(A) The CTD-pep (green) or scr-pep is infused into CpxII ko cells expressing either full-length CpxII (blue) or the truncated Cpx1-100 (orange). (B) Average [Ca2+]i levels (top) and corresponding capacitance responses (bottom) for infusion of either scr-pep or CTD-pep in the indicated groups. Data were collected from the following number of cells: wt + scr pep (black, n = 19), CpxII ko + CpxII + scr-pep (blue, n = 15), CpxII ko+Cpx1-100+scr pep (orange, n = 15) and CpxII ko+Cpx1-100+CT pep (green, n = 18). Flash is at t = 0.5 s. (C) The CTD peptide (green), but not its scrambled variant (orange) recues the RRP and SRP in Cpx1-100 expressing cells, matching nearly the phenotype of full length CpxII expression (blue). The rate of sustained release (SR, fF/s) is slightly reduced with CpxII expression. (D) Neither the time constants of the EB components (τRRP, τSRP) nor the exocytotic delay are altered for the tested groups. (E, F) Contrary to the scrambled peptide the CTD suppresses premature secretion at submicromolar [Ca]i before the flash response (arrow) in Cpx1-100 expressing cells almost like the CpxII protein (same cells as shown in B). (G) Like CpxII expression in CpxII ko cells, the CTD hinders tonic secretion (in response to 19 µM [Ca]i) in Cpx1-100 expressing cells, whereas the scr-pep failed to do so. (H) Total CM after 120 s (upper panel) and amperometric event frequency (lower panel) for the groups in G. Numbers of cells are depicted in the bars. (I) The CTD-pep together with Cpx1-100 increases the fusion pore expansion time (upper panels) and reduces its dynamics (lower panels) like CpxII. Values are determined from the individual parameter’s frequency distribution for each cell. Data are averaged from the cells/events measured for wt + scr pep (13/1558), CpxII ko + CpxII + scr-pep (15/731), CpxII ko+Cpx1-100 + scr pep (20/3292) and CpxII ko+Cpx1-100+CTD pep (18/857) (>20 events/cell). Error bars indicate mean ± SEM. ANOVA followed by Tukey-Kramer post-hoc test. *p<0.05, **p<0.01, ***p<0.001.