Physical Society Colloquium

Single Molecule Measurements of Motor Proteins, In vitro and In vivo

Paul Selvin

Center for Biophysics and Computational Biology
University of Illinois at Urbana-Champaign

The standard diffraction limit of light is about 250 nm, meaning that you cannot “resolve” objects closer than this distance. Despite this, we have come up with a method to measure 1.5 nm in 1-500 msec, using a technique we call Fluorescence Imaging with One Nanometer Accuracy (FIONA). We have chosen to study molecular motors, which are involved in moving things around within the cell, both in purified systems, and inside living cells. There has been a question as to whether molecular motors move things in an “inchworm” fashion, or in a “hand-over-hand” fashion (i.e. by “walking”.) We have definitively determined that myosin, and kinesin, two important motors, walk in a “hand-over-hand” manner in purified systems. In living cells (that is, in Drosophilia, or fruit fly cells), we have seen cargos being moved by individual “conventional” kinesin and dynein. We find that both kinesin and dynein move cargo 8 nm per ATP. Amazingly, these two molecular motors do not engage in a tug-of-war, but appear to be cooperative, taking terms hauling the cargo.