N. Mizushima et al., A new protein conjugation system in human - The counterpart of the yeast Apg12p conjugation system essential for autophagy, J BIOL CHEM, 273(51), 1998, pp. 33889-33892
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.