Molecular motors

The " Molecular Motors " Minisymposium focused mainly on the microtubule-based motors kinesin and dynein and a class V myosin. A common feature of all these motors is that they move processively on their track, meaning that the motor can take multiple steps without dissociating. A common theme of the Minisymposium was motor function in a complex intracellular environment. Kathy Trybus (University of Vermont) reconstituted mRNA transport in vitro, using a class V myosin from budding yeast (Myo4p), and synthesized ASH1 mRNA, the most well-studied localizing mRNA in budding yeast. Myo4p is single-headed, but the mRNA-binding protein She2p recruits two single-headed motors to form a processive complex. Importantly, the mRNA cargo itself is essential to stabilize the complex at physiological ionic strength, providing a checkpoint to ensure that only cargo-bound motors move proces-sively. The most efficient transport was achieved when motor complexes were bound at more than one of the four localization elements (" zip codes ") in ASH1 mRNA, and when the mRNP (messenger ribonucleoprotein complex) walked on bundles of actin filaments, conditions closest to those found in the cell. By building complexity in vitro, one can begin to mimic cellular processes in a controlled way. Paul Selvin (University of Illinois) tackled the question of how ki-nesin and dynein, which move in opposite directions on the micro-tubule, interact when bound to a common cargo. When kinesin walks toward the plus end of the microtubule, dynein walks backward, and thus both motors are engaged. Selvin argued that this scenario is beneficial when a dual motor–bound cargo encounters a roadblock. Kinesin detaches at an obstacle, allowing the dynein to back up, switch tracks, and allow kinesin to reengage and continue forward motion. He termed this a " synergistic tug-of-war. " Based on a comparison of in vivo and in vitro directional stall forces using an optical trap, a similar situation prevails in the cell. A broad range of stall forces is observed for plus end–directed motion (stall force of kinesin minus a variable number of engaged dyneins). For minus end– directed motion, stall forces are multiples of the stall force of dynein, consistent with kinesin not being engaged with the microtubule. Roop Mallik (Tata Institute of Fundamental Research) used optical trapping within cells to show that part of the reason why dynein is so complex may be to allow multiple dyneins to work efficiently together as a team. This complexity appears …