Figure 2.

Processive stepping by myosin VI observed by the dual-beam optical trap. (A) Dual-bead optical trap scheme. An actin filament is held between two 1-μm polystyrene beads. Bead positions are recorded with single nm resolution over 10 kHz. For fixed trap measurements (B and C) the optical load increases with displacement. (B) Sample trace of stepping behavior at 2 mM ATP with a fixed trap. Dwell periods at low load (<1.75 pN) fit single exponential statistics, with a mean stepping rate of 9.1 ± 0.6 s⁻¹ (n = 234). This rate is similar to the 8- to 9-s⁻¹ steady-state ATPase rate for single-headed constructs (9). (C) Oscillatory behavior observed at 2 mM ATP and high load (>2.5 pN).