University of Minnesota
School of Physics & Astronomy

Biophysics Seminar

Wednesday, February 10th 2016
10:10 am:
Biophysics Seminar in 120 PAN
Speaker: Doug Smith, UC San Diego
Subject: Studies of motor-driven viral DNA packaging with optical tweezers: Biophysics of motor function and tight DNA confinement

In many viruses DNA is packed to near-crystalline density into ~50-100 nm prohead shells. The DNA is translocated into empty proheads by an ATP-powered molecular motor via a portal nanochannel, overcoming large forces resisting DNA confinement arising from DNA bending rigidity, electrostatic self-repulsion, and entropy loss. These biomotors are among the most powerful known, generating at least 20ยด higher force than the skeletal muscle myosin motor. In addition to being of biological interest, viral packaging is an experimentally accessible model for investigating effects of spatial confinement on polymer dynamics, a topic of fundamental interest in polymer physics. We use optical tweezers to measure the packaging of single DNA molecules into single viral proheads. Our recent studies of phage phi29 have shown that: (1) The confined DNA undergoes nonequilibrium (glassy) dynamics with a very long relaxation time, causing slowing and pausing of the motor and heterogeneity in the packaging rate; (2) Contrary to theoretical predictions, net attractive DNA-DNA interactions mediated by +3 or +4 ions cause frequent stalling of packaging, which we attribute to a nonequilibrium jamming transition akin to that occurring in colloidal and granular soft matter systems; (3) Motor velocity is regulated not only by load force but also by a novel allosteric mechanism wherein ATP binding and motor pausing is regulated in response to changes in packaged DNA density and conformation. In addition, we investigate the motor mechanism by studying the effect of amino acid changes in the motor proteins. Our recent findings provide evidence for an electrostatic mechanism of force generation in the phage T4 motor and a role of ATP phosphate binding loop residues in mechanochemical coupling in the phage lambda motor, supporting recent structure-based models.

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