Abhay Kotecha

Cryogenic plasma focused ion beam-scanning electron microscopy (cryo-pFIB-SEM) enables nanoscale volume imaging of vitrified cells and tissues, but the fidelity of each newly exposed block-face depends on how ions interact with compositionally heterogeneous biological material. Using xenon, argon, oxygen, and nitrogen plasma beams across a 30-2 kV voltage range, we examined how ion species and accelerating voltage influence block-face quality and ultrastructural fidelity. Curtaining imposed a species-dependent lower limit on accelerating voltage, with xenon and argon supporting uniform milling to 10 kV and oxygen and nitrogen to 15 kV. Monte Carlo simulations predicted substantially reduced ion penetration and atomic displacement under these species-specific low-kV conditions. Experimentally, lowering milling voltage suppressed charging and topographic artifacts, with the largest effects observed at heterogeneous interfaces. Oxygen and nitrogen produced greater membrane sharpness and organelle contrast than the noble gases, and these differences influenced machine-learning-based recognition of fine organelle features in 3D datasets. Together, these findings show that cryo-pFIB-SEM image quality cannot be optimized by accelerating voltage or ion species independently. Instead, milling performance reflects a tradeoff among surface fidelity, charging, contrast, and curtaining that depends on local specimen composition. Species-specific low-kV milling therefore provides a practical framework for matching cryo-pFIB-SEM acquisition conditions to the structural and biophysical properties of the biological target.

EV-A71 has been responsible for recent severe HFMD outbreaks. We report structures for 12 potently neutralizing human anti–EV-A71 monoclonal antibody Fabs, alone and complexed with virus. Most recognize the native antigenic state with epitopes that span interfaces, together covering 85% of the capsid surface. The majority (8 of 12) bind the canyon, while the others cluster around the icosahedral two- and threefold axes. Blocking SCARB2 receptor binding likely contributes to neutralization for all, and a subset induces empty particles. A predominant gene family (IGHV4-39) does not dictate a common binding pose. Long CDR-H3 loops are frequently key to binding, especially at the canyon, suggesting that antigenicity data based on antibodies with shorter CDR3s (e.g., murine) may be misleading. This dataset reveals neutralization mechanisms for recently circulating EV-A71 genotypes, which will inform immunotherapies. We demonstrate synergy in vitro between canyon binding and both two- and threefold binding antibodies to increase neutralization potency.

The signal peptidase complex (SPC) is responsible for cleaving signal peptides (SPs) from approximately 10% of the human proteome. SPs are characterized by a tripartite structure, consisting of an N-terminal n-region, a central helical h-region and a C-terminal c-region, each defined by rather general chemical properties rather than strict sequence conservation. Despite their sequence diversity, SPC recognizes and processes SPs with exquisite specificity. Here, we present a 2.6 Å cryo-EM map of the human SPC-A, one of two SPC paralogs, bound to a model SP. The c-region binds to a hydrophobic binding groove near the active site, a narrow gate marks the transition from c- to h-region, and the h-region localizes in a transmembrane (TM) window. Substrate engagement stabilizes N- and C-terminal helices of Sec11A, which frame the SP and are unresolved in the apo structure. Molecular dynamics (MD) simulations confirm a stable hydrogen-bonding network at the c-region and indicate dynamic interactions within a thinned lipid environment at the TM window. AlphaFold modeling supports this binding mode across physiological SPs. Collectively, our structural and computational analyses explain how the SPC achieves its specificity by combining the selectivity of the luminal binding groove and of the transmembrane window.

Mitochondria regenerate adenosine triphosphate (ATP) through oxidative phosphorylation. This process is carried out by five membrane-bound complexes collectively known as the respiratory chain, working in concert to transfer electrons and pump protons. The precise organization of these complexes in native cells is debated. We used in situ cryo–electron tomography to visualize the native structures and organization of several major mitochondrial complexes in Chlamydomonas reinhardtii cells. ATP synthases and respiratory complexes segregate into curved and flat crista membrane domains, respectively. Respiratory complexes I, III, and IV assemble into a respirasome supercomplex, from which we determined a native 5-angstrom (Å) resolution structure showing binding of electron carrier cytochrome c . Combined with single-particle cryo–electron microscopy at 2.4-Å resolution, we model how the respiratory complexes organize inside native mitochondria.

How cyclin-dependent kinase 7 (CDK7) coordinately regulates the cell cycle and RNA polymerase II transcription remains unclear. Here, high-resolution cryo–electron microscopy revealed how two clinically relevant inhibitors block CDK7 function. In cells, CDK7 inhibition rapidly suppressed transcription, but constitutively active genes were disproportionately affected versus stimulus-responsive. Distinct transcription factors (TFs) regulate constitutive versus stimulus-responsive genes. Accordingly, stimulus-responsive TFs were refractory to CDK7 inhibition whereas constitutively active “core” TFs were repressed. Core TFs (n = 78) are predominantly promoter associated and control cell cycle and proliferative gene expression programs across cell types. Mechanistically, rapid suppression of core TF function can occur through CDK7-dependent phosphorylation changes in core TFs and RB1. Moreover, CDK7 inhibition depleted core TF protein levels within hours, consistent with durable target gene suppression. Thus, a major but unappreciated biological function for CDK7 is regulation of a TF cohort that drives proliferation, revealing an apparent universal mechanism by which CDK7 coordinates RNAPII transcription with cell cycle CDK regulation.

The metabotropic glutamate receptors (mGlus) are class C G protein-coupled receptors (GPCR) that form obligate dimers activated by the major excitatory neurotransmitter L-glutamate. The architecture of mGlu receptor comprises an extracellular Venus-Fly Trap domain (VFT) connected to the transmembrane domain (7TM) through a Cysteine-Rich Domain (CRD). The binding of L-glutamate in the VFTs and subsequent conformational change results in the signal being transmitted to the 7TM inducing G protein binding and activation. The mGlu receptors signal transduction can be allosterically potentiated by positive allosteric modulators (PAMs) binding to the 7TMs, which are of therapeutic interest in various neurological disorders. Here, we report the cryoEM structures of metabotropic glutamate receptor 5 (mGlu₅) purified with three chemically and pharmacologically distinct PAMs. We find that the PAMs modulate the receptor equilibrium through their different binding modes, revealing how their interactions in the 7TMs impact the mGlu₅ receptor conformational landscape and function. In addition, we identified a PAM-free but agonist-bound intermediate state that also reveals interactions mediated by intracellular loop 2. The activation of mGlu₅ receptor is a multi-step process in which the binding of the PAMs in the 7TM modulates the equilibrium towards the active state.

In situ cryo-electron tomography (cryo-ET) has emerged as the method of choice to investigate structures of biomolecules in their native context. However, challenges remain in the efficient production of large-scale cryo-ET datasets, as well as the community sharing of this information-rich data. Here, we applied a cryogenic plasma-based focused ion beam (cryo-PFIB) instrument for high-throughput milling of the green alga Chlamydomonas reinhardtii , a useful model organism for in situ visualization of numerous fundamental cellular processes. Combining cryo-PFIB sample preparation with recent advances in cryo-ET data acquisition and processing, we generated a dataset of 1829 reconstructed and annotated tomograms, which we provide as a community resource to drive method development and inspire biological discovery. To assay the quality of this dataset, we performed subtomogram averaging (STA) of both soluble and membrane-bound complexes ranging in size from >3 MDa to ∼200 kDa, including 80S ribosomes, Rubisco, nucleosomes, microtubules, clathrin, photosystem II, and mitochondrial ATP synthase. The majority of these density maps reached sub-nanometer resolution, demonstrating the potential of this C. reinhardtii dataset, as well as the promise of modern cryo-ET workflows and open data sharing towards visual proteomics.

Cryo-electron microscopy (cryo-EM) has revolutionized structural biology by providing high-resolution insights into biological macromolecules. Here, we present sub-3 Å resolution structures determined using the 100 keV Tundra cryogenic transmission electron microscope (cryo-TEM), equipped with the newly developed Falcon C direct electron detector (DED). Our results demonstrate that this lower voltage microscope, when combined with advanced electron optics and detectors, can achieve high-resolution reconstructions that were previously only attainable with higher voltage systems. The implementation of an extreme-brightness field emission gun (XFEG) and an SP-TWIN objective lens significantly enhanced the spatial and temporal coherence of the system. Furthermore, the semi-automated sample loader minimized contamination and drift, allowing extended data collection sessions without manual intervention. The high detective quantum efficiency (DQE) of Falcon C further improved the signal-to-noise ratio, which is critical for achieving high-resolution structures. We validated the performance of this microscope by determining the structures of various biological samples, including apoferritin, T20S proteasome, GABA _A receptor, haemoglobin, and human transthyretin ranging in size from 440 kDa to 50 kDa. The highest resolutions achieved were 2.1 Å for apoferritin, 2.7 Å for the 20S proteasome, 2.8 Å for the GABAA receptor, 5.0 Å for haemoglobin, and 3.5 Å for transthyretin. We also explored a larger specimen, a 3.9 MDa Adeno-associated virus (AAV9) capsid and resolved it a 2.8 Å. This work highlights the potential of 100 keV TEMs to make high-resolution cryo-EM more accessible to the structural biology community. Furthermore, it sets a precedent for the use of lower voltage TEMs in routine cryo-EM studies, not only for screening grids for single particle analysis but also for achieving high-resolution structures of protein samples.