Kinesin motor mechanics: binding, stepping, tracking, gating, and limping.

This critical review was motivated by the 10 th Biophysical Discussions meeting, " Molecular Motors: Point Counterpoint, " held in Asilomar, California during October 19‐22, 2006. Biophysical Discus‐ sions are meetings that focus on cutting‐edge or emerging topics in biophysics that can benefit from intense discussions. Streaming videos of the speaker presentations at this conference, includ‐ ing a synopsis of this review, are available through the Biophysical Society's website at http://www.biophysics.org/discussions. In keeping with the spirit of a discussions meeting, I pre‐ sent here a personal perspective on the current state of kinesin motor mechanics. Nearly a genera‐ tion has passed since the discovery of the motor named kinesin (Vale et al., 1985), and the subse‐ quent development of the very first single‐molecule gliding‐filament and bead assays for motility (Howard et al., 1989; Block et al., 1990), which helped to establish the modern field of single‐ molecule biophysics. Discrete steps of single molecules were first measured for kinesin (Svoboda et al., 1993), followed shortly thereafter by reports of similar steps for myosin (Finer et al., 1994; Ishi‐ jima et al., 1994). Since then, literally thousands of single‐molecule experiments have been per‐ formed on a whole variety of molecular motors, all with the aim of discovering how these remark‐ able protein machines function. Considerable and impressive progress has been achieved, but key questions still abound, and this remains a very lively field of endeavor. I discuss below my current thinking on several questions concerned with kinesin mechanics, listed in no particular order of precedence. I wade into controversy holding no illusions that everyone will share my views on the answers to these questions, but I do hope to provoke a more thoughtful examination, and set the record straight on at least a few points. By choice, and in keeping with the topic of the meeting ses‐ sion where this was presented (" Motor Walking Mechanisms "), the questions that I've posed relate directly to the nanoscale mechanics of kinesin motion. However, these same questions are inti‐ mately and inevitably linked to other aspects of kinesin structure, biochemistry, and cellular func‐ tion. Does kinesin take sub-steps? If so, over what time and distance scales? In our original paper describing single kinesin stepping, the steps were found to subtend a dis‐ tance of 8 nm, and they took place instantaneously on the time scale of the experiment. Here, the data acquisition rate was …