This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. SUMMARY Bulk degradation of cytoplasmic material is medi-ated by a highly conserved intracellular trafficking pathway termed autophagy. This pathway is characterized by the formation of double-membrane vesicles termed autophagosomes engulfing the sub-strate and transporting it to the vacuole/lysosome for breakdown and recycling. The Atg1/ULK1 kinase is essential for this process; however, little is known about its targets and the means by which it controls autophagy. Here we have screened for Atg1 kinase substrates using consensus peptide arrays and iden-tified three components of the autophagy machinery. The multimembrane-spanning protein Atg9 is a direct target of this kinase essential for autophagy. Phosphorylated Atg9 is then required for the efficient recruitment of Atg8 and Atg18 to the site of autopha-gosome formation and subsequent expansion of the isolation membrane, a prerequisite for a functioning autophagy pathway. These findings show that the Atg1 kinase acts early in autophagy by regulating the outgrowth of autophagosomal membranes. INTRODUCTION