Both wild-type and R979A mutant Syd-1 fully restore NMJ Nrx-1 levels in syd-1 mutants. (A–D”) Third-instar larval NMJ boutons on muscle 4 stained with anti-HRP (blue) and anti-Nrx-1 (green). Bar, 5 µm. All genotypes also contain the motorneuron driver BG380-Gal4. (A–A”) Wild type. (B–B”) syd-1CD/syd-1w46 mutant. (C–C”) syd-1CD/syd-1w46 mutant in which motorneurons express wild-type Syd-1. (D–D”) syd-1CD/syd-1w46 mutant in which motorneurons express R979A mutant Syd-1. (E) Quantification of anti-Nrx-1 staining. As previously reported, loss of syd-1 decreases Nrx-1 levels in presynaptic boutons (BG380-Gal4 alone: 100 ± 16%, n = 13; syd-1 mutant: 47 ± 11%, n = 12, P = 0.0043). Overexpressing either wild-type or R979A mutant Syd-1 in the motorneurons of syd-1 mutants rescues this defect (85 ± 13% (n = 12, P = 0.013) and 91 ± 19% (n = 9, P = 0.021), respectively). Error bars represent SEM. n = animals. ns, not significant; * P < 0.05, ** P < 0.01, based on one-tailed t-tests.