A new flavor of cellular Atg8-family protein lipidation – alternative conjugation to phosphatidylserine during CASM

Abstract

Atg8-family protein lipidation is the most commonly used marker for monitoring autophagy. During macroautophagy, Atg8-family proteins are specifically conjugated to phosphatidylethanolamine (PE) in forming, double-membrane autophagosomes. A distinct, non-canonical autophagy pathway also operates, characterized by the Conjugation of ATG8s to endolysosomal Single Membranes (CASM). In our new study, we show that CASM is associated with the alternative conjugation of Atg8-family proteins to phosphatidylserine (PS), and PE, in response to various cellular stimuli. We also discover differences in the regulation of conjugation to PE and PS by ATG4s, and altered dynamics between the two species. The identification of alternative Atg8-family protein PS lipidation opens up exciting new questions on the roles, regulation and biology of Atg8-family proteins during non-canonical autophagy.

The Pathogenesis of Pancreatitis and the Role of Autophagy

“…In contrast to classical autophagy, the LAPasome is a single membrane vacuole. The term CASM (conjugation of ATG8 to single membranes) was introduced to describe these related pathways [80][81][82]. Detailed descriptions of the autophagy pathway have recently been published [66,83].…”

Autophagy, Acute Pancreatitis and the Metamorphoses of a Trypsinogen-Activating Organelle

“…Notably, unlike LAP, the observed LC3 conjugation to endocytic vacuoles is insensitive to antioxidants and is therefore unlikely to be induced by locally generated reactive oxygen species (ROS) (compare [ 71 ] and [ 9 ]). The LNCA in the acinar cells is probably a subtype of CASM; this conclusion is based on the common single-membrane LC3 conjugation for both processes and on the striking effect of V-ATPase inhibitors on LNCA [ 9 ] (the interaction of LC3-conjugating machinery with V-ATPases is a defining property of CASM [ 73 , 78 , 80 ]). Furthermore, both LNCA in the acinar cells and CASM require ATG16L1 with an intact C-terminal WD domain [ 9 , 75 , 76 ].…”

“…However, within the framework of LAP, ATG8/LC3 conjugates to the single membrane of the phagosome (reviewed in [ 72 ]). Notably, in this case, ATG8/LC3 can bind to both phosphatidylethanolamine and phosphatidylserine [ 73 , 74 ]. Different domains of ATG16L are involved in LAP and canonical autophagy; the WD repeat-containing C-terminal domain (WD CT) of ATG16L1 is essential for LAP but dispensable for canonical autophagy [ 75 , 76 ].…”

“…It was later found that LAP is just one representative of a significant class of cellular processes involving single membrane ATG8/LC3 conjugation including ATG8/LC3 conjugations to structures formed as a result of entosis [ 77 ], micropinocytosis [ 77 ] as well as to vacuoles induced by osmotic imbalance [ 78 ]. The term CASM (conjugation of ATG8 to single membranes) was introduced to describe these related phenomena [ 73 ]. LAP could be considered as a prominent and the first-identified subtype of CASM.…”

LC3B conjugation machinery promotes autophagy-independent HIV-1 entry in CD4+ T lymphocytes

“…In particular, the conjugation of the ATG8/LC3B family members to lipids, recently named "ATG8ylation" by analogy with ubiquitination, has been proposed to be a general mechanism involved in the maintenance of membrane homeostasis during remodeling (17,18). Interestingly, it has been shown that the conjugation of ATG8 proteins is not restricted to PE and that this protein can also be conjugated to phosphatidylserines (PS) (19,20). ATG8ylation involves an E1-like ATG7 protein, two E2-like ATG3 and ATG10 proteins and finally the E3-like complex ATG5−ATG12-ATG16L1.…”