Mechanical Capsid Maturation Facilitates the Resolution of Conflicting Requirements for Herpesvirus Assembly

Abstract

Most viruses undergo a maturation process from a weakly self-assembled, noninfectious particle to a stable, infectious virion. For herpesviruses, this maturation process resolves several conflicting requirements: i) assembly must be driven by weak, reversible interactions between viral particle subunits to reduce errors and minimize energy of self-assembly; ii) the viral particle must be stable enough to withstand tens of atmospheres of DNA pressure resulting from its strong confinement in the capsid. With herpes simplex virus type 1 (HSV-1) as a prototype of human herpesviruses, we demonstrate that this mechanical capsid maturation is mainly facilitated through capsid-binding auxiliary protein UL25, orthologs of which are present in all herpesviruses. Through genetic manipulation of UL25 mutants of HSV-1 combined with interrogation of capsid mechanics with atomic force microscopy nano-indentation, we suggest the mechanism of stepwise binding of distinct UL25 domains correlated with capsid maturation and DNA packaging. These findings demonstrate another paradigm of viruses as elegantly programmed nano-machines, where an intimate relationship between mechanical and genetic information is preserved in UL25 architecture. IMPORTANCE Minor capsid protein UL25 plays a critical role in mechanical maturation of HSV-1 capsid during virus assembly, required for stable DNA packaging. We modulate UL25-capsid interactions by genetically deleting different UL25 regions and quantify the effect on mechanical capsid stability using an atomic force microscopy (AFM) nano-indentation approach. This approach reveals how UL25 regions reinforce the herpesvirus capsid in order to stably package and retain pressurized DNA. Our data suggests a mechanism of stepwise binding of two main UL25 domains timed with DNA packaging.

The herpes simplex virus tegument protein pUL21 is required for viral genome retention within capsids

“…pUL17 is important for genome packaging and the efficient recruitment of pUL25 and pUL36 to capsids [12,14]. pUL25 on capsid vertices prevents the capsid structure from rupturing after packaging [17,18]. Further, recent cryo-EM studies examining pUL25 and its β-and γ-herpesvirus homologs have demonstrated that pUL25 adjacent to the capsid portal is thought to extend over the portal, forming a pentameric portal cap that plugs the capsid portal after genome packaging [14,[19][20][21].…”

The Herpes Simplex Virus Tegument Protein pUL21 is Required for Viral Genome Retention Within Capsids

“…pUL17 is important for genome packaging and the efficient recruitment of pUL25 and pUL36 to capsids (12, 14). pUL25 on capsid vertices prevents the capsid structure from rupturing after packaging (17, 18). Further, pUL25 adjacent to the capsid portal is thought to extend over the portal, thereby plugging it after genome packaging (14).…”