Autophagy is the bulk degradation of cytosolic materials in lysosomes/vacuoles of eukaryotic cells. In the yeast SaccharomYces cerevisiae, 17 Atg proteins are known to be involved in autophagosome formation. Genorne wide analyses have shown that Atgl7 interacts with numerous proteins. Further studies on these interacting proteins may provide further insights into membrane dynamics during autophagy. Here, we identify Cisl/Atg3l as a protein that exhibits similar phenotypes to Atg17. ATG31 null cells were defective in aUtophagy and lost viability under starvation conditions. Localization of Atg3l to p re-a utophago soma I structures (PAS) was dependent on Atgl7. Coirnmunoprecipitation experiments indicated that Atg3l interacts with Atgl7. Together, Atg31 is a novel protein that, in concert with Atgl7, is required for proper autophagosome formation. (c) 2007 Elsevier Inc. All rights reserved.