SDN-1 is enriched at NMJs and is mainly provided by postsynaptic muscle cells. (A) Predicted structure of the SDN-1 protein (288 aa) and the sdn-1 locus (2,921 bp). SP, signal peptide; TM, transmembrane; C1, domain containing the ezrin-binding motif DEGS; C2, domain containing the PDZ-binding motif EFYA. Three putative GAG anchoring sites (black arrowheads) have been predicted on serine 71, 86, and 214. Open arrowheads indicate the insertion sites of fluorescent tags in knock-in strains: mNG, AID-mNG, and SL2::wormScarlet (transcriptional reporter SL2-wrmScarlet). The black line shows the location of the 1,258-bp deletion in sdn-1(zh20) mutant allele, referred to as sdn-1(0). Boxes represent exons. (B) Confocal detection of mNG-SDN-1 in an adult worm. (C) Confocal detection and fluorescence profiles of mNG-SDN-1 and L-AChRs (UNC-29-RFP) along the dorsal cord. (D) Electronic microscopy image of the dorsal part of a C. elegans coronal section. Modified from SW-Worm Viewer, slice 273 (Altun et al., 2021; http://www.wormatlas.org/SW/SW.php). (E) Confocal detection of the transcriptional reporter SDN-1::SL2::wrmScarlet. Arrowheads indicate cell bodies of motor neurons. Dotted lines delineate borders of body-wall muscle cells. (F) Confocal detection of SDN-1-AID-mNG from dorsal nerve cord in animals grown in the absence (control) or presence of auxin to induce SDN-1 depletion in all tissues (Peft-3::tir-1-bfp), neurons (Prab-3::tir-1-bfp), epidermis (Pdpy-7::tir-1-bfp), or body-wall muscles (Pmyo-3::tir-1-bfp). (G–J) Quantification of SDN-1-AID-mNG fluorescence levels when SDN-1 was depleted in all tissues (G), epidermis (H), neurons (I), or body-wall muscles (J). Scale bars, 10 µm. See also Figs. S2, S3, and S4.