Structure of the GP16 ATPase from the φ29 dsDNA packaging motor

Bacillus subtilis bacteriophage φ29 packages double-stranded DNA (dsDNA) into a preformed viral shell, or procapsid, and serves as a model system for studying genome packaging in eukaryotic dsDNA viruses such as poxviruses, herpesviruses and adenoviruses. Encapsidation of bacteriophage φ29 DNA is driven by a phage-encoded molecular motor. This motor is powered by an oligomeric ATPase, gp16, or gene product 16, that converts energy obtained from ATP hydrolysis into translocation of dsDNA. In this study, we solved the gp16 ATPase structure via X-ray crystallography. The resultant monomeric structure indicated that gp16 ATPase adopts a modified Rossmann fold (Rossmann et al., 1974), in which six conserved β-strands form a central β-sheet, and adjacent β-strands are linked by intervening α-helices, and that the protein is a member of the ancient P-loop additional strand catalytic E (ASCE) NTPase superfamily. The active site responsible for ATP hydrolysis is located on one side of the central β-sheet. Superposition of our structure on related ring-forming members of the ASCE superfamily indicated that residue Arg148 protrudes from the other side of the β-sheet and is well-positioned to insert its side chain into the active site of a neighboring ATPase to trigger sequential ATP hydrolysis events around an oligomeric ATPase ring.