Dynamics and Force Generation by Single Motor Complexes in M. Xanthus

Abstract

The gram-negative bacterium Myxococcus xanthus is an important model system for multicellular group formation. These cells move on solid surfaces using a combination of gliding and twitching motility. Whereas pilus retraction drives twitching, the mechanism controlling gliding is not fully understood. We recently identified a new class of molecular motors that power gliding motility [1]. These motors, AglQRS proton channels, assemble within focal adhesion sites that are separated by approximately 480nm along the cell [2]. It is currently unknown how much force the motors can generated and whether they act individually or in cooperative groups.Here, we combined optical tweezers and real-time tracking of motor-driven beads to address these topics. We find that gliding motors produce saltatory movements on two major time scales, 0.4s and 3.5s. These are likely associated with motor kinetics and cargo binding respectively. During bursts of motor activity, we applied a controlled load to slow single motor movement along the cell axis. We show that mechanical force is sufficient to reduce the velocity of a gliding motor complex and that motors stall under forces stronger than 12pN. We reduced the pH gradient through exposure to mild concentrations of the drug nigericin. We find that the characteristic force is independent of the driving proton motive force.[1] Sun et al., PNAS(108)7559, 2011. [2] Mignot et al., Science(315)853, 2007.