Exploring the Balance between DNA Pressure and Capsid Stability in Herpesviruses and Phages

Abstract

We have recently shown in both herpesviruses and phages that packaged viral DNA creates a pressure of tens of atmospheres pushing against the interior capsid wall. For the first time, using differential scanning microcalorimetry, we directly measured the energy powering the release of pressurized DNA from the capsid. Furthermore, using a new calorimetric assay to accurately determine the temperature inducing DNA release, we found a direct influence of internal DNA pressure on the stability of the viral particle. We show that the balance of forces between the DNA pressure and capsid strength, required for DNA retention between rounds of infection, is conserved between evolutionarily diverse bacterial viruses (phages and P22), as well as a eukaryotic virus, human herpes simplex 1 (HSV-1). Our data also suggest that the portal vertex in these viruses is the weakest point in the overall capsid structure and presents the Achilles heel of the virus's stability. Comparison between these viral systems shows that viruses with higher DNA packing density (resulting in higher capsid pressure) have inherently stronger capsid structures, preventing spontaneous genome release prior to infection. This force balance is of key importance for viral survival and replication. Investigating the ways to disrupt this balance can lead to development of new mutation-resistant antivirals. IMPORTANCEA virus can generally be described as a nucleic acid genome contained within a protective protein shell, called the capsid. For many double-stranded DNA viruses, confinement of the large DNA molecule within the small protein capsid results in an energetically stressed DNA state exerting tens of atmospheres of pressures on the inner capsid wall. We show that stability of viral particles (which directly relates to infectivity) is strongly influenced by the state of the packaged genome. Using scanning calorimetry on a bacterial virus (phage ) as an experimental model system, we investigated the thermodynamics of genome release associated with destabilizing the viral particle. Furthermore, we compare the influence of tight genome confinement on the relative stability for diverse bacterial and eukaryotic viruses. These comparisons reveal an evolutionarily conserved force balance between the capsid stability and the density of the packaged genome.A virus can generally be described as a nucleic acid genome contained within a protective protein shell, termed the capsid. The viral replication cycle is composed of two distinct processes: delivery of the genetic material to the host (infection) and the subsequent production of new virions (replication). Between rounds of replication, the virion must be sufficiently stable to ensure that the packaged genome is retained within the capsid. Conversely, during infection it must be unstable enough to allow efficient genome release into the host cell. This balance between genome retention and release is described as viral metastability (1). Viral particles are therefore not inert structures and have not ...

Expansion of Bacteriophages Is Linked to Aggravated Intestinal Inflammation and Colitis

“…Intact bacteriophages harbor various capsid proteins and DNA within the capsid head that could mediate stimulation of the immune system. Bacteriophage treatment with EDTA causes release of DNA from the phage head, leaving the capsid empty but intact (Bauer et al, 2015). Treatment of DCs with DNAase-treated empty capsid did not stimulate IFN-g in culture; however, an over 40-fold induction of IFN-g was initiated upon treatment of DCs with purified phage DNA ( Figure 4B).…”

Spatiotemporal dynamics of HSV genome nuclear entry and compaction state transitions using bioorthogonal chemistry and super-resolution microscopy

“…Heat treatment revealed internal protein VII (by immunofluorescence) and allowed cycloaddition labelling of the genome, which nevertheless remained tightly associated with the capsid [16]. The ejection of the HSV genome (from capsids or heated virions) versus the maintained association of the Adv genome (from heated capsids) most likely reflects differences in the pressurisation status of genomes within capsids [58] and the lack of DNA packaging proteins within HSV compared to Adv where the genome is associated with several core proteins, in particular protein VII [59]. Such differences in internal pressure and protein-genome association are likely reflected in differences in mechanism in nuclear pore engagement and genome import.…”

The Essential Human Cytomegalovirus Proteins pUL77 and pUL93 Are Structural Components Necessary for Viral Genome Encapsidation

“…Although HCMV has the largest genome of all mammalian DNA viruses (ϳ240 kbp); the diameter of its capsid is similar to that of the herpes simplex virus 1 capsid (HSV-1; genome size, 150 kbp). As a consequence, encapsidated HCMV DNA is more densely packed (8), resulting in DNA-filled capsids that are under higher pressure and may thus need additional stabilization (9). Furthermore, the structure of the HCMV inner tegument is distinct from that of HSV-1, insofar as the HCMV tegument contacts both hexons and pentons, whereas in HSV-1, the inner tegument is attached to pentons only (10,11).…”