A new protein conjugation system in human - The counterpart of the yeast Apg12p conjugation system essential for autophagy

Autophagy is an intracellular process for bulk degradation of cytoplasmic c omponents. We recently found a protein conjugation system essential for aut ophagy in the yeast, Saccharomyces cerevisiae. The C-terminal glycine of a novel modifier protein, Apg12p, is conjugated to a lysine residue of Apg5p via an isopeptide bead. This conjugation reaction is mediated by Apg7p, a u biquitin activating enzyme (E1)-like enzyme, and Apg10p, suggesting that it is a ubiquitination-like system (Mizushima, N., Noda, T., Yoshimori, T., T anaka, Y., Ishii, T., George, M. D., Klionsky, D. J,, Ohsumi, Ri., and Ohsu mi, Y. (1998) Nature 395, 395-398). Although autophagy is a ubiquitous proc ess in eukaryotic cells, no molecule involved in autophagy has yet been ide ntified in higher eukaryotes. We reasoned that this conjugation system coul d be conserved. Here we report cloning and characterization of the human ho mologue of Apg12 (hApg12). It is a 140-amino acid protein and possesses 27% identity and 48% similarity with the yeast Apg12p, but no apparent homolog y to ubiquitin. Northern blot analysis showed that its expression was ubiqu itous in human tissues. We found that it was covalently attached to another protein. This target protein was identified to be the human Apg5 homologue (hApg5). Mutagenic analyses suggested that this conjugation was formed via an isopeptide bond between the C-terminal glycine of hApg12 and Lys-130 of hApg5. These findings indicate that the Apg12 system is well conserved and may function in autophagy also in human cells.