Protease-driven remodeling of stabilization networks during adenovirus assembly

Abstract

At least thirteen different proteins form the adenovirus virion, including those bound to the dsDNA genome in the core. To produce infectious particles, the adenovirus protease (AVP) cleaves many of these proteins during genome packaging, using the viral genome as a cofactor. Here, we determine high-resolution structures of two types of adenovirus particles devoid of genome and core proteins and stalled at different AVP processing stages. We find that the N-terminal regions of penton base and internal minor coat protein IIIa are disordered when the core is absent and uncleaved packaging protein L1 52/55 kDa is present, likely enabling correction of early assembly errors at the vertex. Assignment of previously unmodeled densities reveals that proteins IIIa and VIII form long-range bridges linking vertex capsomers to the facet center before proteolytic maturation. Cleavage by AVP remodels these connections, diminishing capsid-wide stabilizing interactions and promoting the metastable state that prepares the mature virion for uncoating. Significance statement Adenoviruses cause disease and are major platforms for gene delivery, yet how their capsids are built and primed for infection remains incompletely understood. High-resolution cryo-EM structures of empty particles arrested at successive proteolytic maturation stages show previously unrecognized interactions that regulate capsid stability. Uncleaved packaging protein L1 52/55 kDa induces disorder at the icosahedral vertex during early assembly, likely facilitating error correction, while minor coat proteins IIIa and VIII serve as transient scaffolds stabilizing the capsid before proteolytic remodeling. Maturation thereby converts a stable assembly intermediate into a metastable particle ready for uncoating. These findings reveal how structural plasticity of minor coat proteins coordinates adenovirus assembly, maturation, and infectivity.