Starvation and ULK1-dependent cycling of mammalian Atg9 between the TGN and endosomes.
Autophagy, fundamentally a lysosomal degradation pathway, functions in cells during normal growth and certain pathological conditions, including starvation, to maintain homeostasis. Autophagosomes are formed through a mechanism that is not well understood, despite the identification of many genes required for autophagy. We have studied the mammalian homologue of Atg9p, a multi-spanning transmembrane protein essential in yeast for autophagy, to gain a better understanding of the function of this ubiquitious protein. We show that both the N- and C-termini of mammalian Atg9 (mAtg9) are cytosolic, and predict that mAtg9 spans the membrane six times. We find that mAtg9 is located in the trans-Golgi network and late endosomes and colocalizes with TGN46, the cation-independent mannose-6-phosphate receptor, Rab7 and Rab9. Amino acid starvation or rapamycin treatment, which upregulates autophagy, causes a redistribution of mAtg9 from the TGN to peripheral, endosomal membranes, which are positive for the autophagosomal marker GFP-LC3. siRNA-mediated depletion of the putative mammalian homologue of Atg1p, ULK1, inhibits this starvation-induced redistribution. The redistribution of mAtg9 also requires PI 3-kinase activity, and is reversed after restoration of amino acids. We speculate that starvation-induced autophagy, which requires mAtg9, may rely on an alteration of the steady-state trafficking of mAtg9, in a Atg1-dependent manner.
Young
Andrew R J
AR
Cancer Research UK London Research Institute, 44 Lincoln's Inn Fields, London, WC2A 3PX, UK.
Chan
Edmond Y W
EY
Hu
Xiao Wen
XW
Köchl
Robert
R
Crawshaw
Samuel G
SG
High
Stephen
S
Hailey
Dale W
DW
Lippincott-Schwartz
Jennifer
J
Tooze
Sharon A
SA
D000818:Animals; D000071189:Autophagy-Related Protein-1 Homolog; D000071183:Autophagy-Related Proteins; D011992:Endosomes / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D000090802:rab7 GTP-Binding Proteins; D021601:trans-Golgi Network / Q000378:metabolism* / Q000648:ultrastructure
C503283:ATG9B protein, human; D000071183:Autophagy-Related Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D000090802:rab7 GTP-Binding Proteins; C000716463:rab7 GTP-binding proteins, human; C000716466:rab7 GTP-binding proteins, rat; D049452:Green Fluorescent Proteins; D000071189:Autophagy-Related Protein-1 Homolog; D017346:Protein Serine-Threonine Kinases; C484754:ULK1 protein, human; D020691:rab GTP-Binding Proteins