Uchikawa, E. | bioRxiv Science

bioRxiv2025

Structure and Activation Mechanism of a Lamassu Phage Defence System

Lamassu is a diverse family of defence systems that protect bacteria, including pandemic strains of Vibrio cholerae, against phage infection. They target essential cellular processes, aborting infection and preventing phage propagation by terminating the infected host. The mechanisms by which Lamassu efectors are activated when needed and otherwise suppressed are unknown. Here, we present structures of a Lamassu defence system from Salmonella enterica. We show that an oligomerization domain of the nuclease efector, LmuA, is sequestered by two tightly-folded SMC-like LmuB protomers and LmuC. Upon activation, liberated LmuA proteins assemble into a cyclic homo-tetramer, in which two of four nuclease domains are brought into proximity to create an active site capable of cleaving DNA. We propose tetramer formation is likely a one-way switch that establishes a threshold to limit potential spontaneous activation and cell death. Our findings reveal a mechanism of cellular defence, involving liberation and oligomerization of immune efectors, and shed light on how Lamassu systems balance potent immune responses with self-preservation.

microbiology↗

bioRxiv2022

Cryo-EM structures and binding of mouse ACE2 to SARS-CoV-2 variants of concern

Investigation of potential hosts of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is crucial to understanding future risks of spillover and spillback. SARS-CoV-2 has been reported to be transmitted from humans to various animals after requiring relatively few mutations.[1] There is significant interest in describing how the virus interacts with mice as they are well adapted to human environments, are used widely as infection models and can be infected.[2] Structural and binding data of the mouse ACE2 receptor with the Spike protein of newly identified SARS-CoV-2 variants are needed to better understand the impact of immune system evading mutations present in variants of concern (VOC). Previous studies have developed mouse-adapted variants and identified residues critical for binding to heterologous ACE2 receptors.[3,4] Here we report the cryo-EM structures of mouse ACE2 bound to trimeric Spike ectodomains of four different VOC: Beta, Omicron BA.1, Omicron BA.2.12.1 and Omicron BA.4/5. These variants represent the oldest to the newest variants known to bind the mouse ACE2 receptor. Our high-resolution structural data complemented with bio-layer interferometry (BLI) binding assays reveal a requirement for a combination of mutations in the Spike protein that enable binding to the mouse ACE2 receptor. AUTHOR SUMMARYThe SARS-CoV-2 virus can infect different types of animals beyond humans. The virus uses its Spike protein on its surface to bind to cells. These cells have a protein called ACE2 that the Spike protein recognizes. Animals have slightly different ACE2 receptors compared to humans. Mice are widely used as a research animal and live in the same environments as humans so scientists are particularly interested. Understanding how Spike proteins binds to the mouse ACE2 receptor allows us to understand the impact of immune evading mutations found in new variants. We use a high resolution imaging technique called cryo-electron microscopy to look at how different Spike variants bind to the ACE2 receptor from mouse at a resolution where we can see the amino acids. We can see directly the individual amino acids and mutations on the Spike protein that interact with the mouse ACE2 receptor. Many of the mutations found in variants of concern also increase the strength of binding to the mouse ACE2 receptor. This result suggests that mutations in the Spike protein of future variants may have an additional effect in influencing how it binds to not only human ACE2 receptors but to mice and also different animals.

immunology↗