Association of APP with the syntaxin 1 microdomain through X11–Munc18 is controlled by cdk5 in N2a cells. (a) The effect of cdk5 on APP–syntaxin 1 and APP–BACE1 copatching. Cdk5/p35 expression caused switching of APP association in copatching. Results are means + SD of 20–31 measurements. (b) Effects of Munc18-1 mutants at the cdk5 phosphorylation site on APP–BACE1 copatching. Phosphorylation-resistant Munc18-1 T574A abolished the effect of cdk5, suggesting that the effect of cdk5 is mainly mediated by Munc18-1. A phosphorylation-mimetic mutant with reduced affinity to syntaxin 1, Munc18-1 T574E, had an effect similar to cdk5, indicating an involvement of syntaxin 1 and Munc18 in the regulation of the APP–BACE1 interaction. Data are means + SD based on three independent experiments (n = 32–44). (c) Effects of cdk5/p35 on interaction between syntaxin 1 and APP. N2a cells expressing Venus–syntaxin 1a with cdk5/p35 were used for IP with anti-GFP after cell lysis. Cdk5 reduced the association of Munc18, X11, and APP with syntaxin 1. **, P < 0.01.