An unexpected twist in viral capsid maturation

Abstract

The lambda-like double-stranded (ds) DNA bacteriophage HK97 is a favourable system for studying viral capsid maturation since it can be assembled in Escherichia coli from the expression of just two viral gene products and maturation is easily triggered and analysed in vitro. Various mature viral capsid structures have been determined but until now no procapsids have been available for a dsDNA virus or bacteriophage. Gertsman et al. now report the high-resolution structure of the HK97 procapsid providing insight into the capsid assembly process leading to infectious virions. The knowledge gained from this structure is relevant for related viruses such as the human herpesvirus. This paper reports the high-resolution structure of the double-stranded DNA bacteriophage HK97 procapsid, providing insight into the capsid assembly process leading to infectious virions. The knowledge gained from this structure is relevant for related viruses such as human herpesviruses. Lambda-like double-stranded (ds) DNA bacteriophage undergo massive conformational changes in their capsid shell during the packaging of their viral genomes. Capsid shells are complex organizations of hundreds of protein subunits that assemble into intricate quaternary complexes that ultimately are able to withstand over 50 atm of pressure during genome packaging1. The extensive integration between subunits in capsids requires the formation of an intermediate complex, termed a procapsid, from which individual subunits can undergo the necessary refolding and structural rearrangements needed to transition to the more stable capsid. Although various mature capsids have been characterized at atomic resolution, no such procapsid structure is available for a dsDNA virus or bacteriophage. Here we present a procapsid X-ray structure at 3.65 Å resolution, termed prohead II, of the lambda-like bacteriophage HK97, the mature capsid structure of which was previously solved to 3.44 Å (ref. 2). A comparison of the two largely different capsid forms has unveiled an unprecedented expansion mechanism that describes the transition. Crystallographic and hydrogen/deuterium exchange data presented here demonstrate that the subunit tertiary structures are significantly different between the two states, with twisting and bending motions occurring in both helical and β-sheet regions. We also identified subunit interactions at each three-fold axis of the capsid that are maintained throughout maturation. The interactions sustain capsid integrity during subunit refolding and provide a fixed hinge from which subunits undergo rotational and translational motions during maturation. Previously published calorimetric data of a closely related bacteriophage, P22, showed that capsid maturation was an exothermic process that resulted in a release of 90 kJ mol-1 of energy3. We propose that the major tertiary changes presented in this study reveal a structural basis for an exothermic maturation process probably present in many dsDNA bacteriophage and possibly viruses such as herpesvirus, which share the HK97 subunit fold4.