Perez-Berna AJ, Ortega-Esteban A, Menendez-Conejero R, Winkler DC, Menendez M, Steven AC, Flint SJ, de Pablo PJ, San Martín C.
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. Cryo-electron 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 viral core is completely released from cracked capsids in mature
virions, but remains connected to shell fragments in the immature
particle. The extra stability of immature adenovirus does not equate
with greater rigidity, since 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..