Apg7p is a unique E1 enzyme which is essential for both the Apg12p- and Apg8p-modification systems, and plays indispensable roles in yeast autophagy. A cDNA encoding murine Apg7p homolog (mApg7p) was isolated from a mouse brain cDNA library. The predicted amino acid sequence of the clone shows a significant homology to human Apg7p and yeast Apg7p. Murine Apg12p as well as the three mammalian Apg8p homologs co-immunoprecipitate with mApg7p. Site-directed mutagenesis revealed that an active-site cysteine within mApg7p is Cys(567), indicating that mApg7p is an authentic El enzyme for murine Apg12p and mammalian Apg8p homologs. The mutagenesis study also revealed that Apg12p has a substrate preference for mApg7p over the three Apg8p homologs, suggesting that the Apg12p conjugation by Apg7p occurs preferentially in mammalian cells compared with the modification of the three Apg8p homologs. We also report here on the ubiquitous expression of human APG7 mRNA in human adult and fetal tissues and of rat Apg7p in adult tissues. (C) 2002 Elsevier Science (USA).