Biology subjects

Metskas, L. A.

Publications and source records attributed to Metskas, L. A..

12 recordsLinked to original sources

bioRxiv2026

Using Cryogenic Electron Tomography (cryoET) to Determine Rubisco Polymerization Constants in α-Carboxysomes

Bacteria microcompartments (BMCs) are pseudo-organelles comprised of a self-assembling, semi-permeable protein shell, most commonly enclosing components of enzymatic pathways. -Carboxysomes (-CBs) are anabolic BMCs known for their role in sequestering Rubisco, the enzyme responsible for carbon fixation in plants, algae and bacteria, along with an upstream enzyme and an assembly protein. Rubisco has low selectivity for its substrate, CO2, and has a slow enzymatic turnover rate, resulting in an inefficient metabolic pathway. Within the -CB, Rubisco has been observed at a range of concentrations and with either a liquid-like assembly or a pseudo-lattice of polymerized fibrils. The biophysical origins of the fibril ultrastructure organization are unclear; however, it is only observed inside -CBs. Quantitative knowledge of the binding constants and energies for assembly and maintenance of these fibrils is critical for understanding this organization and Rubisco regulation, but quantitative methods for in situ analysis of Rubisco polymerization have been lacking. Here, we present an approach to convert tomography-derived -CB volumes and Rubisco particle positions into polymerization binding curves. We used this procedure to determine the Rubisco polymerization constants, including the nucleus size (n) and equilibrium polymerization constant (Kpol). The adopted modeling approach is consistent with in situ constraints, such as concentration-dependent binding interactions and confinement. This approach offers a powerful tool to evaluate both in vitro and potentially in vivo biomolecular interactions, both of Rubisco and of other proteins and polymers suitable for analysis by cryo-electron tomography. Significance StatementCryogenic electron tomography (cryoET) is a powerful method to resolve structures of proteins in their native environment at subnanometer-level resolution. Because tomography data retains spatial relationships of all particles, it intrinsically contains information about component (e.g., protein) binding interactions. Here, we use Rubisco polymerization in -carboxysomes as a model system to demonstrate that quantitative, biochemical binding analysis is possible with cryoET.

biophysics↗

bioRxiv2025

Identifying differential effects from eleven mixing techniques on mRNA lipid nanoparticle physicochemistry and biological performance

Lipid nanoparticle (LNP) formulation requires a mixing step. Many studies, especially from academic groups, utilize either microfluidic mixers or hand mixing to prepare LNPs, but commercial-scale processes use turbulent-flow mixers. This discrepancy in mixing techniques has been underexplored, as LNPs made by different techniques may exhibit different performance, such that bench-scale results cannot be replicated using materials manufactured at scale. We here isolate and interrogate the effect of primary mixing. Lipid nanoparticles are produced from ten mixers (one used in two ways), holding all other formulation parameters constant, to directly compare across techniques. LNPs produced from the different mixers exhibit widely different physical properties and biological performance. Notably, manual pipetting common in academic practice yields particles that do not resemble those produces by turbulent-flow mixers. Findings are connected mechanistically to physicochemical characteristics that arise from the different flow regimes. Further establishing the relationship between mixing and LNP properties is critical. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=132 SRC="FIGDIR/small/687311v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@79cac0org.highwire.dtl.DTLVardef@1ee037eorg.highwire.dtl.DTLVardef@12763eforg.highwire.dtl.DTLVardef@111e394_HPS_FORMAT_FIGEXP M_FIG C_FIG

bioengineering↗

bioRxiv2025

Systematic Characterization of Optical Aberrations Reveals Cryo-FLM Localization Fidelity

Cryo-correlative light and electron microscopy (cryo-CLEM) facilitates in situ imaging and structural analysis by combining the molecular specificity of fluorescence microscopy with the ultrastructural resolution of cryo-electron microscopy. By further combining single molecule localization with cryo-CLEM, molecular positions of individual emitters can be revealed in the context of the electron density map of a cell, providing unique insights to profound questions in cell biology and virology. However, cryogenic fluorescence light microscopy (cryo-FLM) suffers from severe and spatially heterogeneous optical aberrations that distort the point spread function, limiting the accuracy of molecular localizations as well as downstream cryo-transmission electron microscopy workflows. Here, we present a systematic and quantitative analysis of optical aberrations in a commercial cryo-FLM system, uncovering the sources of significant distortions such as system imperfections, refractive index mismatches, and sample-induced heterogeneities. These system and sample induced aberrations lead to localization errors up to 90 nm laterally and over 300 nm axially, challenging the feasibility of precise molecular positioning within the vitrified specimen. We demonstrate that these errors are partially mitigated by spatially matched or adaptive point spread function models pushing the error rate down to ten nanometers or less, offering practical guidance for aberration-aware cryo-FLM and cryo-CLEM strategies. Our findings highlight the necessity of accurate, in situ point spread function modeling to achieve nanometer-scale localization in cryo-FLM. The experimental pipeline developed in this work establishes a novel tool to assess optical performance in cryo-CLEM and cryogenic focused ion beam milling workflows as the field strives toward accurate and precise molecular localization.

bioinformatics↗

bioRxiv2025

A toolkit to characterize protein polymerization from cryo-electron tomography data

BackgroundCryogenic electron tomography (cryo-ET) is a powerful method to study protein structures and macromolecular complexes. These studies can provide structural information at the nanometer scale, allowing for the visualization of ultrastructures and access to subnanometer information through localized, subtomogram averaging (STA). STA alignments can provide analysts with an opportunity to quantify relationships between particles; however, the analytical tools to accomplish this are often lab or system specific. We offer a robust MATLAB script package that can be applied to a wide array of systems. MethodsMany tomographic analyses require extensive segmentation and image classification, which though useful can have poorly characterized error rates and user biases. We present a method for unbiased, numerical classification of head-to-tail polymerization in STA datasets with no outside information or user influence required. We provide this code in a modular MATLAB script package for ease of adaptation to other projects, with analyses including volumes, concentrations, binding, and fibril bundling. ResultsWe demonstrate the robust analysis possible with this script package using a model system of Rubisco in -carboxysomes (-CBs), showcasing the codes ability to evaluate global data such as volume and overall organization, polymerization data such as twist and bend, and lattice data such as lateral fibril distances and angles. DiscussionOur script package offers structural biologists a toolkit to conduct a robust biophysical analysis on STA data in an unbiased manner. The information generated will provide new insights into protein-protein interactions and the conditions favorable for larger ultrastructures. Particles can also be classified for further STA processing. This script package can be used for scientists studying proteins within isolated compartments or with clearly defined regions of interest.

biophysics↗

bioRxiv2022

Structure of Anabaena flos-aquae gas vesicles revealed by cryo-ET

Gas vesicles (GVs) are gas-filled protein nanostructures employed by several species of bacteria and archaea as flotation devices to enable access to optimal light and nutrients. The unique physical properties of GVs have led to their use as genetically-encodable contrast agents for ultrasound and MRI. Currently, however, the structure and assembly mechanism of GVs remain unknown. Here we employ cryo-electron tomography to reveal how the GV shell is formed by a helical filament of highly conserved GvpA subunits. This filament changes polarity at the center of the GV cylinder--a site that may act as an elongation center. High-resolution subtomogram averaging reveals a corrugated pattern of the shell arising from polymerization of GvpA into a {beta}-sheet. The accessory protein GvpC forms a helical cage around the GvpA shell, providing structural reinforcement. Together, our results help explain the remarkable mechanical properties of GVs and their ability to adopt different diameters and shapes.

molecular biology↗

bioRxiv2022

Rubisco forms a lattice inside alpha-carboxysomes

Bacteria employ microcompartments to sequester enzymatic processes, either for purposes of protecting cellular contents from reactive intermediates or as a way of increasing reaction efficiency. In these structures, a cargo of enzymes and accessory proteins is encased within a semi-permeable protein shell that permits passage of substrates and products but restricts movement of intermediates. In addition to their importance as a component of many bacterial species metabolisms, microcompartments have recently become a target of protein engineering. The shells can be reassembled from purified proteins, and the full operons can be functionally expressed outside their native prokaryotes and can remain functional following purification. Despite the importance of microcompartments in prokaryotic biology and bioengineering, structural heterogeneity has prevented a complete understanding of their architecture, ultrastructure, and spatial organization. Here, we employ cryo electron tomography to image -carboxysomes, a pseudo-icosahedral microcompartment responsible for carbon fixation. We have solved a high-resolution subtomogram average of the Rubisco cargo in situ, and determined a novel arrangement of the enzyme. We find that the H. neapolitanus Rubisco polymerizes in vivo, mediated by the small Rubisco subunit. These fibrils can further pack to form a lattice with six-fold pseudo-symmetry. This arrangement preserves freedom of motion and accessibility around the Rubisco active site and the binding sites for two other carboxysome proteins, CsoSCA (a carbonic anhydrase) and the disordered CsoS2, even at Rubisco concentrations exceeding 800 M. This characterization of Rubisco cargo inside the -carboxysome provides new insight into the balance between order and disorder in microcompartment organization.

biophysics↗

bioRxiv2021

In situ imaging of bacterial membrane projections and associated protein complexes using electron cryo-tomography

The ability to produce membrane projections in the form of tubular membrane extensions (MEs) and membrane vesicles (MVs) is a widespread phenomenon among bacteria. Despite this, our knowledge of the ultrastructure of these extensions and their associated protein complexes remains limited. Here, we surveyed the ultrastructure and formation of MEs and MVs, and their associated protein complexes, in tens of thousands of electron cryo-tomograms of [~] 90 bacterial species that we have collected for various projects over the past 15 years (Jensen lab database), in addition to data generated in the Briegel lab. We identified MEs and MVs in 13 species and classified several major ultrastructures: 1) tubes with a uniform diameter (with or without an internal scaffold), 2) tubes with irregular diameter, 3) tubes with a vesicular dilation at their tip, 4) pearling tubes, 5) connected chains of vesicles (with or without neck-like connectors), 6) budding vesicles and nanopods. We also identified several protein complexes associated with these MEs and MVs which were distributed either randomly or exclusively at the tip. These complexes include a secretin-like structure and a novel crown-shaped structure observed primarily in vesicles from lysed cells. In total, this work helps to characterize the diversity of bacterial membrane projections and lays the groundwork for future research in this field.

molecular biology↗

bioRxiv2021

Measuring gas vesicle dimensions by electron microscopy

Gas vesicles (GVs) are cylindrical or spindle-shaped protein nanostructures filled with air and used for flotation by various cyanobacteria, heterotrophic bacteria, and Archaea. Recently, GVs have gained interest in biotechnology applications due to their ability to serve as imaging agents and actuators for ultrasound, magnetic resonance and several optical techniques. The diameter of GVs is a crucial parameter contributing to their mechanical stability, buoyancy function and evolution in host cells, as well as their properties in imaging applications. Despite its importance, reported diameters for the same types of GV differ depending on the method used for its assessment. Here, we provide an explanation for these discrepancies and utilize electron microscopy (EM) techniques to accurately estimate the diameter of the most commonly studied types of GVs. We show that during air drying on the EM grid, GVs flatten, leading to a ~1.5-fold increase in their apparent diameter. We demonstrate that GVs diameter can be accurately determined by direct measurements from cryo-EM samples or alternatively indirectly derived from widths of flat collapsed and negatively stained GVs. Our findings help explain the inconsistency in previously reported data and provide accurate methods to measure GV dimensions.

biochemistry↗

bioRxiv2020

A Prokaryotic Membrane Sculpting BAR Domain Protein

Bin/Amphiphysin/RVS (BAR) domain proteins belong to a superfamily of coiled-coil proteins influencing membrane curvature in eukaryotes and are associated with vesicle biogenesis, vesicle-mediated protein trafficking, and intracellular signaling. Here we report the first prokaryotic BAR domain protein, BdpA, from Shewanella oneidensis MR-1, known to produce redox-active membrane vesicles and micrometer-scale outer membrane extensions (OMEs). BdpA is required for uniform size distribution of membrane vesicles and scaffolding OMEs into a consistent diameter and curvature. Cryogenic transmission electron microscopy reveals a strain lacking BdpA produces lobed, disordered OMEs rather than membrane tubes produced by the wild type strain. Overexpression of BdpA promotes OME formation during conditions where they are less common. Heterologous expression results in OME production in Marinobacter atlanticus and Escherichia coli. Based on the ability of BdpA to alter membrane curvature in vivo, we propose that BdpA and its homologs comprise a newly identified class of prokaryotic BAR (P-BAR) domains.

microbiology↗

bioRxiv2018

Rapid Tilt-Series Acquisition for Electron Cryotomography

Using a new Titan Krios stage equipped with a single-axis holder, we developed two methods to accelerate the collection of tilt-series. We demonstrate a continuous-tilting method that can record a tilt-series in seconds (about 100x faster than current methods), but with loss of details finer than [~]4 nm. We also demonstrate a fast-incremental method that can record a tilt-series about 10x faster than current methods and with similar resolution. We characterize the utility of both methods in real biological electron cryotomography workflows. We identify opportunities for further improvements in hardware and software and speculate on the impact such advances could have on structural biology.

biophysics↗

bioRxiv2018

Fluorescence-Based Detection of Fusion State on a Cryo-EM Grid using Correlated Cryo-Fluorescence and Cryo-Electron Microscopy

Correlated light and electron microscopy (CLEM) has become a popular technique for combining the protein-specific labeling of fluorescence with electron microscopy, both at room and cryogenic temperatures. Fluorescence applications at cryo-temperature have typically been limited to localization of tagged protein oligomers due to known issues of extended triplet state duration, spectral shifts, and reduced photon capture through cryo-CLEM objectives. Here, we consider fluorophore characteristics and behaviors that could enable more extended applications. We describe how dialkylcarbocanine DiD and its autoquenching by resonant energy transfer can be used to distinguish the fusion state of a lipid bilayer at cryo-temperatures. By adapting an established fusion assay to work under cryo-CLEM conditions, we identified areas of fusion between influenza virus-like particles and fluorescently labeled lipid vesicles on a cryo-EM grid. This result demonstrates that cryo-CLEM can be used to localize functions in addition to tagged proteins, and that fluorescence autoquenching by resonant energy transfer can be incorporated successfully into cryo-CLEM approaches. In the case of membrane fusion applications, this method provides both an orthogonal confirmation of functional state independent of the morphological description from cryo-EM and a way to bridge room-temperature kinetic assays and the cryo-EM images.

biophysics↗

bioRxiv2017

Conformational changes in Arp2/3 complex induced by ATP, WASp-VCA and actin filaments

We used fluorescence spectroscopy and electron microscopy to determine how binding of ATP, nucleation-promoting factors (NPF), actin monomers and actin filaments change the conformation of Arp2/3 complex during the process that nucleates an actin filament branch. We mutated subunits of Schizosaccharomyces pombe Arp2/3 complex for labeling with fluorescent dyes at either the C-termini of Arp2 and Arp3 or ArpC1 and ArpC3. We measured Forster resonance energy transfer (FRET) efficiency (ETeff) between the dyes in the presence of the various ligands. We also computed class averages from electron micrographs of negatively stained specimens. ATP binding made small conformational changes of the nucleotide binding clefts of the Arp subunits. WASp-VCA, WASp-CA, and WASp-actin-VCA changed the ETeff between the dyes on the Arp2 and Arp3 subunits much more than between dyes on ArpC1 and ArpC3. Ensemble FRET detected a different structural change that involves bringing ArpC1 and ArpC3 closer together when Arp2/3 complex bound actin filaments. Each of the ligands that activates Arp2/3 complex changes the structure in different ways, each leading progressively to fully activated Arp2/3 complex on the side of a filament.

biophysics↗