Table 4.

Optical trap step kinetics and step displacement fit parameters

			Step kinetics				Step sizes
Motor	[MgATP]	Load, pN	A1	k1, s−1	A2	k2, s−1	Aback	μback, nm	Aforward	μforward, nm
KIF3AA	1 mM	1 pN hindering	1.0	18	—	—	0.04	−7.1 ± 4.8	0.96	8.7 ± 2.9
KIF3AA	1 mM	1 pN assisting	1.0	24	—	—	0.12	−11 ± 6.4	0.88	11 ± 5.5
KIF3CC	1 mM	1 pN hindering	0.61	6.6	0.39	0.6	0.39	−6.7 ± 5.2	0.61	7.8 ± 4.4
KIF3CC	1 mM	1 pN assisting	0.78	18	0.22	2.2	0.20	−9.8 ± 5.1	0.80	11 ± 7.5
KIF3AC	10 μM	1 pN hindering	1.0	5.9	—	—	0.35	−5.0 ± 7.2	0.65	7.8 ± 2.9
KIF3AC	1 mM	1 pN hindering	0.96	11	0.04	2.4	0.21	−7.4 ± 5.0	0.79	9.2 ± 4.5
KIF3AC	1 mM	1 pN assisting	0.88	24	0.12	2.7	0.27	−9.4 ± 7.4	0.73	11 ± 7.3

Dwell time distributions were fit by either a single exponential function or the sum of two exponential functions to determine step kinetics. 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. Amplitude and rate parameters are shown. Justification of a second exponential was determined by the log-likelihood ratio test. For KIF3CC, P < 1 × 10⁻¹⁶ at 1-pN hindering load, and P < 1 × 10⁻¹⁶ at 1-pN assisting load. For KIF3AC, P < 1 × 10⁻¹⁶ at 1-pN hindering load, and P < 1 × 10⁻¹⁶ at 1-pN assisting load. Step-size distributions were fit by the sum of two Gaussian distributions. Relative amplitudes are reported; step sizes are reported as means ± σ of the fit curve, where σ indicates the width of Gaussian peak and not uncertainty in peak position.