Biology subjects

Bravo, J. P. K.

Publications and source records attributed to Bravo, J. P. K..

8 recordsLinked to original sources

bioRxiv2026

The bacterial antiphage protein Tiamat is an ATPase and DNase whose homologs are found across the tree of life

Bacteria and archaea encode hundreds of antiviral defense systems, some with homologs found in eukaryotic innate immunity. GHL ATPases like plant MORCs and prokaryotic Tiamat defend against viruses and phage, though their architecture and biochemical activities have remained unclear. Searching thousands of proteomes, we find Tiamat is rare in prokaryotes yet enriched across eukaryotes, including green plants, aquatic chordates, sponges, placozoans, and water molds. We resolve the central DUF3684 and flanking region into three predicted folds (TPR-like repeats, an uncharacterized conserved fold, and an inactive metalloprotease-like fold) that occupy the substrate-binding position of other GHL proteins. We show that purified Bacillus cereus Tiamat hydrolyzes ATP, inefficiently nicks duplex DNA, and cleaves single-stranded DNA site-specifically. We identify Tiamat-like proteins whose C-terminal nuclease is often swapped for helicases, peptidases, or other effectors. Additionally, we show Tiamat genes neighbor helicases, DNA methyltransferases, and nucleases, suggesting some Tiamats degrade foreign nucleic acid within a multi-component complex. Eukaryotic Tiamat homologs like the close Arabidopsis homolog NO VEIN and the distant human homolog SACSIN have acquired critical non-defense roles, leaving open whether these homologs remain antiviral or have been exapted.

biochemistry↗

bioRxiv2023

Lipid droplets as substrates for protein phase separation

Membrane-associated protein phase separation plays critical roles in cell biology, driving essential cellular phenomena from immune signaling to membrane traffic. Importantly, by restricting diffusion to a two-dimensional surface, lipid bilayers can nucleate phase separation at far lower concentrations compared to those required for phase separation in solution. How might other intracellular lipid substrates, such as lipid droplets, contribute to nucleation of phase separation? Distinct from bilayer membranes, lipid droplets consist of a phospholipid monolayer surrounding a core of neutral lipids, and they are energy storage organelles that protect cells from lipotoxicity and oxidative stress. Here, we show that intrinsically disordered proteins can undergo phase separation on the surface of synthetic and cell-derived lipid droplets. Specifically, we find that model disordered domains, FUS LC and LAF1-RGG, separate into protein-rich and protein-depleted phases on the surfaces of lipid droplets. Owing to the hydrophobic nature of interactions between FUS LC proteins, increasing ionic strength drives an increase in its phase separation on droplet surfaces. The opposite is true for LAF1-RGG, owing to the electrostatic nature of its interprotein interactions. In both cases, protein-rich phases on the surfaces of synthetic and cell-derived lipid droplets demonstrate molecular mobility indicative of a liquid-like state. Our results show that lipid droplets can nucleate protein condensates, suggesting that protein phase separation could be key in organizing biological processes involving lipid droplets.

biophysics↗

bioRxiv2022

A single 2'-O-methylation of ribosomal RNA gates assembly of a functionalribosome

RNA modifications are widespread in biology, and particularly abundant in ribosomal RNA. However, the significance of these modifications is not well understood. We show that methylation of a single universally conserved nucleotide, in the catalytic center of the large subunit, gates ribosome assembly. Massively parallel mutational scanning of the essential nuclear GTPase Nog2 identified important interactions with ribosomal RNA helix 92, particularly with the methylated A-site base Gm2922. We found that 2-O-methylation of G2922 is needed for efficient nuclear export of the large subunit. Critically, we identified single amino acid changes in Nog2 that completely bypass its dependence on G2922 methylation. By solving the cryo-EM structure of the unmodified nascent subunit, we reveal how methylation flips Gm2922 into the active site channel of Nog2. This work demonstrates that a single RNA modification is a critical checkpoint in ribosome biogenesis, and suggests that RNA modifications can play an important role in regulation and assembly of macromolecular machines.

molecular biology↗

bioRxiv2021

SCOPE: Flexible targeting and stringent CARF activation enables type III CRISPR-Cas diagnostics

Characteristic properties of type III CRISPR-Cas systems include recognition of target RNA (rather than DNA) and the subsequent induction of a multifaceted immune response. This involves sequence-specific cleavage of a target RNA and production of cyclic oligoadenylate (cOA) second messenger molecules that may trigger dormancy or cell death. In this study, we discovered that a largely exposed seed region at the 3 end of the crRNA is essential for target RNA binding and cleavage, whereas base pairing at a unique region at the 5 end of the guide is required to trigger cOA production. Moreover, we uncovered that the natural variation in the composition of type III complexes within a single host results in different guide lengths, and hence variable seed regions. This shifting seed may prevent escape by invading genetic elements, while controlling cOA production very tightly to prevent unnecessary damage to the host. Lastly, we used these findings to develop a new diagnostic tool, named SCOPE, which was used for the specific detection of SARS-CoV-2 from human nasal swab samples, showing sensitivities in the atto-molar range.

biochemistry↗

bioRxiv2020

Rotavirus Replication Factories Are Complex Ribonucleoprotein Condensates

RNA viruses induce formation of subcellular organelles that provide microenvironments conducive to their replication. Here we show that replication factories of rotaviruses represent protein-RNA condensates that are formed via liquid-liquid phase separation. We demonstrate that rotavirus proteins NSP5 and NSP2 undergo phase separation in vitro and form RNA-rich condensates in vivo that can be reversibly dissolved by aliphatic diols. During infection, these RNA-protein condensates became less dynamic and impervious to aliphatic diols, indicating a transition from a liquid to solid state. Some aspects of assembly of rotavirus replication factories mirror the formation of cytoplasmic ribonucleoprotein granules, while the selective enrichment of viral transcripts appears to be a unique feature of these condensates. Such complex RNA-protein condensates that underlie replication of RNA viruses represent an attractive target for developing novel therapeutic approaches.

biophysics↗

bioRxiv2020

Structural basis of rotavirus RNA chaperone displacement and RNA annealing

Rotavirus genomes are distributed between eleven distinct RNA molecules, all of which must be selectively co-packaged during virus assembly. This likely occurs through sequence-specific RNA interactions facilitated by the RNA chaperone NSP2. Here, we report that NSP2 auto-regulates its chaperone activity through its C-terminal region (CTR) that promotes RNA-RNA interactions by limiting its helix-unwinding activity. Unexpectedly, structural proteomics data revealed that the CTR does not directly interact with RNA, whilst accelerating RNA release from NSP2. Cryo-electron microscopy reconstructions of an NSP2-RNA complex reveal a highly conserved acidic patch poised towards RNA. Virus replication was abrogated by charge-disrupting mutations within the acidic patch but completely restored by charge-preserving mutations. Mechanistic similarities between NSP2 and the unrelated bacterial RNA chaperone Hfq suggest that accelerating RNA dissociation whilst promoting inter-molecular RNA interactions may be a widespread strategy of RNA chaperone recycling.

biophysics↗

bioRxiv2020

Structure of a type IV CRISPR-Cas effector complex

We reveal the structure of a type IV-B CRISPR effector (Csf) complex at 3.9 [A] resolution using cryo-electron microscopy. The complex resembles the type III-A CRISPR Csm effector complex, but lacks subunits for RNA processing and target DNA cleavage, and is surprisingly assembled upon heterogeneous non-CRISPR RNA. These findings provide the first glimpse into the assembly and function of enigmatic type IV CRISPR systems.

biochemistry↗

bioRxiv2018

Stability of local secondary structure determines selectivity of viral RNA chaperones

To maintain genome integrity, segmented double-stranded RNA viruses of the Reoviridae family must accurately select and package a complete set of up to a dozen distinct genomic RNAs. It is thought that the high fidelity segmented genome assembly involves multiple sequence-specific RNA-RNA interactions between single-stranded RNA segment precursors. These are mediated by virus-encoded non-structural proteins with RNA chaperone-like activities, such as rotavirus NSP2 and avian reovirus {sigma}NS. Here, we compared the abilities of NSP2 and {sigma}NS to mediate sequence-specific interactions between rotavirus genomic segment precursors. Despite their similar activities, NSP2 successfully promotes inter-segment association, while {sigma}NS fails to do so. To understand the mechanisms underlying such selectivity in promoting inter-molecular duplex formation, we compared RNA-binding and helix-unwinding activities of both proteins. We demonstrate that octameric NSP2 binds structured RNAs with high affinity, resulting in efficient intramolecular RNA helix disruption. Hexameric {sigma}NS oligomerises into an octamer that binds two RNAs, yet it exhibits only limited RNA-unwinding activity compared to NSP2. Thus, the formation of intersegment RNA-RNA interactions is governed by both helix-unwinding capacity of the chaperones and stability of RNA structure. We propose that this protein-mediated RNA selection mechanism may underpin the high fidelity assembly of multi-segmented RNA genomes in Reoviridae.

biophysics↗