Dual Functions of Syntaxin 17 in Mitochondrial Division and Autophagosome Formation Are Coordinated by MAP1B-LC1

In fed cells, syntaxin 17 (Stx17) is associated with microtubules and promotes mitochondrial fission by determining the localization and function of Drp1. Upon starvation, Stx17 dissociates from MTs and Drp1, and binds to Atg14L to facilitate autophagosome formation, but the mechanism underlying this phenomenon remains unknown. Here we identify MAP1BLC1 (microtubule-associated protein 1B-light chain 1) not only as a link between Stx17 and microtubules, but also as a critical regulator for Stx17 function. Depletion of MAP1B-LC1 causes Stx17-dependent autophagosome accumulation even under nutrient-rich conditions, whereas its overexpression blocks starvation-induced autophagosome formation. MAP1B-LC1 links microtubules and Stx17 in fed cells, and starvation causes the dephosphorylation of MAP1B-LC1 at Thr217, allowing Stx17 to dissociate from MAP1B-LC1 and bind to Atg14L. Our results also provide an explanation for the effect of the microtubuledepolymerizing reagent vinblastine that causes autophagosome formation in fed cells. Vinblastine, but not nocodazole, causes the dissociation of Stx17 from MAP1B-LC1.