Movie illustrating the pseudoatomic model for the fully mature capsid of HPV16 based on the 9.1-Å cryo-EM reconstruction
Giovanni Cardonea, Adam L. Moyerb, Naiqian Chenga, Cynthia D. Thompsonb, Israel Dvoretzkyc, Douglas R. Lowyb, John T. Schillerb, Alasdair C. Stevena, Christopher B. Buckb, Benes L. Trusd
aLaboratory of Structural Biology, National Institute for Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health (NIH), Bethesda, Maryland, USA
bLaboratory of Cellular Oncology, National Cancer Institute, NIH, Bethesda, Maryland, USA
cYale University, New Haven, Connecticut, USA
dImaging Sciences Laboratory, Center for Information Technology, NIH, Bethesda, Maryland, USA
Time-lapse cryo-electron microscopy and image analysis was used to study the maturation of HPV16 capsids assembled in mammalian cells and exposed to an oxidizing environment after cell lysis. Initially, the virion is a loosely connected procapsid that, under in vitro conditions, condenses over several hours into the more familiar 60-nm-diameter papillomavirus capsid. In this process, the procapsid shrinks by ~5% in diameter, its pentameric capsomers change in structure (most markedly in the axial region), and the interaction surfaces between adjacent capsomers are consolidated. A C175S mutant that cannot achieve normal inter-L1 disulfide cross-links shows maturation-related shrinkage but does not achieve the fully condensed 60-nm form. Pseudoatomic modeling based on a 9-Å resolution reconstruction of fully mature capsids revealed C-terminal disulfide-stabilized "suspended bridges" that form intercapsomeric cross-links. The data suggest a model in which procapsids exist in a range of dynamic intermediates that can be locked into increasingly mature configurations by disulfide cross-linking, possibly through a Brownian ratchet mechanism.