Regulation of a Viral Packaging Motor’s Grip on DNA

ATP-powered viral DNA packaging motors are among the most powerful biomotors known. Here, we quantify how nucleotide binding regulates the motor’s grip on DNA via optical tweezers measurements with rapid solution exchange. In the apo state (with no nucleotide) there is almost no detectable grip. With low applied force the DNA usually slips at ~2000 bp/s. In contrast, with non-hydrolyzable ATP mimic bound, the motor grips the DNA strongly. Transient slips that occur when ATP dissociates are notably slower (~40 bp/s) than in the apo state, showing that multiple ATP-bound subunits exert friction on the DNA . Although the grip in the ATP-bound state can be ruptured by application of force, with a 30 pN force, the estimated maximum resistance at high prohead filling, slipping is low enough that the motor can still function. With bound ADP three states are observed: one that grips strongly like in with bound ATP, one with virtually no grip like apo, and one where the DNA slips at an intermediate speed (~740 bp/s). Although in the apo and ADP states the motor usually has little grip, when the end of the DNA is about to exit the capsid slipping suddenly arrests. This unique "clamp" state is highly stable and packaging resumes when ATP is added.