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never been observed. Some studies suggest that in mammalian cells, the
autophagosomal membrane originates from the endoplasmic reticulum
(
Axe et al., 2008; Dunn, 1990
). In addition, more recent research suggests
that autophagosome formation involves membrane derived from the mito-
chondria or the plasma membrane (
Hailey et al., 2010; Ravikumar, Moreau,
Jahreiss, Puri, & Rubinsztein, 2010
).
Formation of the autophagosomal membrane requires phosphorylation
of phosphatidylinositol. In yeast, this is accomplished by a class III PI3K com-
plex consisting of Vps30/Atg6 /Beclin1, Vps34/ class III PI3K, Atg14, and
Vps15 (
Kametaka, Okano, Ohsumi, & Ohsumi, 1998; Kihara, Noda,
Ishihara, &Ohsumi, 2001; Suzuki et al., 2001
). Atg6 also forms a complex re-
quired for thevacuolar protein sorting (VPS) pathway inyeast,whichconsists of
Atg6, Vps35, Vps15, andVps38 (
Kihara et al., 2001
). TheBeclin1-Vps34 com-
plex is similar in mammalian cells; however, it contains additional regulators,
including UVRAG, Bif1, Ambra1, and Barkor (
Fimia et al., 2007; Liang
et al., 2006; Sun et al., 2008; Takahashi et al., 2007
). As in yeast, it has been
suggested that Beclin1 forms at least two distinct complexes in animal cells that
play different roles
in membrane trafficking (
Itakura, Kishi, Inoue, &
Mizushima, 2008
).
2.2. Autophagosome formation
Genetic studies in yeast have identified several
Atg
genes that are required for
autophagy (
Harding, Hefner-Gravink, Thumm, & Klionsky, 1996; Harding,
Morano, Scott, & Klionsky, 1995; Klionsky et al., 2003; Thumm et al., 1994;
Tsukada&Ohsumi, 1993
).Many of these genes are involved in two conserved
ubiquitin-like conjugation systems that are required for autophagosome forma-
tion, Atg12 and Atg8 (LC3 in mammals) (
Klionsky & Emr, 2000; Ohsumi,
2001
). Atg12 and Atg8 are both activated by the E1-like enzyme Atg7.
Atg12 is then transferred to the E2-like enzyme Atg10. Finally, Atg12 is con-
jugated to Atg5 and forms a complex with Atg16 on the isolation membrane
(
Kuma, Mizushima, Ishihara, &Ohsumi, 2002; Mizushima, Noda, &Ohsumi,
1999; Mizushima et al., 1998; Shintani et al., 1999; Tanida et al., 1999
). Atg8 is
transferred to the E2-like enzyme Atg3 and is then conjugated to the phospho-
lipid anchor phosphatidylethanolamine (PE) (
Ichimura et al., 2000
). This final
conjugation results in the anchoring of Atg8-PE to the isolationmembrane and
is thought to regulate the elongation of the isolation membrane (
Nakatogawa,
Ichimura, &Ohsumi, 2007
). In addition to Atg7 and Atg3, Atg8 modification
requires Atg4, a cysteine protease that processes Atg8 before conjugation and
