Fig. 4. Dynein force production in the presence of tau.
Histograms of stalling forces with no tau on MTs (a), ~0.2 4RL tau/tubulin dimer (b), and ~0.2 3RS tau/tubulin dimer (c) feature three identifiable peaks (at ~1.1 pN, ~1.8 pN, and ~2.6 pN). The decrease in counts on the high-force side of the ~2.6 pN peak are not shown here (but see expanded histogram in supplement). Black bars represent counts for stall forces outside the linear range of the optical trap. Example stall shapes corresponding to the three peaks are shown in (g-i) respectively. Notably, higher stall force are significantly suppressed (but still observed) in the 3RS tau assay (c). Indeed, the reduction is also seen in overall force production (not just stall events). Representative bead displacement in an optical trap (panels d-f) for one bead is shown for each assay. The force production for no tau (d) and high 4RL tau (e) events are similar. The frequency of moderate and high force events in the high 3RS assay is significantly reduced (f). Because the motors need to move extremely short distances to generate such forces, this decrease reflects an effect of tau on the motor’s on-rate, confirming the general picture derived from the stall-force histograms (a-c). We conclude that high amounts of 3RS tau can significantly reduce the average number of engaged motors in our dynein assay, but that multiple motors can still engage in transport.