Single Reconstituted Neuronal SNARE Complexes Zipper in Three Distinct Stages

Ying Gao, Sylvain Zorman, Gregory Gundersen, Zhiqun Xi, Lu Ma, George Sirinakis, James E. Rothman, Yongli Zhang

SNARE proteins drive membrane fusion by assembling into a four-helix bundle in a zippering process. Here, we used optical tweezers to observe in a cell-free reconstitution experiment in real time a long-sought SNARE assembly intermediate in which only the membrane-distal N-terminal half of the bundle is assembled. Our findings support the zippering hypothesis, but suggest that zippering proceeds through three sequential binary switches, not continuously, in the N- and C-terminal halves of the bundle and the linker domain. The half-zippered intermediate was stabilized by externally applied force that mimicked the repulsion between apposed membranes being forced to fuse. This intermediate then rapidly and forcefully zippered, delivering free energy of 36 kBT to mediate fusion.

DOI

Measurement of the Instantaneous Velocity of a Brownian Particle

Tongcang Li, Simon Kheifets, David Medellin, Mark G. Raizen

Brownian motion of particles impacts many branches of science. We report on the Brownian motion of micron-sized beads of glass held in air in an optical tweezer, over a wide range of pressures, and measure the instantaneous velocity of a Brownian particle. Our results provide direct verification of the energy equipartition theorem for a Brownian particle. For short times, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. We discuss the applications of these methods towards cooling the center of mass motion of a bead in vacuum to the quantum ground motional state.

DOI