Faculty Bibliography

Members of the Mammalian Cell Entry (MCE) superfamily interact with other proteins to form diverse architectures for the transport of hydrophobic molecules across the cell envelope in Gram-negative bacteria. Some of these trans-envelope MCE protein complexes include a PqiC-like outer membrane (OM) lipoprotein component. The best-studied member of this group of OM lipoproteins is E. coli PqiC, from the PqiABC system, which can form an octameric ring. How PqiC-like lipoproteins interact with their MCE protein binding partners to facilitate transport is not well understood. Here we report the cryo-electron microscopy structures of Pseudomonas aeruginosa PA3214, a homolog of PqiC, in the context of the full MCE transport PA3211-PA3214 system. Our structure provides insight into the biological assembly of the lipoprotein and interactions with its binding partner, MCE protein PA3213. We utilize deep mutational scanning to identify functionally important sites in E. coli PqiC in an unbiased manner. Through phenotypic and biochemical experiments, we characterize the interactions of the lipoproteins PqiC and PA3214 with their associated MCE proteins PqiB and PA3213, thus providing a model for how some MCE proteins employ a C-terminal peptide to mediate key interactions with their cognate lipoproteins at the OM.

Cryo-electron tomography (cryo-ET) has emerged as a transformative technique for visualizing the native ultrastructure of eukaryotic parasites, from proteins to cellular architecture. Recent technical advances in sample preparation, data collection, and computational analysis have enabled unprecedented insights into structural cell biology of medically important pathogens including Toxoplasma gondii, Plasmodium falciparum, Trypanosoma brucei and Trypanosoma cruzi, Cryptosporidium parvum, and Microsporidia. This review highlights the range of resolutions and cellular structures accessible by cryo-ET, and the kinds of biological insights that may be obtained, using eukaryotic parasites as case studies. Lower-resolution data (20-30 Å) provide structural information on organelles, whole-cells, and cell-cell interactions, while at the higher-resolution end, near-atomic structures can be resolved in situ using subtomogram averaging, typically for large, abundant particles such as ribosomes. Combined with orthogonal techniques, cryo-ET is a powerful tool for studying the structural cell biology of parasites.

Membrane transport proteins translocate diverse cargos, ranging from small sugars to entire proteins, across cellular membranes^1-3. A few structurally distinct protein families have been described that account for most of the known membrane transport processes^4-6. However, many membrane proteins with predicted transporter functions remain uncharacterized. Here we determined the structure of Escherichia coli LetAB, a phospholipid transporter involved in outer membrane integrity, and found that LetA adopts a distinct architecture that is structurally and evolutionarily unrelated to known transporter families. LetA localizes to the inner membrane, where it is poised to load lipids into its binding partner, LetB, a mammalian cell entry (MCE) protein that forms an approximately 225 Å long tunnel for lipid transport across the cell envelope. Unexpectedly, the LetA transmembrane domains adopt a fold that is evolutionarily related to the eukaryotic tetraspanin family of membrane proteins, including transmembrane AMPA receptor regulatory proteins (TARPs) and claudins. Through a combination of deep mutational scanning, molecular dynamics simulations, AlphaFold-predicted alternative states and functional studies, we present a model for how the LetA-like family of membrane transporters facilitates the transport of lipids across the bacterial cell envelope.

Directional interactions that generate regular coordination geometries are a powerful means of guiding molecular and colloidal self-assembly, but implementing such high-level interactions with proteins remains challenging due to their complex shapes and intricate interface properties. Here we describe a modular approach to protein nanomaterial design inspired by the rich chemical diversity that can be generated from the small number of atomic valencies. We design protein building blocks using deep learning-based generative tools, incorporating regular coordination geometries and tailorable bonding interactions that enable the assembly of diverse closed and open architectures guided by simple geometric principles. Experimental characterization confirms the successful formation of more than 20 multicomponent polyhedral protein cages, two-dimensional arrays and three-dimensional protein lattices, with a high (10%-50%) success rate and electron microscopy data closely matching the corresponding design models. Due to modularity, individual building blocks can assemble with different partners to generate distinct regular assemblies, resulting in an economy of parts and enabling the construction of reconfigurable networks for designer nanomaterials.

Microsporidia are single-celled intracellular parasites that cause opportunistic diseases in humans. Encephalitozoon intestinalis is a prevalent human-infecting species that invades the small intestine. Macrophages are potential reservoirs of infection, and dissemination to other organ systems is also observed. The macrophage response to infection and the developmental trajectory of the parasite are not well studied. Here we use single cell RNA sequencing to investigate transcriptional changes in both the parasite and the host during E. intestinalis infection of human macrophages in vitro. The parasite undergoes large transcriptional changes throughout the life cycle, providing a blueprint for parasite development. While a small population of infected macrophages mount a response, most remain transcriptionally unchanged, suggesting that the majority of parasites may avoid host detection. The stealthy microsporidian lifestyle likely allows these parasites to harness macrophages for replication. Together, our data provide insights into the host response in primary human macrophages and the E. intestinalis developmental program.

Microsporidia are divergent fungal pathogens that employ a unique harpoon-like apparatus called the polar tube (PT) to invade host cells. The long PT is fired out of the microsporidian spore over the course of just a few hundred milliseconds. Once fired, the PT is thought to pierce the plasma membrane of a target cell and act as a conduit for the transfer of the parasite into the host cell, which initiates infection. The PT architecture and its association with neighboring organelles within the parasite cell remain poorly understood. Here, we use cryoelectron tomography to investigate the structural cell biology of the PT in dormant spores from the human-infecting microsporidian species, Encephalitozoon intestinalis. Segmentation and subtomogram averaging of the PT reveal at least four layers: two protein-based layers surrounded by a membrane layer and filled with a dense core. Regularly spaced protein filaments form the structural skeleton of the PT. Combining cryoelectron tomography with cellular modeling, we propose a model for the three-dimensional organization of the polaroplast, an organelle that surrounds the PT and is continuous with the outermost, membranous layer of the PT. Our results reveal the ultrastructure of the microsporidian invasion apparatus in situ, laying the foundation for understanding infection mechanisms.

The outer membrane of Gram-negative bacteria provides a formidable barrier, essential for both pathogenesis and antimicrobial resistance. Biogenesis of this complex structure necessitates the transport of phospholipids across the cell envelope. Recently, YhdP was implicated as a major protagonist in the trafficking of inner membrane phospholipids to the outer membrane; however the molecular mechanism of YhdP mediated transport remains elusive. Here, utilising AlphaFold, we observe YhdP to form an elongated assembly of 60 β-strands that curve to form a continuous hydrophobic groove. This architecture is consistent with our negative stain electron microscopy data which reveals YhdP to be approximately 250 Å in length and thus sufficient to span the bacterial cell envelope. Furthermore, molecular dynamics simulations and bacterial growth assays indicate essential helical regions at the N- and C-termini of YhdP, that may embed into the inner and outer membranes respectively, reinforcing its envelope spanning nature. Our in vivo crosslinking data reveal phosphate-containing substrates captured along the length of the YhdP groove, providing direct evidence that YhdP interacts with a phosphate-containing substrate, which we propose to be phospholipids. This finding is congruent with our molecular dynamics simulations which demonstrate the propensity for inner membrane lipids to spontaneously enter the groove of YhdP. Collectively, our results support a model in which YhdP bridges the cell envelope, providing a hydrophobic environment for the transport of phospholipids to the outer membrane.

We describe a modular bond-centric approach to protein nanomaterial design inspired by the rich diversity of chemical structures that can be generated from the small number of atomic valencies and bonding interactions. We design protein building blocks with regular coordination geometries and bonding interactions that enable the assembly of a wide variety of closed and opened nanomaterials using simple geometrical principles. Experimental characterization confirms successful formation of more than twenty multi-component polyhedral protein cages, 2D arrays, and 3D protein lattices, with a high (10-50 %) success rate and electron microscopy data closely matching the corresponding design models. Because of the modularity, individual building blocks can assemble with different partners to generate distinct regular assemblies, resulting in an economy of parts and enabling the construction of reconfigurable systems.

In Mycobacterium tuberculosis (Mtb), proteins that are posttranslationally modified with a prokaryotic ubiquitin-like protein (Pup) can be degraded by bacterial proteasomes. A single Pup-ligase and depupylase shape the pupylome, but the mechanisms regulating their substrate specificity are incompletely understood. Here, we identified a depupylation regulator, a protein called CoaX, through its copurification with the depupylase Dop. CoaX is a pseudopantothenate kinase that showed evidence of binding to pantothenate, an essential nutrient Mtb synthesizes, but not its phosphorylation. In a ∆coaX mutant, pantothenate synthesis enzymes including PanB, a substrate of the Pup-proteasome system (PPS), were more abundant than in the parental strain. In vitro, CoaX specifically accelerated depupylation of Pup~PanB, while addition of pantothenate inhibited this reaction. In culture, media supplementation with pantothenate decreased PanB levels, which required CoaX. Collectively, we propose CoaX regulates PanB abundance in response to pantothenate levels by modulating its vulnerability to proteolysis by Mtb proteasomes.

Volume electron microscopy encompasses a set of electron microscopy techniques that can be used to examine the ultrastructure of biological tissues and cells in three dimensions. Two block face techniques, focused ion beam scanning electron microscopy (FIB-SEM) and serial block face scanning electron microscopy (SBF-SEM) have often been used to study biological tissue samples. More recently, these techniques have been adapted to in vitro tissue culture samples. Here we describe step-by-step protocols for two sample embedding methods for in vitro tissue culture cells intended to be studied using SBF-SEM. The first focuses on cell pellet embedding and the second on en face embedding. En face embedding can be combined with light microscopy, and this CLEM workflow can be used to identify specific biological events by light microscopy, which can then be imaged using SBF-SEM. We systematically outline the steps necessary to fix, stain, embed and image adherent tissue culture cell monolayers by SBF-SEM. In addition to sample preparation, we discuss optimization of parameters for data collection. We highlight the challenges and key steps of sample preparation, and the consideration of imaging variables.