome-targeted motor of the same directionality activates peroxisome transport in the opposite direction. However, motility-deficient versions of motors, which retain the ability to bind microtubules and hydrolyze adenosine triphosphate, do not activate peroxisome motility. Thus, any pair of opposite-polarity motors, provided they move along microtubules, can activate one another. These results demonstrate that mechanical interactions between opposite-polarity motors are necessary and sufficient for bidirectional organelle transport in live cells.
This study investigated the relative roles of visuospatial versus linguistic working memory (WM) systems in the online generation of bridging inferences while viewers comprehend visual narratives. We ...contrasted these relative roles in the
visuospatial primacy
hypothesis versus the
shared
(
visuospatial & linguistic
)
systems
hypothesis, and tested them in 3 experiments. Participants viewed picture stories containing multiple target episodes consisting of a beginning state, a bridging event, and an end state, respectively, and the presence of the bridging event was manipulated. When absent, viewers had to infer the bridging-event action to comprehend the end-state image. A pilot study showed that after viewing the end-state image, participants’ think-aloud protocols contained more inferred actions when the bridging event was
absent
than when it was present. Likewise, Experiment
1
found longer viewing times for the end-state image when the bridging-event image was absent, consistent with viewing times revealing online inference generation processes. Experiment
2
showed that both linguistic and visuospatial WM loads attenuated the inference viewing time effect, consistent with the shared systems hypothesis. Importantly, however, Experiment
3
found that articulatory suppression did not attenuate the inference viewing time effect, indicating that (sub)vocalization did not support online inference generation during visual narrative comprehension. Thus, the results support a shared-systems hypothesis in which both visuospatial and linguistic WM systems support inference generation in visual narratives, with the linguistic WM system operating at a deeper level than (sub)vocalization.
The behavior and microscale processes associated with freely suspended organisms, along with sinking particles underlie key ecological processes in the ocean. Mechanistically studying such multiscale ...processes in the laboratory presents a considerable challenge for microscopy: how to measure single cells at microscale resolution, while allowing them to freely move hundreds of meters in the vertical direction? Here we present a solution in the form of a scale-free, vertical tracking microscope, based on a 'hydrodynamic treadmill' with no bounds for motion along the axis of gravity. Using this method to bridge spatial scales, we assembled a multiscale behavioral dataset of nonadherent planktonic cells and organisms. Furthermore, we demonstrate a 'virtual-reality system for single cells', wherein cell behavior directly controls its ambient environmental parameters, enabling quantitative behavioral assays. Our method and results exemplify a new paradigm of multiscale measurement, wherein one can observe and probe macroscale and ecologically relevant phenomena at microscale resolution. Beyond the marine context, we foresee that our method will allow biological measurements of cells and organisms in a suspended state by freeing them from the confines of the coverslip.
Which region of the visual field is most useful for recognizing scene gist, central vision (the fovea and parafovea) based on its higher visual resolution and importance for object recognition, or ...the periphery, based on resolving lower spatial frequencies useful for scene gist recognition, and its large extent? Scenes were presented in two experimental conditions: a "Window," a circular region showing the central portion of a scene, and blocking peripheral information, or a "Scotoma," which blocks out the central portion of a scene and shows only the periphery. Results indicated the periphery was more useful than central vision for maximal performance (i.e., equal to seeing the entire image). Nevertheless, central vision was more efficient for scene gist recognition than the periphery on a per-pixel basis. A critical radius of 7.4 degrees was found where the Window and Scotoma performance curves crossed, producing equal performance. This value was compared to predicted critical radii from cortical magnification functions on the assumption that equal V1 activation would produce equal performance. However, these predictions were systematically smaller than the empirical critical radius, suggesting that the utility of central vision for gist recognition is less than predicted by V1 cortical magnification.
This study investigated the relevance of the revised 2-factor study process questionnaire (R-SPQ-2F) for exploring medical students' approaches to learning in Qatar and identify how factors like ...gender, age, educational attainment, and prior experience with health care influence students' adoption of deep approaches to learning.
The sample consisted of 108 medical students (44% male, 56% female) from all four years of medical school at Weill Cornell Medicine-Qatar (WCM-Q). Participants completed the 20-item R-SPQ-2F questionnaire to measure their learning approaches through a structural model contrasting deep and surface learning. Participants also completed a survey collecting demographic information.
Statistical analysis revealed significant differences in deep learning approaches across year levels for both men and women. Additionally, educational attainment played a significant role in students' approaches to learning.
Based on structural equation modeling, this cross-verification study supports the R-SPQ-2F instrument and offers additional evidence for its robustness and application to medical education. These findings may help educational and program leaders in Qatar better understand medical students' learning approaches to enhance their pedagogical practices.
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