Novel links of ATG4 proteases to cytoskeletal adapters of the obscurin protein family

The obscurin family, containing the giant protein OBSCN (obscurin, cytoskeletal calmodulin and titin-interacting RhoGEF) and its closely related OBSL1 (obscurin like cytoskeletal adaptor 1) as well as SPEG (striated muscle enriched protein kinase) are a group of intracellular proteins that contain serially linked immunoglobulin (Ig) and fibronectin type III (Fn3) domains, along with signaling modules such as protein kinase domains. Hence, obscurins harbor multi-faceted roles for the architecture and organization of cell- and organelle membrane proteins. Besides mediating cellular signaling and promoting protein homeostasis, obscurin proteins are also proposed to act as versatile cytoskeletal linkers. Due to their close homology, many functions for OBSCN are evolutionary conserved in OBSL1 and SPEG. However, their expression patterns differ widely, with OBSL1 being ubiquitously expressed in all cell types, while OBSCN and SPEG are more restricted to cross-striated muscles. Recent evidence indicates that autophagy-linked peptidases of the ATG4 family interact with the cytoskeletal adapter proteins OBSL1 and OBSCN. Peptidases of the ATG4 family process Atg8-family proteins (e.g. LC3) in their immature state (i.e. as pro-peptides like pro-LC3) or their bioactive lipidated state (i.e. LC3-II) and facilitate their conversion to the delipidated state (i.e. LC3-I). Loss of interaction between ATG4 peptidases and obscurin family proteins affects cellular macroautophagy/autophagy and mitophagy, leading in situations of cellular stress to depletion of ATG4 and accumulation of the lipidated state for Atg8-family proteins (e.g. LC3-II).