Table S2.

ΔF/F-V fit parameters

Cell-type/constructs	ΔF/F-V
	ΔF/Fmax	VF, mV	kF, mV
Stac3 heterozygous myotubes	1.43 ± 0.21 (6)	−7.4 ± 0.4	6.9 ± 0.4
Stac3 KO myotubes	0.08 ± 0.01 (5)	−30.0 ± 1.4	13.3 ± 1.3
Stac3 KO myotubes + Stac3-BFP	1.26 ± 0.15 (6)	−8.6 ± 0.5	7.6 ± 0.4
Stac3 KO myotubes + Stac3W280S-BFP	0.17 ± 0.01 (5)	−20.2 ± 2.3	13.5 ± 2.0

Average values of parameters obtained by fits of Eq. 4 to ΔF/F-V data from individual cells (Materials and Methods). Data are given as mean ± SEM, with the numbers in parentheses indicating the number of myotubes tested. The average parameters listed above were used to produce the smooth curves through the ΔF/F-V data shown in Figs. 4 and 6.