Book of Abstracts: Albany 2003
June 17-21 2003
Grabbing the Cat by the Tail: Studies of DNA Packaging by Single φ29 Bacteriophage Particles Using Optical Tweezers
I will present our recent results on the packaging of DNA by the connector motor at the base of the head of bacteriophage φ29. As part of their infection cycle, many viruses must package their newly replicated genomes inside a protein capsid to insure its proper transport and delivery to other host cells. Bacteriophage φ29 packages its 6.6 mm long double-stranded DNA into a 42 nm dia. x 54 nm high capsid via a portal complex that hydrolyses ATP. This process is remarkable because entropic, electrostatic, and bending energies of the DNA must be overcome to package the DNA to near-crystalline density. We have used optical tweezers to pull on single DNA molecules as they are packaged, thus demonstrating that the portal complex is a force generating motor. We find that this motor can work against loads of up to ~57 picoNewtons on average, making it one of the strongest molecular motors ever reported. Movements of over 5 mm are observed, indicating high processivity. Pauses and slips also occur, particularly at higher forces. We establish the force-velocity relationship of the motor and find that the rate-limiting step of the motor's cycle is force dependent even at low loads. Interestingly, the packaging rate decreases as the prohead is filled, indicating that an internal pressure builds up due to DNA compression. We estimate that at the end of the packaging the capsid pressure is ~15 MegaPascals, corresponding to an internal force of ~50 pN acting on the motor. The biological implications of this internal pressure and the mechano-chemical efficiency of the engine are discussed.
University of California