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Published on Sep 8, 2011
Air date: Wednesday, September 07, 2011, 3:00:00 PM Time displayed is Eastern Time, Washington DC Local Category: Wednesday Afternoon Lectures Description: 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 phi29 packages its 6.6 mm long double-stranded DNA into a 42 nm dia. x 54 nm high capsid using a multimeric ring motor that belongs to the ASCE (Additional Strand, Conserved E) superfamily of ATPases.
A number of fundamental questions remain as to the coordination of the various subunits in these multimeric rings. The portal motor in phi29 is ideal to investigate these questions. Using optical tweezers, 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. Interestingly, the packaging rate decreases as the prohead is filled, indicating that an internal pressure builds up due to DNA compression. The capsid pressure at the end of the packaging is ~6 MegaPascals, corresponding to an internal force of ~52 pN acting on the motor.
We have identified where in the mechanochemical cycle the chemical energy of ATP is converted into mechanical work. Using ultra-high resolution optical tweezers, we have performed the first direct measurement of the step size of a translocating ring ATPase. What emerges is a surprising mechanism that involves a step size with a non-integer number of base pairs and that reveals an unexpected degree of coordination among the individual subunits that has not been proposed previously for a ring ATPase.