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

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.