Biology subjects

Oltrogge, L. M.

Publications and source records attributed to Oltrogge, L. M..

16 recordsLinked to original sources

bioRxiv2026

Activity and specificity trade-offs in adenine base editors

Adenine base editors (ABEs) are CRISPR effectors that introduce A-T to G-C transitions in the genome using a nucleotide deaminase fused to a Cas protein. ABEs have been evolved to have very high editing efficiency, but off-target editing effects compromise their precision and hinder their applications. Here we explore the activity and specificity relationship of ABEs using a combination of machine learning-guided design and high-throughput screening. We designed a diverse library of 12,000 variants and built quantitative bacterial selection systems that allowed us to simultaneously measure their on-target and off-target editing. We found that the ABEs were fully described by the single dimension of intrinsic deaminase activity with no evidence for independent specialization with respect to local sequence context, editing window width, RNA editing, or genotoxicity. These results were supported by in vitro studies and consistent with editing experiments in mammalian cells. Finally, the activity and specificity trade-offs were recapitulated among previously reported engineered variants and a selection of library variants spanning the activity spectrum. Our results suggest that fundamental architectural improvements will be necessary to transcend the activity and specificity limitations for the next generation of ABEs.

molecular biology↗

bioRxiv2026

Structural Insights into the Coupling Mechanism of Vectorial CO2 Uptake by DAB1

CO2 transporters enable bacterial carbon-concentrating mechanisms by catalyzing directional hydration of CO2, yet the basis of this vectorial carbonic anhydrase (CA) activity remains an open question. We used cryo-EM to determine the structure of the DAB1 complex from Thermocrinis albus to 2.13 [A], revealing a heterotrimer in which a deeply buried {beta}-CA active site in DabA is structurally coupled to the proton-translocating subunit DabB. Two conformational states define distinct solvent channels for substrate entry and product exit. A suppressor screen identifies mutations that disrupt coupling while retaining CA activity, underlying the importance of conserved residues that link proton translocation to active-site remodeling. These results support a model in which proton-driven conformational changes regulate substrate access to the active site, enabling vectorial CO2 hydration.

biochemistry↗

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

Exploring the deletion landscape of S. aureus Cas9 with SABER

Profiling tolerated amino acid deletions in proteins can elucidate structure-function relationships, reconstruct intermediate stages in protein evolution, and be used to engineer minimized versions of proteins with size-sensitive biotechnology applications. Despite advances in deletion library construction techniques over the past several decades, there are presently few methods available that are simultaneously efficient, precise, and easy to implement. Here we present SABER, a novel approach which utilizes SpRYCas9, a near-PAMless engineered SpCas9 variant, as a molecular biological tool for building deletion libraries with unprecedented speed and ease. We applied this technique to the small and structurally divergent Cas9 from Staphylococcus aureus (SaCas9) and mapped the set of deletions tolerated for DNA binding activity. We proceeded to use this information to design a set of minimal SaCas9-based effectors capable of CRISPRi transcriptional repression in bacterial cells. Our findings provide new insights into the function of certain structural elements in SaCas9, and we anticipate that our dSaCas9 deletion map may prove useful in further efforts to develop minimal Cas9-based effectors and gene editors.

molecular biology↗

bioRxiv2023

Mapping the biochemical landscape of rubisco

Rubisco is the primary CO2 fixing enzyme of the biosphere yet has slow kinetics. The roles of evolution and chemical mechanism in constraining the sequence landscape of rubisco remain debated. In order to map sequence to function, we developed a massively parallel assay for rubisco using an engineered E. coli where enzyme function is coupled to growth. By assaying >99% of single amino acid mutants across CO2 concentrations, we inferred enzyme velocity and CO2 affinity for thousands of substitutions. We identified many highly conserved positions that tolerate mutation and rare mutations that improve CO2 affinity. These data suggest that non-trivial kinetic improvements are readily accessible and provide a comprehensive sequence-to-function mapping for enzyme engineering efforts.

biochemistry↗

bioRxiv2023

α-carboxysome size is controlled by the disordered scaffold protein CsoS2

Carboxysomes are protein microcompartments that function in the bacterial CO2 concentrating mechanism (CCM) to facilitate CO2 assimilation. To do so, carboxysomes assemble from thousands of constituent proteins into an icosahedral shell which encapsulates the enzymes rubisco and carbonic anhydrase to form structures typically >100 nm and >300 megadaltons. Although many of the protein interactions driving the assembly process have been determined, it remains unknown how size and composition are precisely controlled. Here we show that the size of -carboxysomes is controlled by the disordered scaffolding protein CsoS2. CsoS2 contains two classes of related peptide repeats which bind to the shell in a distinct fashion, and our data indicate that size is controlled by the relative number of these interactions. We propose an energetic and structural model wherein the two repeat classes bind at the junction of shell hexamers but differ in their preferences for the shell contact angles, and thus the local curvature. In total, this model suggests that a set of specific and repeated interactions between CsoS2 and shell proteins collectively achieve the large size and monodispersity of -carboxysomes.

biochemistry↗

bioRxiv2023

Conserved and repetitive motifs in an intrinsically disordered protein drive α-carboxysome assembly

All photosynthetic bacteria and some chemoautotrophic bacteria fix CO2 into sugars in specialized proteinaceous compartments called carboxysomes. Carboxysomes enclose the enzymes Rubisco and carbonic anhydrase inside a layer of shell proteins to increase the CO2 concentration for efficient carbon fixation by Rubisco. In the -carboxysome lineage, a disordered and highly repetitive protein named CsoS2 is essential for carboxysome formation and function. Without it, the bacteria are unable to fix enough carbon to grow in air. How a protein lacking structure serves as the architectural scaffold for such a vital cellular compartment remains unknown. In this study, we identify key residues in CsoS2 that are necessary for building functional -carboxysomes in vivo. These highly conserved and repetitive residues, VTG and Y, contribute to the interaction between CsoS2 and shell proteins. We also demonstrate in vitro reconstitution of the -carboxysome into spherical condensates with CsoS2, Rubisco, and shell proteins, and show the utility of reconstitution as a biochemical tool to study carboxysome biogenesis. The precise self-assembly of thousands of proteins is crucial for carboxysome formation, and understanding this process could enable their use in alternative biological hosts or industrial processes as effective tools to fix carbon.

molecular biology↗

bioRxiv2022

Autotrophic biofilms sustained by deeply-sourced groundwater host diverse CPR bacteria implicated in sulfur and hydrogen metabolism

BackgroundCandidate Phyla Radiation (CPR) bacteria are commonly detected yet enigmatic members of diverse microbial communities. Their host associations, metabolic capabilities, and potential roles in biogeochemical cycles remain under-explored. We studied chemoautotrophically-based biofilms that host diverse CPR bacteria and grow in sulfide-rich springs using bulk geochemical analysis, genome-resolved metagenomics and scanning transmission x-ray microscopy (STXM) at room temperature and 87{degrees} K. ResultsCPR-affiliated Gracilibacteria, Absconditabacteria, Saccharibacteria, Peregrinibacteria, Berkelbacteria, Microgenomates, and Parcubacteria are members of two biofilm communities dominated by chemolithotrophic sulfur-oxidizing bacteria including Thiothrix or Beggiatoa. STXM imaging revealed ultra-small cells along the surfaces of filamentous bacteria that we interpret are CPR bacterial episymbionts. STXM and NEXAFS spectroscopy at carbon K and sulfur L2,3 edges show protein-encapsulated elemental sulfur spherical granules associated with filamentous bacteria, indicating that they are sulfur-oxidizers, likely Thiothrix. Berkelbacteria and Moranbacteria in the same biofilm sample are predicted to have a novel electron bifurcating group 3b [NiFe]-hydrogenase, putatively a sulfhydrogenase, potentially linked to sulfur metabolism via redox cofactors. This complex could potentially underpin a symbiosis involving Berkelbacteria and/or Moranbacteria and filamentous sulfur-oxidizing bacteria such as Thiothrix that is based on cryptic sulfur cycling. One Doudnabacteria genome encodes adjacent sulfur dioxygenase and rhodanese genes that may convert thiosulfate to sulfite. We find similar conserved genomic architecture associated with CPR bacteria from other sulfur-rich subsurface ecosystems. ConclusionsOur combined metagenomic, geochemical, spectromicroscopic and structural bioinformatics analyses link some CPR bacteria to sulfur-oxidizing Proteobacteria, likely Thiothrix, and indicate roles for CPR bacteria in sulfur and hydrogen cycling.

microbiology↗

bioRxiv2022

Exploring masses and internal mass distributions of single carboxysomes in free solution using fluorescence and interferometric scattering in an anti-Brownian trap

Carboxysomes are self-assembled bacterial microcompartments that facilitate carbon assimilation by co-localizing the enzymes of CO2 fixation within a protein shell. These microcompartments can be highly heterogeneous in their composition and filling, so measuring the mass and loading of an individual carboxysome would allow for better characterization of its assembly and function. To enable detailed and extended characterizations of single nanoparticles in solution, we recently demonstrated an improved Interferometric Scattering Anti-Brownian ELectrokinetic (ISABEL) trap, which tracks the position of a single nanoparticle via its scattering of a near-infrared beam and applies feedback to counteract its Brownian motion. Importantly, the scattering signal can be related to the mass of nanoscale proteinaceous objects, whose refractive indices are well-characterized. We calibrate single-particle scattering cross-section measurements in the ISABEL trap and determine individual carboxysome masses in the 50-400 MDa range by analyzing their scattering cross-sections with a core-shell model. We further investigate carboxysome loading by combining mass measurements with simultaneous fluorescence reporting from labeled internal components. This method may be extended to other biological objects, such as viruses or extracellular vesicles, and can be combined with orthogonal fluorescence reporters to achieve precise physical and chemical characterization of individual nanoscale biological objects. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=103 SRC="FIGDIR/small/505029v1_ufig1.gif" ALT="Figure 1"> View larger version (21K): org.highwire.dtl.DTLVardef@18e9f99org.highwire.dtl.DTLVardef@900912org.highwire.dtl.DTLVardef@4398baorg.highwire.dtl.DTLVardef@1dff103_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

bioRxiv2022

Trajectories for the evolution of bacterial CO2-concentrating mechanisms

Cyanobacteria rely on CO2 concentrating mechanisms (CCMs) that depend on {approx}15 genes to produce two protein complexes: an inorganic carbon (Ci) transporter and a 100+ nm carboxysome compartment that encapsulates rubisco with a carbonic anhydrase (CA) enzyme. Mutations disrupting CCM components prohibit growth in todays atmosphere (0.04% CO2), indicating that CCMs evolved to cope with declining environmental CO2. Indeed, geochemical data and models indicate that atmospheric CO2 has been generally decreasing from high concentrations over the last {approx}3.5 billion years. We used a synthetic reconstitution of a bacterial CCM in E. coli to study the co-evolution of CCMs with atmospheric CO2. We constructed strains expressing putative ancestors of modern CCMs -- strains lacking one or more CCM components -- and evaluated their growth in a variety of CO2 concentrations. Partial forms expressing CA or Ci uptake genes grew better than controls in intermediate CO2 levels ({approx}1%); we observed similar phenotypes in genetic studies of two autotrophic bacteria, H. neapolitanus and C. necator. To explain how partial CCMs improve growth, we advance a model of co-limitation of autotrophic growth by CO2 and HCO3-, as both are required to produce biomass. Our model and results delineated a viable trajectory for bacterial CCM evolution where decreasing atmospheric CO2 induces an HCO3- deficiency that is alleviated by acquisition of CAs or Ci uptake genes, thereby enabling the emergence of a modern CCM. This work underscores the importance of considering physiology and environmental context when studying the evolution of biological complexity. SignificanceThe greenhouse gas content of the ancient atmosphere is estimated using models and measurements of geochemical proxies. Some inferred high ancient CO2 levels using models of biological CO2 fixation to interpret the C isotopes found in preserved organic matter. Others argued that elevated CO2 could reconcile a faint young Sun with evidence for liquid water on Earth. We took a complementary "synthetic biological" approach to understanding the composition of the ancient atmosphere by studying present-day bacteria engineered to resemble ancient autotrophs. By showing that it is simpler to rationalize the emergence of modern bacterial autotrophs if CO2 was once high, these investigations provided independent evidence for the view that CO2 concentrations were significantly elevated in the atmosphere of early Earth.

microbiology↗

bioRxiv2022

Ratiometric Sensing of Redox Environments Inside Individual Carboxysomes Trapped in Solution

Diffusion of biological nanoparticles in solution impedes our ability to continuously monitor individuals and measure their physical and chemical properties. To overcome this, we previously developed the Interferometric Scattering Anti-Brownian ELectrokinetic (ISABEL) trap, which uses scattering to localize a particle and applies electrokinetic forces which counteract Brownian motion, thus enabling extended observation. Here, we present an improved ISABEL trap that incorporates a near-infrared scatter illumination beam and rapidly interleaves 405 and 488 nm fluorescence excitation reporter beams. With the ISABEL trap, we monitor the internal redox environment of individual carboxysomes labeled with the ratiometric redox reporter roGFP2. Carboxysomes widely vary in scattering contrast (reporting on size) and redox-dependent ratiometric fluorescence. Further, we used redox sensing to explore the chemical kinetics within intact carboxysomes, where bulk measurements may contain unwanted contributions from aggregates or interfering fluorescent proteins. Overall, we demonstrate the ISABEL traps ability to sensitively monitor nanoscale biological objects, enabling new experiments on these systems. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=167 SRC="FIGDIR/small/484789v1_ufig1.gif" ALT="Figure 1"> View larger version (45K): org.highwire.dtl.DTLVardef@1fccfacorg.highwire.dtl.DTLVardef@a639b6org.highwire.dtl.DTLVardef@116fff6org.highwire.dtl.DTLVardef@fe95fd_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

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

The encapsulin from Thermatoga maritima is a flavoprotein with a symmetry matched ferritin-like cargo protein

Bacterial nanocompartments, also known as encapsulins, are an emerging class of protein-based organelles found in bacteria and archaea. Encapsulins are virus-like icosahedral particles comprising a [~]25-50 nm shell surrounding a specific cargo enzyme. Compartmentalization is thought to create a unique chemical environment to facilitate catalysis and isolate toxic intermediates. Many questions regarding nanocompartment structure-function remain unanswered, including how shell symmetry dictates cargo loading and to what extent the shell facilitates enzymatic activity. Here, we explore these questions using the model T. maritima nanocompartment known to encapsulate a redox-active ferritin-like protein. Biochemical analysis revealed the encapsulin shell to possess a flavin binding site located at the interface between capsomere subunits, suggesting the shell may play a direct and active role in the function of the encapsulated cargo. Furthermore, we used cryoEM to show that cargo proteins use a form of symmetry-matching to facilitate encapsulation and define stoichiometry. In the case of the T. maritima encapsulin, the decameric cargo protein with 5-fold symmetry preferentially binds to the pentameric-axis of the icosahedral shell. Taken together, these observations suggest the shell is not simply a passive barrier--it also plays a significant role in the structure and function of the cargo enzyme.

biochemistry↗

bioRxiv2020

Discovery and characterization of a novel family of prokaryotic nanocompartments involved in sulfur metabolism

Prokaryotic nanocompartments, also known as encapsulins, are a recently discovered proteinaceous organelle in prokaryotes that compartmentalize cargo enzymes. While initial studies have begun to elucidate the structure and physiological roles of encapsulins, bioinformatic evidence suggests that a great diversity of encapsulin nanocompartments remains unexplored. Here, we describe a novel encapsulin in the freshwater cyanobacterium Synechococcus elongatus PCC 7942. This nanocompartment is upregulated upon sulfate starvation and encapsulates a cysteine desulfurase enzyme via an N-terminal targeting sequence. Using cryoelectron microscopy, we have determined the structure of the nanocompartment complex to 2.2 [A] resolution. Lastly, biochemical characterization of the complex demonstrated that the activity of the cysteine desulfurase is enhanced upon encapsulation. Taken together, our discovery, structural analysis, and enzymatic characterization of this prokaryotic nanocompartment provide a foundation for future studies seeking to understand the physiological role of this encapsulin in various bacteria.

biochemistry↗

bioRxiv2019

α-carboxysome formation is mediated by the multivalent and disordered protein CsoS2

Carboxysomes are bacterial microcompartments that function as the centerpiece of the bacterial CO2-concentrating mechanism, feeding high concentrations of CO2 to the enzyme Rubisco for fixation. The carboxysome self-assembles from thousands of individual proteins into icosahedral-like particles with a dense enzyme cargo encapsulated within a proteinaceous shell. In the case of the -carboxysome, there is little molecular insight into protein-protein interactions which drive the assembly process. Here we show that the N-terminus of CsoS2, an intrinsically disordered protein found in the -carboxysome, possesses a repeated peptide sequence that binds Rubisco. X-ray structural analysis of the peptide bound to Rubisco reveals a series of conserved electrostatic interactions that are only made with properly assembled hexadecameric Rubisco. Although biophysical measurements indicate this single interaction is weak, its implicit multivalency induces high-affinity binding through avidity. Taken together, our results indicate CsoS2 acts as an interaction hub to condense Rubisco and enable efficient -carboxysome formation.

biochemistry↗

bioRxiv2018

Genome-wide screening reveals a novel class of carbonic anhydrase-like inorganic carbon transporters in chemoautotrophic bacteria

Many bacterial autotrophs rely on CO2 concentrating mechanisms (CCMs) to assimilate carbon. Although many CCM proteins have been identified, including a 200+ MDa protein organelle called the carboxysome, a systematic screen of CCM components has not been carried out. Here, we performed a genome-wide barcoded transposon screen to identify essential and CCM-related genes in the {gamma}-proteobacterium H. neapolitanus. Our screen revealed an operon encoding a domain of unknown function (PFAM:PF10070) and putative cation transporter subunit (PFAM:PF00361) is critical for CCM function. These two proteins, which we name DabA and DabB for \"DABs accumulate bicarbonate,\" function as a heterodimeric, energy-coupled inorganic carbon pump in E. coli. Furthermore, DabA has distant homology to a {beta}-carbonic anhydrase and binds a zinc ion necessary for activity. Based on these results, we propose that DABs function as vectorial CAs coupled to cation gradients and serve as inorganic carbon pumps throughout prokaryotic phyla.

molecular biology↗