Convergence Of Secretory,Endosomal And Autophagic Routes In Drosophila Trans-Golgi-associated Lysosomes

Traffic between ER and cis-Golgi is tightly canalized,with many known proteins regulating ER-Golgi exchanges.Compared to ER-Golgi traffic,genetic screenings have produced very little insight into the functional organization of the trans-Golgi and molecular events regulating traffic there.Indeed,ample evidence shows complex traffic connections between the trans-Golgi and the endolysosomal system additional to the main Golgi-to-plasma membrane secretory route.Besides,more and more reports support for a strong relation between autophagy and secretion.Work by many groups has focused on contributions of ERES and the vertebrate ER-Golgi intermediate compartment to the growth of autophagic vesicles.Recently,multiple other pieces of evidence link secretory pathway to macroautophagy at the level of the Golgi and TGN.This study started from three hits in a secretion screen in Drosophila melanogaster,which displayed secretory cargo accumulation in autophagic vesicles: SNARE-binding Rop,ESCRT-III Vps20 and lysosomal pump subunit Vha PPA1-1.We found that Rop,Vps20 and lysosomes localized near the Golgi.Furthermore,we documented that the vicinity of the trans-Golgi was the main cellular location for lysosomes,an association that live imaging indicated was mostly stable.We also documented that early,late and recycling endosomes associate as well with a trans-Golgi-associated degradative compartment where basal microautophagy of secretory cargo and other materials occurs.Disruption of this compartment leads to secretory cargo accumulation in our hits.Besides basal microautophagy,we showed that the trans-Golgi-associated degradative compartment contributed to the growth of autophagic vesicles in developmental and starvation-induced macroautophagy.Finally,we showed that Rab-GTPases in secretion and endosomal pathway contributed to the growth of autophagic vesicles in developmental macroautophagy as well.By using several imaging techniques,Drosophila genetics,immunostaining and inhibitor treatment,our research documents that the vicinity of the trans-Golgi is the main location for lysosomes in Drosophila cells and shows that the convergence of secretory and endosomal traffic not only gives rise to a trans-Golgi-associated degradative compartment in homeostatic conditions but also contributes to autophagic vesicle growth upon macroautophagy induction.Our study suggests that the fly trans-Golgi is the gravitational center of the whole endomembrane system.