An Arginine Finger Regulates the Sequential Action of Asymmetrical Hexameric ATPase in the Double-Stranded DNA Translocation Motor

What is it about?

Biological motors are ubiquitous in living systems. Currently, how the motor components coordinate the unidirectional motion is still elusive in most cases. Here, we report that the sequential action of the ring in a viral DNA packaging motor is regulated by an arginine finger that extends from one ring subunit to the adjacent one to form a noncovalent dimer. Mutation of the arginine finger resulted in the failure of motor ring formation, ATP binding, ATP hydrolysis, and DNA translocation. Dimer formation reappeared when arginine-free mutants were mixed with other subunits that can offer the arginine to promote their interaction. The hexametric motor ring was composed of four monomers and one dimer. ATP binding resulted in the change of the subunit into a new conformational with high affinity for DNA binding; ATP hydrolysis resulted in the conformational reversal of the subunit, leading to low affinity for DNA, thus pushing the DNA into the adjacent subunit with high affinity for DNA. It is concluded that the arginine finger regulates sequential action of the ring subunit of the motor by alternatively promoting the formation of the dimer inside the hexamer. The finding of asymmetrical hexameric organization is supported by structural evidence of many other biomotor systems showing the presence of one noncovalent dimer and four monomer subunits. All of these provide clues for why the asymmetrical hexameric ATPase motor of ϕ29 was previously reported as a pentameric configuration by cryo-electron microscopy (cryo-EM) since the contact by the arginine finger renders two adjacent ATPase subunits closer than the other subunits. Thus, the asymmetrical hexamer would appear as a pentamer by cryo-EM, a technology that acquires the average of many images.

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