Figure 5.

Phospholipid-binding mutants decrease the apparent Ca²⁺ affinity but do not affect the Ca²⁺ cooperativity of release. A, EJPs were evoked in P[syt^WT], P[syt^A-RQ] line 8, and P[syt^B-RQ] lines 3 and 4 by 0.05 Hz stimulation, and 10 sweeps were averaged for each fiber at each [Ca²⁺]. The responses from lines 3 and 4 of P[syt^B-RQ] were not significantly different, so the results were pooled. For all genotypes at all [Ca²⁺], n = 10–18 muscle fibers, except for 1.5 mm Ca²⁺, at which n = 40–49 muscle fibers. The Hill equation was fit to the data. Error bars are SEM. B, The EJP amplitudes within the nonsaturating range of Ca²⁺ were plotted on a double-log plot, and a linear regression line was used to determine the slope (n) (P[syt^WT]: n = 3.2, r = 0.999; P[syt^A-RQ]: n = 3.0, r = 0.999; P[syt^B-RQ]: n = 2.9, r = 0.933). C, The EJP amplitudes at each Ca²⁺ concentration were normalized to the maximum predicted by the Hill equation for each genotype and replotted to illustrate the shift in EC₅₀. P[syt^wt]: EC₅₀ = 1.4 ± 0.1 mm; P[syt^A-RQ]: EC₅₀ = 2.0 ± 0.1 mm; P[syt^B-RQ]: EC₅₀ = 2.3 ± 0.2 mm. For all panels, black filled circles indicate P[syt^WT], gray open squares indicate P[syt^A-RQ], and gray open diamonds indicate P[syt^B-RQ].