Abstract
Adenovirus assembly concludes with proteolytic processing of several capsid and core proteins. Immature virions containing precursor proteins lack infectivity because they cannot properly uncoat, becoming trapped in early endosomes. Structural studies have shown that precursors increase the network of interactions maintaining virion integrity. Using different biophysical techniques to analyze capsid disruption in vitro, we show that immature virions are more stable than the mature ones under a variety of stress conditions and that maturation primes adenovirus for highly cooperative DNA release. Cryoelectron tomography reveals that under mildly acidic conditions mimicking the early endosome, mature virions release pentons and peripheral core contents. At higher stress levels, both mature and immature capsids crack open. The virus core is completely released from cracked capsids in mature virions, but it remains connected to shell fragments in the immature particle. The extra stability of immature adenovirus does not equate with greater rigidity, because in nanoindentation assays immature virions exhibit greater elasticity than the mature particles. Our results have implications for the role of proteolytic maturation in adenovirus assembly and uncoating. Precursor proteins favor assembly by establishing stable interactions with the appropriate curvature and preventing premature ejection of contents by tightly sealing the capsid vertices. Upon maturation, core organization is looser, particularly at the periphery, and interactions preserving capsid curvature are weakened. The capsid becomes brittle, and pentons are more easily released. Based on these results, we hypothesize that changes in core compaction during maturation may increase capsid internal pressure to trigger proper uncoating of adenovirus.
Highlights
Adenovirus proteolytic maturation is required for correct uncoating in the cell
The minor transition (T2) appearing at 67.3 °C for WT and 68.9 °C for ts1 seems to be related to hexon stabilization upon Group of Nine (GON) formation, as it is absent in polypeptide IXdeleted recombinant viruses [42]
The three low temperature transitions may correspond to capsid disassembly and reflect changes undergone by adenovirus during uncoating in the cell
Summary
Adenovirus proteolytic maturation is required for correct uncoating in the cell. Results: Maturation makes the virion metastable and facilitates penton and peripheral core protein release, as well as cooperative genome ejection. How Maturation Primes Adenovirus for Uncoating remaining minor capsid protein, polypeptide VI, could not be traced in this study, but it has been assigned to density within the internal cavity of each hexon trimer [4, 5]. The virus is internalized, and disassembly continues in the early endosome with release of some internal components, such as minor coat proteins IIIa, VI, VIII, and core polypeptide V [9, 12]. This is a crucial step for infection, as polypeptide VI plays the key role of altering the endosomal membrane to facilitate virus escape to the cytosol [13]. We report the differences between mature and immature virions and discuss their implications for assembly and uncoating
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