Table 1.

iSCAT step kinetics and displacement fit parameters

		Step kinetics				Step sizes
Motor	[MgATP], mM	A1	k1, s−1	A2	k2, s−1	Aback	μback, nm	Aforward1	μforward1 (nm)	Aforward2	μforward2, nm
KIF3AA	1	0.83	34	0.17	6.1	0.12	−14 ± 7.2	0.49	9.1 ± 3.5	0.39	16 ± 15
KIF3CC	1	0.68	1.3	0.32	0.44	0.18	−8.2 ± 7.2	0.66	6.9 ± 2.9	0.16	19 ± 8.2
KIF3AC	1	1.0	11	—	—	0.15	−12 ± 8.2	0.60	9.3 ± 3.4	0.25	16 ± 6.4

The relative amplitudes and rates of each exponential fit are shown. Single exponential fit equation: $y=k_{\mathit{1}}{e}^{-k_{\mathit{1}}t}$ and sum of two exponential functions fit equation: $y=A{k}_{\mathit{1}}{e}^{-k_{\mathit{1}}t}+\left(1-A\right)k_{\mathit{2}}{e}^{-k_{\mathit{2}}t}$ where the amplitude A is reported as A₁ and (1 – A) is reported as A₂. For step displacements, A_back and μ_back describe backstepping, A_forward1 and μ_forward1 the dominant forward step size peak near 8 nm, and A_forward2 and μ_forward2 the additional forward peak near 16 nm. Backward and forward step-size distributions were fit independently. For the KIF3AA dwell time fit, the log-likelihood test gives P = 1.4 × 10⁻⁷, justifying a double exponential fit, and for the KIF3CC dwell-time fit the log-likelihood ratio test gives P = 0.036. Means are shown ± σ of the fit curve where σ indicates the width of the Gaussian peak and not the uncertainty in peak position. Dashes, not applicable.