A v100 mutant that rescues neurotransmission but causes cell death at high levels. (A) Alignment of 17 amino acids surrounding the arginine at position 755 (R755) shows 100% conservation across species. (B) Neurotransmitter release rescue experiments. Representative ERG traces from left to right show control (ctrl; yellow), v100-null mutant (eyFLP eye mosaic; blue), v100^R755A expression in WT photoreceptors (red), and v100^R755A expression in v100 mutant photoreceptors (green). Note that rescue with v100^R755A exhibits opposite phenotypes from the null mutant: rescue of the on transient (red circles) and a strong reduction of the response amplitude (depolarization). (C) Quantification of on transients for all four genotypes. (D) Quantification of depolarization for all four genotypes. v100^R755A in a WT background (red) and v100^R755A in a mutant background (green) compared with control (orange) and mutant (blue) are shown. (E–I) Eye pictures show dosage-dependent loss of photoreceptors upon v100^R755A expression in the mutant (H and I) but not upon v100^R755A expression in WT (G). Error bars indicate SEM. Asterisks denote statistical significance in pairwise comparisons with control (P < 0.001).