Elucidating the structural mechanisms of capsid stability and assembly using a hyperthermophilic bacteriophage

Nearly all viruses encapsulate their genomes in protective protein shells known as capsids. Capsids self-assemble from repeating protein subunits, which surround the viral genome. Many viruses use a powerful biomotor to pump DNA into preformed capsid shells. Therefore, not only does the capsid protect the genome from environmental stress, it additionally stabilizes against high internal pressure caused by the tightly-packaged genome inside. To understand how capsids remain stable despite extreme conditions, I use thermophilic bacteriophage P74-26 as a model to probe the structural mechanisms that govern capsid assembly and stability. P74-26 capsids have a similar architecture to capsids of mesophilic tailed bacteriophages, allowing direct comparison to elucidate the structural basis of enhanced thermostability. Here I determine the structure of the P74-26 capsid decoration protein, which contains a core beta-barrel domain termed the ‘beta-tulip’ domain. The beta-tulip domain is conserved in structural proteins from both Herpesviruses and phage, as well as a broad-spectrum Cas9 inhibitor, providing evidence of shared evolutionary ancestry. Additionally, my high-resolution structure of the P74-26 virion capsid reveals unique interdigitated architectural features that contribute to enhanced stability in the thermophile. P74-26 has a significantly larger capsid than related mesophiles yet retains the same icosahedral geometry, demonstrating a novel mechanism for increasing capsid capacity. Furthermore, my thesis work explores capsid assembly and maturation mechanisms in vitro, establishing P74-26 as a platform for future development of novel nanoparticles and therapeutic delivery systems. Taken together, this work illuminates the incredible stability of a thermophilic virus and illustrates its utility as a powerful tool for studying viral maturation

Observing Lense-Thirring Precession in Tidal Disruption Flares

When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted X-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review Letters. Minor changes made to match proof

Loss Cone Shielding

A star wandering close enough to a massive black hole (MBH) can be ripped apart by the tidal forces of the black hole. The advent of wide-field surveys at many wavelengths has quickly increased the number of tidal disruption events (TDEs) observed, and has revealed that i) observed TDE rates are lower than theoretical predictions and ii) E+A galaxies are significantly overrepresented. This overrepresentation further worsens the tension between observed and theoretically predicted TDEs for non-E+A galaxies. Classical loss cone theory focuses on the cumulative effect of many weak scatterings. However, a strong scattering can remove a star from the distribution before it can get tidally disrupted. Most stars undergoing TDEs come from within the radius of influence, the densest environments of the universe. In such environments, close encounters rare elsewhere become non-negligible. We revise the standard loss cone theory to take into account classical two-body interactions as well as strong scattering, collisions, tidal captures, and study under which conditions close encounters can shield the loss cone. We i) analytically derive the impact of strong scattering and other close encounters, ii) compute time-dependent loss cone dynamics including both weak and strong encounters, and iii) derive analytical solutions to the Fokker-Planck equation with strong scattering. We find that i) TDE rates can be reduced to up to an order of magnitude and ii) strong shielding preferentially reduces deeply plunging stars. We also show that stellar overdensities, one possible explanation for the E+A preference, can fail to increase TDE rates when taking into account strong scattering.Comment: 14 pages, 10 figures, Accepted for publication MNRA

Principles for enhancing virus capsid capacity and stability from a thermophilic virus capsid structure

The capsids of double-stranded DNA viruses protect the viral genome from the harsh extracellular environment, while maintaining stability against the high internal pressure of packaged DNA. To elucidate how capsids maintain stability in an extreme environment, we use cryoelectron microscopy to determine the capsid structure of thermostable phage P74-26 to 2.8-A resolution. We find P74-26 capsids exhibit an overall architecture very similar to those of other tailed bacteriophages, allowing us to directly compare structures to derive the structural basis for enhanced stability. Our structure reveals lasso-like interactions that appear to function like catch bonds. This architecture allows the capsid to expand during genome packaging, yet maintain structural stability. The P74-26 capsid has T = 7 geometry despite being twice as large as mesophilic homologs. Capsid capacity is increased with a larger, flatter major capsid protein. Given these results, we predict decreased icosahedral complexity (i.e. T \u3c /= 7) leads to a more stable capsid assembly

Principles for enhancing virus capsid capacity and stability from a thermophilic virus capsid structure [preprint]

The capsids of double-stranded DNA viruses protect the viral genome from the harsh extracellular environment, while maintaining stability against the high internal pressure of packaged DNA. To elucidate how capsids maintain stability in an extreme environment, we used cryoelectron microscopy to determine the capsid structure of the thermostable phage P74-26. We find the P74-26 capsid exhibits an overall architecture that is very similar to those of other tailed bacteriophages, allowing us to directly compare structures to derive the structural basis for enhanced stability. Our structure reveals lasso-like interactions that appear to function like catch bonds. This architecture allows the capsid to expand during genome packaging, yet maintain structural stability. The P74-26 capsid has T=7 geometry despite being twice as large as mesophilic homologs. Capsid capacity is increased through a novel mechanism with a larger, flatter major capsid protein. Our results suggest that decreased icosahedral complexity (i.e. lower T number) leads to a more stable capsid assembly

Structure of CYRI-B (FAM49B), a key regulator of cellular actin assembly.

In eukaryotes, numerous fundamental processes are controlled by the WAVE regulatory complex (WRC) that regulates cellular actin polymerization, crucial for cell motility, cell-cell adhesion and epithelial differentiation. Actin assembly is triggered by interaction of the small GTPase Rac1 with CYFIP1, a key component of the WRC. Previously known as FAM49B, CYRI-B is a protein that is highly conserved across the Eukaryota and has recently been revealed to be a key regulator of Rac1 activity. Mutation of CYRI-B or alteration of its expression therefore leads to altered actin nucleation dynamics, with impacts on lamellipodia formation, cell migration and infection by intracellular pathogens. In addition, knockdown of CYRI-B expression in cancer cell lines results in accelerated cell proliferation and invasiveness. Here, the structure of Rhincodon typus (whale shark) CYRI-B is presented, which is the first to be reported of any CYRI family member. Solved by X-ray crystallography, the structure reveals that CYRI-B comprises three distinct α-helical subdomains and is highly structurally related to a conserved domain present in CYFIP proteins. The work presented here establishes a template towards a better understanding of CYRI-B biological function