The majority of ATG8 proteins are expressed with C-terminal amino acids shielding the glycine residue needed for lipidation.

ATG8 lipidation requires the removal of the C-terminal extension by ATG4 proteases, referred to as priming.

The potential role of these C-terminal extensions in macroautophagy and the reason for maintaining these extra amino acids in evolution remain unknown.

In search for an answer, we discuss possible explanations by combining the literature and our own experiments.

ATG8 proteins form a family of small ubiquitin-like modifiers, well-known for their importance in both macroautophagy and autophagy-independent processes. A unique feature of this protein family is their conjugation to membrane lipids through the covalent attachment of a glycine residue at the C-terminus of ATG8 proteins. Notably, most ATG8 proteins are expressed with additional amino acids at their C-terminus, shielding the key glycine residue. Consequently, lipidation requires the activation of the ATG8 precursors through proteolytic cleavage, known as priming. ATG4 proteases catalyze this priming process, and under physiological conditions, unprimed forms of ATG8 are not detected. This raises the question about the purpose of the C-terminal extension of ATG8 proteins. While the roles of lipidated and free, primed ATG8 proteins have been extensively studied, the potential function of their precursor form or the priming process itself remains largely unexplored. Here, we summarize information from existing literature and our own experiments to contribute to the understanding of these neglected amino acids.

© 2024 The Author(s). Published by Elsevier Ltd.