Regulation of the ordinal DNA translocation cycle in bacteriophage Φ29 through trans-subunit interactions.

Certain viruses such as tailed bacteriophages and herpes simplex virus package double-stranded DNA into empty procapsids via powerful, ring-shaped molecular motors. High-resolution structures and force measurements on the DNA packaging motor of bacteriophage Φ29 revealed that its five ATPase subunits coordinate ATP hydrolysis with each other to maintain the proper cyclic sequence of DNA translocation steps about the ring. Here, we explore how the Φ29 motor regulates translocation by timing key events, namely ATP binding/hydrolysis and DNA gripping, through trans-subunit interactions. We used subunit dimers bound to DNA as our model system, a minimal system that still captures the conformation and trans-subunit interactions of the full pentameric motor complex. Molecular dynamics simulations of all-ATP and mixed ATP-ADP dimers revealed that the nucleotide occupancy of one subunit strongly affects the ability to hydrolyze ATP in the adjacent subunit by altering the free energy landscape of its catalytic glutamate approaching the gamma phosphate of ATP. Specifically, one ATP-bound subunit donates residues in trans that sterically block the neighboring subunit's catalytic glutamate. This steric hindrance is resolved when the first subunit hydrolyzes ATP and is ADP bound. This obstructive mechanism is supported by functional mutagenesis and appears to be conserved across several Φ29 relatives. Mutual information analysis of our simulations revealed intersubunit signaling pathways, via the trans-acting obstructive residues, that allow for sensing and communication between the binding pockets of adjacent subunits. This work reveals how the sequential order of DNA translocation events among subunits is preserved through trans-subunit interactions and pathways.