Table 3.

Native thin filament motility over native thick filament motility under various experimental conditions

	pCa 7.5			pCa 5
				Single-phase trajectories			Dual-phase trajectories					Dual-phase runs, %
Thick filament preparation	Velocity, µm/s	Displacement, µm	n	Velocity, µm/s	Displacement, µm	n	Fast-phase velocity, µm/s	Fast-phase displacement, µm	Slow-phase velocity, µm/s	Slow-phase displacement, µm	n
Low calpain
SOL	0.39 ± 0.16	0.35 ± 0.14	205	0.59 ± 0.20	0.79 ± 0.04	106	0.79 ± 0.32	0.41 ± 0.15	0.33 ± 0.18	0.40 ± 0.14	197	65
EDL	0.46 ± 0.19	0.36 ± 0.12	135	1.11 ± 0.50	0.75 ± 0.09	133	1.19 ± 0.51	0.35 ± 0.16	0.39 ± 0.25	0.47 ± 0.15	187	58
High calpain
SOL	NA	NA	NA	0.57 ± 0.24	0.78 ± 0.06	172	0.72 ± 0.26	0.35 ± 0.14	0.30 ± 0.13	0.50 ± 0.13	134	44

Native thin and thick filaments were isolated using low calpain treatment from rat slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) skeletal muscles. The motility of thin filament shards (Materials and Methods) over their respective thick filaments was observed at low Ca²⁺ (pCa 7.5) and high Ca²⁺ (pCa 5) concentrations. At pCa 7.5, only short displacements at a constant velocity were observed. At pCa 5, trajectories were described by a single velocity and displacement (single-phase trajectories) or by dual-phase trajectories having an initial fast velocity phase followed by a slow velocity phase, with the displacements for each phase reported. Following high calpain treatment that proteolytically cleaves only the slow-type MyBP-C N terminus, there was a reduction in the percentage of dual-phase trajectories over SOL native thick filaments. n, the number of thin filament trajectories. NA denotes that no trajectories were observed under the indicated experimental conditions.