Syntaxin 4 interacts with AP-1 and is essential for lysosome exocytosis. (A) Endogenous syntaxin 4 (Stx4) or syntaxin 2 (Stx2) were immunoprecipitated from wild-type (WT), μ1AKD and μ1BKD MDCK cell lysates (300 µg each) and immunoblotted for γ-adaptin. Inputs: 30 µg each of total lysates probed for γ-adaptin, syntaxin 4 and syntaxin 2. (B) Immunostaining for endogenous syntaxin 4 (green) in wild-type, μ1AKD and μ1BKD cells. ZO-1 is in red. (C) Representative western blot showing shRNA-mediated knockdown of endogenous syntaxin 4 in wild-type, μ1AKD and μ1BKD cells; Scr, scrambled sequence. Actin was the loading control. (D) β-hex release from cells transfected with either scrambled or syntaxin 4 shRNA constructs. *P<0.05, compared with basolateral release from corresponding Scr cells; #P<0.05 compared with apical release from corresponding Scr cells.