**Run lengths of chimeras reveal a key role for the myosin X tail in selecting bundles.** The domain composition of each construct is shown for visual clarity of their size. The Kaplan-Meier estimate of the run length survivor function is shown for each processive construct on fascin-actin bundles (*blue*) and single actin filaments (*red*) at 2 mm ATP. Events are left-truncated at 0.2 μm and are right-censored at track ends. Run lengths are estimated from single exponential fits to the empirical survivor function (*dotted lines*). Run length decay constants (±S.E.) are: *A*, myosin X, filaments: 0.17 ± 0.05 μm (*n* = 24); bundles: 0.63 ± 0.08 μm (*n* = 100). Data are from Ref. . *B*, myosin V, filaments: 0.66 ± 0.05 μm (*n* = 231); bundles: 0.57 ± 0.06 μm (*n* = 134). Data are from Ref. . *C*, VXX, filaments: 0.29 ± 0.01 μm (*n* = 779); bundles: 0.49 ± 0.02 μm (*n* = 571). *D*, XVV, filaments: 0.27 ± 0.02 μm (*n* = 129); bundles: 0.31 ± 0.02 μm (*n* = 334). *E*, XVX, filaments: not determined (*n* = 2); bundles: 0.21 ± 0.01 μm (*n* = 323). *F*, VXV: not determined. *G*, XXV: not determined. *H*, VVX, filaments: 0.45 ± 0.02 μm (*n* = 556); bundles: 0.76 ± 0.02 μm (*n* = 817). The VXV and XXV chimeras were nonprocessive on both structures. Note the longer run lengths on bundles for the constructs containing the myosin X tail. The differences in run lengths are significant for VXX and VVX (*p* = 6 × 10^{−21} and 2 × 10^{−20}, respectively; using the Kolmogorov-Smirnov test) and apparent for XVX, despite the low number of observed events on single filaments. The mean velocities measured in this single-molecule TIRF assay are (nm/s ± S.D.): VXX, filaments: 320 ± 100; bundles: 280 ± 90. XVV, filaments: 350 ± 110; bundles: 320 ± 110. XVX, bundles: 280 ± 100. VVX, filaments: 290 ± 110; bundles: 300 ± 120.

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