**3. Receptor proteins required for the selective degradation of organelles by autophagy**

Damaged or superfluous organelles, such as mitochondria and peroxisomes, are also selectively degraded by autophagy. To date, several receptor proteins which function in these selective types of autophagy have been identified.

#### **3.1 Receptor proteins in mitophagy**

The mitochondrion is an organelle that produces energy through oxidative phosphorylation and simultaneously generates reactive oxygen species (ROS), causing oxidative damage to mitochondrial DNA, protein and lipids, and often inducing cell death. Therefore, an appropriate quality control of mitochondria is important to maintain proper cellular homeostasis. Selective degradation of mitochondria via autophagy, known as mitophagy, is the primary mechanism for mitochondrial quality control. Two groups independently identified Atg32 as a receptor protein for mitophagy in yeast (Kanki et al., 2009; Okamoto et al., 2009). Atg32 is a single-pass mitochondrial outer membrane protein, and its N- and Cterminal domains are oriented towards the cytoplasm and the intermembrane space, respectively. Mitochondria-anchored Atg32 binds Atg11 during mitophagy to recruit mitochondria to the PAS. When mitophagy is induced, Atg32 is phosphorylated, for which Ser-114 and Ser-119 of Atg32 are required. , The phosphorylation of Atg32 is required for Atg32-Atg11 interaction and mitophagy (Aoki et al., 2011). By controlling the activity and/or localization of the kinase that phosphorylates Atg32, cells may regulate the amount of mitochondria or remove damaged or aged mitochondria through mitophagy. Similarly to other receptor proteins, Atg32 binds to Atg8 using the AIM sequence, 86-WQAI-89, and this binding is required for the efficient sequestration of mitochondria by the autophagosome. The Atg32-Atg8 interaction may restrict autophagosome formation to the neighborhood of the targeted mitochondrion by gathering surrounding Atg proteins. Although no homolog of Atg32 has been identified in mammals, Nix was recently shown to be a mammalian mitophagy receptor protein (Novak et al., 2010). Similarly to Atg32, Nix has a transmembrane domain in its C terminus that can target the protein to the mitochondrial outer membrane and has a functional AIM, 35-WVEL-38, that directly interacts with mammalian Atg8 homologs. Thus Nix may fulfill the function of Atg32 in mammals.

#### **3.2 Receptor protein in pexophagy**

Peroxisomes have diverse functions, including the decomposition of hydrogen peroxide and the oxidation of fatty acids. The specific degradation of peroxisomes by autophagy, pexophagy, is conserved from yeast to humans and is triggered physiologically to allow cells to clear an excess of peroxisomes. The study of methylotrophic yeasts, particularly *P. pastoris* and *Hansenula polymorpha*, has led to the current understanding of the molecular mechanism governing pexophagy. Farre *et al.* identified Atg30 as a receptor protein for pexophagy in *P. pastoris* (Farre et al., 2008). PpAtg30 interacts with peroxisomes via two peroxisomal membrane proteins, Pex3 and Pex14, and with autophagy machinery via PpAtg11 and PpAtg17, which organize the PAS. Several residues on PpAtg30 are phosphorylated under pexophagy conditions and, similarly to Atg32, such phosphorylation, especially that of Ser-112, is required for PpAtg11 interaction. The isolation membrane then expands and surrounds the PpAtg30-localizing peroxisomes, in

**3. Receptor proteins required for the selective degradation of organelles by** 

Damaged or superfluous organelles, such as mitochondria and peroxisomes, are also selectively degraded by autophagy. To date, several receptor proteins which function in

The mitochondrion is an organelle that produces energy through oxidative phosphorylation and simultaneously generates reactive oxygen species (ROS), causing oxidative damage to mitochondrial DNA, protein and lipids, and often inducing cell death. Therefore, an appropriate quality control of mitochondria is important to maintain proper cellular homeostasis. Selective degradation of mitochondria via autophagy, known as mitophagy, is the primary mechanism for mitochondrial quality control. Two groups independently identified Atg32 as a receptor protein for mitophagy in yeast (Kanki et al., 2009; Okamoto et al., 2009). Atg32 is a single-pass mitochondrial outer membrane protein, and its N- and Cterminal domains are oriented towards the cytoplasm and the intermembrane space, respectively. Mitochondria-anchored Atg32 binds Atg11 during mitophagy to recruit mitochondria to the PAS. When mitophagy is induced, Atg32 is phosphorylated, for which Ser-114 and Ser-119 of Atg32 are required. , The phosphorylation of Atg32 is required for Atg32-Atg11 interaction and mitophagy (Aoki et al., 2011). By controlling the activity and/or localization of the kinase that phosphorylates Atg32, cells may regulate the amount of mitochondria or remove damaged or aged mitochondria through mitophagy. Similarly to other receptor proteins, Atg32 binds to Atg8 using the AIM sequence, 86-WQAI-89, and this binding is required for the efficient sequestration of mitochondria by the autophagosome. The Atg32-Atg8 interaction may restrict autophagosome formation to the neighborhood of the targeted mitochondrion by gathering surrounding Atg proteins. Although no homolog of Atg32 has been identified in mammals, Nix was recently shown to be a mammalian mitophagy receptor protein (Novak et al., 2010). Similarly to Atg32, Nix has a transmembrane domain in its C terminus that can target the protein to the mitochondrial outer membrane and has a functional AIM, 35-WVEL-38, that directly interacts with

mammalian Atg8 homologs. Thus Nix may fulfill the function of Atg32 in mammals.

Peroxisomes have diverse functions, including the decomposition of hydrogen peroxide and the oxidation of fatty acids. The specific degradation of peroxisomes by autophagy, pexophagy, is conserved from yeast to humans and is triggered physiologically to allow cells to clear an excess of peroxisomes. The study of methylotrophic yeasts, particularly *P. pastoris* and *Hansenula polymorpha*, has led to the current understanding of the molecular mechanism governing pexophagy. Farre *et al.* identified Atg30 as a receptor protein for pexophagy in *P. pastoris* (Farre et al., 2008). PpAtg30 interacts with peroxisomes via two peroxisomal membrane proteins, Pex3 and Pex14, and with autophagy machinery via PpAtg11 and PpAtg17, which organize the PAS. Several residues on PpAtg30 are phosphorylated under pexophagy conditions and, similarly to Atg32, such phosphorylation, especially that of Ser-112, is required for PpAtg11 interaction. The isolation membrane then expands and surrounds the PpAtg30-localizing peroxisomes, in

these selective types of autophagy have been identified.

**3.1 Receptor proteins in mitophagy** 

**3.2 Receptor protein in pexophagy** 

**autophagy** 

order to selectively degrade surplus peroxisomes. Unlike other receptor proteins, PpAtg30 has no AIMs, so that PpAtg30 is unable to interact directly with PpAtg8. It is important to understand how the isolation membrane expands around the peroxisome surface while excluding cytosolic contents, and this is speculated to involve the interaction of PpAtg30 with an unidentified protein in the isolation membrane, or the interaction of PpAtg8 with another protein on the peroxisome surface.
