Biology subjects

Barad, B. A.

Publications and source records attributed to Barad, B. A..

11 recordsLinked to original sources

bioRxiv2026

Spatial confinement shapes organelle architecture and remodeling in axons

Axons are extended cellular compartments that mediate neuronal connectivity over extraordinary distances, placing unique demands on local organelle organization and trafficking. How these geometric constraints influence organelle architecture remains poorly understood. Here, we used cryo-electron tomography and quantitative morphometric analyses to define the three-dimensional ultrastructural organization of dorsal root ganglion axons in a near-native state. We identify distinct vesicle populations with tightly versus broadly constrained size distributions and reveal a continuum of endolysosomal and autophagic intermediates that highlight the dynamic nature of membrane remodeling in growing axons. Unexpectedly, we observe vesicular structures enclosed within the lumen of the endoplasmic reticulum, suggesting a previously undescribed mechanism of ER membrane remodeling. Across multiple organelle classes, morphology and size are constrained by axonal geometry. This principle is most evident in mitochondria, which undergo dramatic narrowing and remodeling at varicosity - axon boundaries to traverse confined axonal segments. Together, these findings reveal spatial confinement as a fundamental organizing principle of axonal cell biology. SUMMARYCryo-electron tomography establishes a quantitative framework for organelle organization in axons. Diverse membrane trafficking pathways, including endolysosomal intermediates and mitochondria, exhibit structural adaptations to axonal geometry, identifying spatial confinement as a fundamental organizing principle of axonal cell biology.

neuroscience↗

bioRxiv2025

Prohibitin complexes associate with unique membrane microdomains in cells

Prohibitins are multi-subunit protein complexes implicated in a variety of roles in mitochondria, including maintaining cristae architecture, regulating lipid metabolism, and mediating protein quality control. Prohibitins form large, ring-like structures within the mitochondrial inner membrane, and it has been proposed that this unique architecture enables them to organize distinct membrane environments that support their diverse functions. However, the molecular mechanisms driving these structure-function relationships within the native cellular context remain poorly defined. In this work, we combine cellular cryo-electron tomography, subtomogram averaging, and Surface Morphometrics analysis to systematically characterize the structure and local membrane environment of prohibitin complexes in mouse embryonic fibroblast (MEF) cells. We show that prohibitins are enriched in the inner boundary membrane subdomain of the inner mitochondrial membrane. Moreover, we find that a subset of prohibitin complexes exhibits a unique association with a matrix-facing density that resembles an m-AAA protease. Further, we show that prohibitins consistently localize to regions of reduced lipid bilayer thickness across all inner mitochondrial membrane subdomains, suggesting that they locally remodel the membrane to support their function. Together, these findings provide direct structural evidence that prohibitin complexes are associated with unique membrane microdomains in cells and establish a framework for uncovering how the local cellular context enables prohibitin function.

cell biology↗

bioRxiv2025

Surface morphometrics reveals local membrane thickness variation in organellar subcompartments

Lipid bilayers form the basis of organellar architecture, structure, and compartmentalization in the cell. Decades of biophysical, biochemical, and imaging studies on purified or in vitro reconstituted liposomes have shown that variations in lipid composition influence the physical properties of membranes, such as thickness and curvature. However, similar studies characterizing these membrane properties within the native cellular context have remained technically challenging. Recent advancements in cellular cryo-electron tomography (cryo-ET) imaging enable high-resolution, three-dimensional views of native organellar membrane architecture preserved in near-native conditions. We previously developed a Surface Morphometrics pipeline that generates surface mesh reconstructions to model and quantify cellular membrane ultrastructure from cryo-ET data. Here, we expand this pipeline to measure the distance between the phospholipid head groups (PHG) of the membrane bilayer as a readout of membrane thickness. Using this approach, we demonstrate thickness variations both within and between distinct organellar membranes. We also demonstrate that membrane thickness positively correlates with other features, such as membrane curvedness. Further, we show that subcompartments of the mitochondrial inner membrane exhibit varying membrane thicknesses that are independent of network morphology (i.e., fragmented versus elongated networks). Finally, we demonstrate that large membrane-associated macromolecular complexes exhibit distinct density profiles that correlate with local variations in membrane thickness. Overall, our updated Surface Morphometrics pipeline provides a framework for investigating how changes in membrane composition in various cellular and disease contexts affect organelle ultrastructure and function.

biochemistry↗

bioRxiv2024

Cytoplasmic ribosomes on mitochondria alter the local membrane environment for protein import

Most of the mitochondria proteome is nuclear-encoded, synthesized by cytoplasmic ribosomes, and targeted to mitochondria post-translationally. However, a subset of mitochondrial-targeted proteins is imported co-translationally, although the molecular mechanisms governing this process remain unclear. We employ cellular cryo-electron tomography to visualize interactions between cytoplasmic ribosomes and mitochondria in Saccharomyces cerevisiae. We use surface morphometrics tools to identify a subset of ribosomes optimally oriented on mitochondrial membranes for protein import. This allows us to establish the first subtomogram average structure of a cytoplasmic ribosome on the surface of the mitochondria in the native cellular context, which showed three distinct connections with the outer mitochondrial membrane surrounding the peptide exit tunnel. Further, this analysis demonstrated that cytoplasmic ribosomes primed for mitochondrial protein import cluster on the outer mitochondrial membrane at sites of local constrictions of the outer and inner mitochondrial membrane. Overall, our study reveals the architecture and the spatial organization of cytoplasmic ribosomes at the mitochondrial surface, providing a native cellular context to define the mechanisms that mediate efficient mitochondrial co-translational protein import. SUMMARYChang et al. present a membrane-guided approach for identifying a subset of cytoplasmic ribosomes oriented for protein import on the mitochondrial surface in Saccharomyces cerevisiae using cryo-electron tomography. They show that ribosomes cluster, make multiple contacts with, and induce local changes to the mitochondrial membrane ultrastructure at import sites.

cell biology↗

bioRxiv2023

Overcoming Resolution Attenuation During Tilted Cryo-EM Data Collection

Structural biology efforts using cryogenic electron microscopy are frequently stifled by specimens adopting "preferred orientations" on grids, leading to anisotropic map resolution and impeding structure determination. Tilting the specimen stage during data collection is a generalizable solution but has historically led to substantial resolution attenuation. Here, we develop updated data collection and image processing workflows and demonstrate, using multiple specimens, that resolution attenuation is negligible or significantly reduced across tilt angles. Reconstructions with and without the stage tilted as high as 60{degrees} are virtually indistinguishable. These strategies allowed the reconstruction to 3 [A] resolution of a bacterial RNA polymerase with preferred orientation. Furthermore, we present a quantitative framework that allows cryo-EM practitioners to define an optimal tilt angle for dataset acquisition. These data reinforce the utility of employing stage tilt for data collection and provide quantitative metrics to obtain isotropic maps.

biophysics↗

bioRxiv2022

Mapping Protein Dynamics at High-Resolution with Temperature-Jump X-ray Crystallography

Understanding and controlling protein motion at atomic resolution is a hallmark challenge for structural biologists and protein engineers because conformational dynamics are essential for complex functions such as enzyme catalysis and allosteric regulation. Time-resolved crystallography offers a window into protein motions, yet without a universal perturbation to initiate conformational changes the method has been limited in scope. Here we couple a solvent-based temperature jump with time-resolved crystallography to visualize structural motions in lysozyme, a dynamic enzyme. We observed widespread atomic vibrations on the nanosecond timescale, which evolve on the sub-millisecond timescale into localized structural fluctuations that are coupled to the active site. An orthogonal perturbation to the enzyme, inhibitor binding, altered these dynamics by blocking key motions that allow energy to dissipate from vibrations into functional movements linked to the catalytic cycle. Because temperature-jump is a universal method for perturbing molecular motion, the method demonstrated here is broadly applicable for studying protein dynamics.

biophysics↗

bioRxiv2022

A surface morphometrics toolkit to quantify organellar membrane ultrastructure using cryo-electron tomography

Cellular cryo-electron tomography (cryo-ET) enables 3-dimensional reconstructions of organelles in their native cellular environment at subnanometer resolution. However, quantifying ultrastructural features of pleomorphic organelles in three dimensions is challenging, as is defining the significance of observed changes induced by specific cellular perturbations. To address this challenge, we established a semi-automated workflow to segment organellar membranes and reconstruct their underlying surface geometry in cryo-ET. To complement this workflow, we developed an open source suite of ultrastructural quantifications, integrated into a single pipeline called the surface morphometrics toolkit. This toolkit allows detailed mapping of spacing, curvature, and orientation onto reconstructed membrane meshes, highlighting subtle organellar features that are challenging to detect in three dimensions and allowing for statistical comparison across many organelles. To demonstrate the advantages of this approach, we combine cryo-ET with cryo-fluorescence microscopy to correlate bulk mitochondrial network morphology (i.e., elongated versus fragmented) with membrane ultrastructure of individual mitochondria in the presence and absence of endoplasmic reticulum (ER) stress. Using our toolkit, we demonstrate ER stress promotes adaptive remodeling of ultrastructural features of mitochondria including spacing between the inner and outer membranes, local curvature of the inner membrane, and spacing between mitochondrial cristae. We show that differences in membrane ultrastructure correlate to mitochondrial network morphologies, suggesting that these two remodeling events are coupled. Our toolkit offers opportunities for quantifying changes in organellar architecture on a single-cell level using cryo-ET, opening new opportunities to define changes in ultrastructural features induced by diverse types of cellular perturbations.

cell biology↗

bioRxiv2021

Nanoscale details of mitochondrial fission revealed by cryo-electron tomography

Mitochondrial fission is required for proper segregation during cell division, quality control, and cellular homeostasis (metabolism and energy production). Despite its importance, models of the process remain speculative. Here we apply cryogenic electron tomography to image the nanoscale architecture of mitochondrial fission in mammalian cells. We find that constriction of the inner and outer membranes is coordinated, suggesting that force on both membranes is applied externally. While we observe ER at constriction sites, it did not encircle constrictions. Instead, we find long bundles of both unbranched actin and septin filaments enriched at constrictions. Actin bundles align with the central region of division bridges and septin bundles with the necks on either side. Septin bundles appear to guide microtubules to constriction sites, suggesting, along with autolysosomes observed in the vicinity, a pathway for mitophagy. Together, our results rule out several existing models for mitochondrial fission and provide empirical parameters to inform the development of realistic coarse-grained models in the future.

cell biology↗

bioRxiv2020

Outcomes of the 2019 EMDataResource model challenge: validation of cryo-EM models at near-atomic resolution

This paper describes outcomes of the 2019 Cryo-EM Map-based Model Metrics Challenge sponsored by EMDataResource (www.emdataresource.org). The goals of this challenge were (1) to assess the quality of models that can be produced using current modeling software, (2) to check the reproducibility of modeling results from different software developers and users, and (3) compare the performance of current metrics used for evaluation of models. The focus was on near-atomic resolution maps with an innovative twist: three of four target maps formed a resolution series (1.8 to 3.1 [A]) from the same specimen and imaging experiment. Tools developed in previous challenges were expanded for managing, visualizing and analyzing the 63 submitted coordinate models, and several novel metrics were introduced. The results permit specific recommendations to be made about validating near-atomic cryo-EM structures both in the context of individual laboratory experiments and holdings of structure data archives such as the Protein Data Bank. Our findings demonstrate the relatively high accuracy and reproducibility of cryo-EM models derived from these benchmark maps by 13 participating teams, representing both widely used and novel modeling approaches. We also evaluate the pros and cons of the commonly used metrics to assess model quality and recommend the adoption of multiple scoring parameters to provide full and objective annotation and assessment of the model, reflective of the observed density in the cryo-EM map.

biochemistry↗

bioRxiv2019

Dissecting the chitinolytic activity of mammalian chitinases

Chitin is an abundant polysaccharide used by a large range of organisms for structural rigidity and water repulsion. As such, the insoluble crystalline structure of chitin poses significant challenges for enzymatic degradation. Vertebrates do not produce chitin, but do express chitin degrading enzymes. Acidic mammalian chitinase, the primary enzyme involved in the degradation of environmental chitin in mammalian lungs, is a processive glycosyl hydrolase that may be able to make multiple hydrolysis events for each binding event. Mutations to acidic mammalian chitinase have been associated with asthma, and genetic deletion of the enzyme in mice results in significantly increased morbidity and mortality with age. We initially set out to reverse this phenotype by engineering hyperactive acidic mammalian chitinase variants. Using a directed evolution screening approach using commercial fluorogenic substrates, we identified mutations with consistent increases in activity. To determine whether the activity increases observed with oligomeric substrates were consistent with more biologically relevant chitin substrates, we developed new assays to quantify chitinase activity with colloidal crystalline chitin, and identified a high throughput fluorogenic assay that gives sufficient signal to noise advantages to quantify changes to activity due to the addition or removal of a chitin binding domain to the enzyme. We show that the activity increasing mutations derived from our directed evolution screen were lost when crystalline substrates were used. In contrast, naturally occurring gain-of-function mutations gave similar results with oligomeric and crystalline substrates. We also show that the activity differences between acidic mammalian chitinase and chitotriosidase are reduced in the context of crystalline substrate, suggesting that previously reported activity differences with oligomeric substrates may have been largely driven by differential substrate specificity for the oligomers. These results highlight the need for assays against more physiological substrates when engineering complex metabolic enzymes, and provide a new approach that may be broadly applicable to engineering glycosyl hydrolases.

biochemistry↗

bioRxiv2018

Temperature-Jump Solution X-ray Scattering Reveals Distinct Motions in a Dynamic Enzyme

Correlated motions of proteins and their bound solvent molecules are critical to function, but these features are difficult to resolve using traditional structure determination techniques. Time-resolved methods hold promise for addressing this challenge but have relied on the exploitation of exotic protein photoactivity, and are therefore not generalizable. Temperature-jumps (T-jumps), through thermal excitation of the solvent, have been implemented to study protein dynamics using spectroscopic techniques, but their implementation in X-ray scattering experiments has been limited. Here, we perform T-jump small- and wide-angle X-ray scattering (SAXS/WAXS) measurements on a dynamic enzyme, cyclophilin A (CypA), demonstrating that these experiments are able to capture functional intramolecular protein dynamics. We show that CypA displays rich dynamics following a T-jump, and use the resulting time-resolved signal to assess the kinetics of conformational changes in the enzyme. Two relaxation processes are resolved, which can be characterized by Arrhenius behavior. We also used mutations that have distinct functional effects to disentangle the relationship of the observed relaxation processes. A fast process is related to surface loop motions important for substrate specificity, whereas a slower process is related to motions in the core of the protein that are critical for catalytic turnover. These results demonstrate the power of time-resolved X-ray scattering experiments for characterizing protein and solvent dynamics on the s-ms timescale. We expect the T-jump methodology presented here will be useful for understanding kinetic correlations between local conformational changes of proteins and their bound solvent molecules, which are poorly explained by the results of traditional, static measurements of molecular structure.

biophysics↗