Biology subjects

Letham, S. C.

Publications and source records attributed to Letham, S. C..

4 recordsLinked to original sources

bioRxiv2026

Intracellular barriers and receptor masking limit success of a Pseudomonas aeruginosa clinical phage family

Bacteriophage therapy is needed to treat antibiotic resistant infections; however, when a clinical isolate resists a given phage, it is often unclear why. It is therefore currently unknown how to rationally fortify phage therapies to circumvent a priori resistance. Using a family of broad host range therapeutic Pseudomonas aeruginosa phages (Pbunaviruses), we show that cell surface receptor masking and intracellular defenses are both common barriers in distinct clinical isolates. In some cases these barriers can be bypassed by intrafamily phage engineering. Using unbiased genome-wide CRISPRi screens, we reveal that the broadly conserved L-Rhamnose in the core polysaccharide is the receptor for Pbunavirus family. This molecule is often masked by diverse O-antigen structures. In other isolates with the L-Rha receptor accessible, internal defense mechanisms commonly prevent Pbunavirus DNA replication. A single anti-defense locus often encoding 8-11 different genes within the Pbunavirus family is required for optimal host range, providing anti-defense genes that enable replication of both Pbunavirus phages and phages of other families. Our work demonstrates the importance of both internal and surface defense mechanisms in clinical isolates causally antagonizing a commonly used phage therapeutic and presents phage engineering strategies to circumvent a priori resistance.

microbiology↗

bioRxiv2022

Biophysical basis of phage liquid crystalline droplet-mediated antibiotic tolerance in pathogenic bacteria

Inoviruses are abundant filamentous phages infecting numerous prokaryotic phyla, where they can symbiotically promote host fitness and increase bacterial virulence. Due to their unique properties, inoviruses have also been utilised in biotechnology for phage display and as models for studying phase behaviour of colloidal rods. Inoviral phages secreted by bacteria can self-assemble into liquid crystalline droplets that protect bacterial cells in biofilms from antibiotics, however, factors governing the formation of such droplets and the mechanism of antibiotic protection are poorly understood. Here, we investigate the structural, biophysical, and protective properties of liquid crystalline droplets formed by Pseudomonas aeruginosa and Escherichia coli inoviral phages. We report a cryo-EM structure of the capsid from the highly studied E. coli fd phage, revealing distinct biochemical properties of fd compared to Pf4 phage from P. aeruginosa. We show that fd and Pf4 form liquid crystalline droplets with diverse morphologies governed by the underlying phage particle geometry and biophysics, rather than their surface biochemical properties. Finally, we show that these morphologically diverse droplets made of either phage can protect rod-shaped bacteria from antibiotic treatment, despite differing modes of association with cells. This study advances our understanding of phage assembly into liquid crystalline droplets, and provides insights into how filamentous molecules protect bacteria from extraneous molecules under crowding conditions, which are found in biofilms or on infected host tissues.

microbiology↗

bioRxiv2021

Relative Affinities of Protein-Cholesterol Interactions from Equilibrium Molecular Dynamics Simulations

Specific interactions of lipids with membrane proteins contribute to protein stability and function. Multiple lipid interactions surrounding a membrane protein are often identified in molecular dynamics (MD) simulations and are, increasingly, resolved in cryo-EM densities. Determining the relative importance of specific interaction sites is aided by determination of lipid binding affinities by experimental or simulation methods. Here, we develop a method for determining protein-lipid binding affinities from equilibrium coarse-grained MD simulations using binding saturation curves, designed to mimic experimental protocols. We apply this method to directly obtain affinities for cholesterol binding to multiple sites on a range of membrane proteins and compare our results with free energies obtained from density-based equilibrium methods and with potential of mean force calculations, getting good agreement with respect to the ranking of affinities for different sites. Thus, our binding saturation method provides a robust, high-throughput alternative for determining the relative consequence of individual sites seen in e.g. cryo-EM derived membrane protein structures surrounded by a plethora of ancillary lipid densities.

biophysics↗

bioRxiv2019

Structure of D. melanogaster ARC1 reveals a repurposed molecule with characteristics of retroviral Gag

The tetrapod neuronal protein ARC and its D. melanogaster homologue, dARC1, have important but differing roles in neuronal development. Both are thought to originate through exaptation of ancient Ty3/Gypsy retrotransposon Gag genes, with their novel function relying on an original capacity for self-assembly and encapsidation of nucleic acids. Here, we present the crystal structure of dARC1 CA and examine the relationship between dARC1, mammalian ARC and the CA protein of circulating retroviruses. We show that whilst the overall architecture is highly related to that of orthoretroviral and spumaretroviral CA, there are significant deviations in both N- and C-terminal domains, potentially affecting recruitment of partner proteins and particle assembly. The degree of sequence and structural divergence suggests that Ty3/Gypsy Gag has been exapted on two separate occasions and that, although mammalian ARC and dARC1 share functional similarity, the structures have undergone different adaptations after appropriation into the tetrapod and insect genomes.

evolutionary biology↗