Fig. 1.

Changes in the number of engaged motors are revealed by the analysis of stall forces. Comparison of single motor no tau baseline case (a) with single motor assays with moderate amounts of 4RL (b) or 3RS (c) tau shows that tau does not affect the amount of force a single motor can produce against external load. Note that as tau is added to the assay, increased numbers of binding attempts are required to observe one stall event (serving as an effective experimental limitation for further increases in tau concentration). In contrast to the single-motor case (a–c), stall forces in the ≈2 motor assay are strongly affected by tau. The bare MT assay (d) shows contributions from both single motor (*) and 2 motor events (**), with rare contributions from 3 or more motor events (gray bar). Comparing the bare MT assay (d) with assays featuring progressively higher 4RL (e and f) and 3RS (g and h) tau concentrations reveals that the frequency of two-motor stall events is gradually suppressed. The high-force (>7.0 pN) events account for 72.7% of the total in the no tau case (d), but the percentage is significantly reduced in e and g (28.7% and 20.0% respectively). The solid lines are fits to Gaussian form (the peak locations are reported in each subplot). The molar ratio of tau to tubulin dimer as well as the isoform of tau used are shown, as appropriate.