A new role for ATM in selective autophagy of peroxisomes (pexophagy)

Abstract

Peroxisomes are autonomously replicating and highly metabolic organelles necessary for β-oxidation of fatty acids, a process that generates large amounts of reactive oxygen species (ROS). Maintaining a balance between biogenesis and degradation of peroxisomes is essential to maintain cellular redox balance, but how cells do this has remained somewhat of a mystery. While it is known that peroxisomes can be degraded via selective autophagy (pexophagy), little is known about how mammalian cells regulate pexophagy to maintain peroxisome homeostasis. We have uncovered a mechanism for regulating pexophagy in mammalian cells that defines a new role for ATM (ATM serine/threonine kinase) kinase as a "first responder" to peroxisomal ROS. ATM is delivered to the peroxisome by the PEX5 import receptor, which recognizes an SRL sequence located at the C terminus of ATM to localize this kinase to peroxisomes. In response to ROS, the ATM kinase is activated and performs 2 functions: i) it signals to AMPK, which activates TSC2 to suppresses MTORC1 and phosphorylates ULK1 to induce autophagy, and ii) targets specific peroxisomes for pexophagy by phosphorylating PEX5 at Ser141, which triggers ubiquitination of PEX5 at Lys209 and binding of the autophagy receptor protein SQSTM1/p62 to induce pexophagy.

Figure 1.

Working model depicting the mechanism of pexophagy in the mammalian system. ATM signaling to the TSC signaling node to repress MTORC1, phosphorylation of PEX5 to induce ubiquitination. by the PEX2-PEX10-PEX.12 E3 ligase, and recognition of Ub-PEX5 by SQSTM1 to induce pexophagy in response to ROS. Phagophores are formed that sequester a .peroxisome; the resulting autophagosomes fuse with a lysosome, and lysosomal enzymes degrade the peroxisome to complete the process of pexophagy.