Figure 4.

Motility assays for the MT locked mutant compared to control M6 dimer. (a) Plots of log landing rate versus log motor density with lines depicting the processive model and the nonprocessive model superimposed on the data. The control M6 dimer data match the processive model (number of motors for attachment = 1.0), and the MT locked data more closely match the nonprocessive model (number of motors for attachment = 2.0). (b) Plots of the probability of a filament moving further than its length versus log motor density. As described for a, models for processive and nonprocessive movement were superimposed over the data, with the control M6 dimer being in better agreement with the one-motor model (r² processive model = 0.98, r² nonprocessive model = 0.95) and the MT locked mutant being in better agreement with the two-motor model (r² processive model = 0.93, r² nonprocessive model = 0.97). (c) Histogram of measured nonprocessive displacements for the MT locked mutant in a dual-beam optical trap assay. The mean displacement is indicated and represents the length of the power stroke of the motor. All error bars are standard errors.