Packaging Motor Relies on a High Degree of Coordination among Five Ring Subunits

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The cover of the latest Biophysical Journal issue features an artist’s rendition of the packaging motor of bacteriophage ø29, which uses the energy stored in ATP (colored green in the image) to pump double-stranded DNA into the viral capsid. The active core of the packaging motor consists of five identical ATPase subunits that form a ring though which DNA is translocated. The normal operation of this motor relies on a high degree of coordination among individual ring subunits. The hydraulic pistons and mechanical grippers were used to illustrate the intricate mechanism governing the operation of this molecular complex. Two former members of our laboratory, Jeffrey Moffitt and Yann Chemla, conceived the idea of this illustration. The scientific animation studio XVIVO created the artwork.

For over a decade our laboratory has been using single-molecule techniques to study the ø29 packaging motor and related ring ATPases. In this issue of the Biophysical Journal we review recent advances from our group and other groups in the understanding of ring-shaped biological motors that were made possible by single-molecule experiments. ATP-powered ring motors are among the blue-collar workers of the cell, and are responsible for many essential mechanical and transport tasks. The enzyme responsible for unwinding DNA at the replication fork, the replicative helicase, is a hexameric ring ATPase that translocates single-stranded DNA though its central pore to enable the polymerase to copy the genetic information. The molecular complex responsible for recycling proteins in the cell, the proteasome, contains a ring-shaped molecular motor that unravels proteins and pumps the unfolded polypeptide chain into a degradation cavity, again using the energy of ATP hydrolysis. The enzyme that produces the universal energy currency of the cell, the ATP synthase, contains a ring-shaped motor with a central shaft that rotates. Depending on which direction the central shaft spins, the enzyme can either produce ATP using a proton gradient as source of energy or it can hydrolyze ATP to generate a proton gradient across a lipid bilayer membrane. Although ring-shaped molecular motors perform a wide range of cellular tasks, they share a common set of basic mechanisms. Through the use of single-molecule methods, we are now just beginning to understand how the different ring motors adapt their design and subunit coordination to optimize the performance of their specific tasks.

To familiarize yourself with the most recent research from our lab, please visit: http://physics.berkeley.edu/research/bustamante/ or http://www.hhmi.org/scientists/carlos-j-bustamante

- Carlos Bustamante

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