(A) Schematic illustration of the sPC1. κIgG: κ IgG light chain signal peptide; Flag, Flag antibody epitope tag; NTF, N-terminal fragment; CTF, C-terminal fragment; CTT, intracellular C-terminal tail. Cleavage at GPS site (G-protein coupled-receptor domain proteolytic site) results in NTF and CTF. NTF contains multiple adhesion domains for protein and carbohydrate interaction and a partial GAIN domain sequence before GPS. CTF contains stalk peptide region, 11 transmembrane domains, and the C-terminal tail (CTT). The enlarged region shows GAIN domain (GPCR auto-proteolysis inducing, blue) that contains GPS and stalk peptide (yellow). Full-length PC1 is 4,303 amino acids; NTF is 3,048 amino acids; CTF is 1,255 amino acids; CTT is ~186 amino acids. Not drawn to scale. (B) Scheme of sPC1 deletion constructions used. (C–G) Representative I-V curves for PC2 alone or with different sPC1 deletion constructs recorded in bath solution containing (in mM) 100 NaCl (black), 100 KCl (green), 100 CaCl₂ (red), or 100 NMDG-Cl (blue). Selected regions of I-V curves are enlarged to show reversal potentials. Reversal potential shifts from CaCl₂ to NaCl [labeled Δ(E_Ca-E_Na)] and to KCl [Δ(E_Ca-E_K)] are shown. In E, red arrows indicate that Ca²⁺ entry through heteromers leads to increased inward current size and show that Ca²⁺-activated Cl^– current contributes to increases in outward current size (E versus other panels). TMEM16A channel inhibitor, T16A(inh)-A01 (10 μM), does not cause complete inhibition of endogenous Ca²⁺-activated Cl^– currents. (H and I) The relative permeability of Ca²⁺ versus Na⁺ (P_Ca/P_Na) and Ca²⁺ versus K⁺ (P_Ca/P_K) were calculated from potential shifts upon switching extracellular solution. Individual n numbers for each group are shown in Supplemental Table 1. *P < 0.001, **P < 0.002, ***P < 0.01, by 2-tailed unpaired Student’s t test.