Abstract
Double-stranded DNA viruses package their genomes into pre-assembled capsids using virally-encoded ASCE ATPase ring motors. We present the first atomic-resolution crystal structure of a multimeric ring form of a viral dsDNA packaging motor, the ATPase of the asccφ28 phage, and characterize its atomic-level dynamics via long timescale molecular dynamics simulations. Based on these results, and previous single-molecule data and cryo-EM reconstruction of the homologous φ29 motor, we propose an overall packaging model that is driven by helical-to-planar transitions of the ring motor. These transitions are coordinated by inter-subunit interactions that regulate catalytic and force-generating events. Stepwise ATP binding to individual subunits increase their affinity for the helical DNA phosphate backbone, resulting in distortion away from the planar ring towards a helical configuration, inducing mechanical strain. Subsequent sequential hydrolysis events alleviate the accumulated mechanical strain, allowing a stepwise return of the motor to the planar conformation, translocating DNA in the process. This type of helical-to-planar mechanism could serve as a general framework for ring ATPases.
Highlights
The Additional Strand, Conserved Glutamate (ASCE) superfamily is an ancient and ubiquitous class of NTPases, encompassing subfamilies such as AAA+ motors, RecA-and FtsK-like ATPases, and ABC transporters [1]
Double-stranded DNA viruses, such as herpes, adeno- and pox viruses, as well all tailed bacteriophages, encode for ASCE segregation motors that they use to package their genomes into preformed procapsids during virus replication [2,3,4]
Atomic resolution structures of all individual 29 motor components are available [12,13,14,15] and medium resolution structures of motors assembled on capsids in various stages of assembly and/or packaging have been determined [15,16,17,18,19]. These results indicate that the DNA packaging motor consists of a dodecameric portal protein, a pentameric prohead RNA and a pentameric ATPase that assemble as co-axial rings at a unique vertex of the 29 capsid
Summary
FtsK-like ATPases, and ABC transporters [1] These motors convert energy from NTP binding and/or hydrolysis into mechanical work, and typically perform biological segregation tasks such as proton transport, chromosomal segregation, DNA or RNA strand separation, and protein degradation. Double-stranded DNA (dsDNA) viruses, such as herpes-, adeno- and pox viruses, as well all tailed bacteriophages, encode for ASCE segregation motors that they use to package their genomes into preformed procapsids during virus replication [2,3,4]. Among ASCE ATPases, viral packaging motors generate high forces (>50 pN) to overcome the entropy loss, electrostatic repulsion and DNA stiffness that oppose DNA confinement [7,8,9,10]. Viral dsDNA packaging motors provide a unique window into the mechanochemistry of force-generation found in this broad class of molecular motors
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