Phosphoregulated SMCR8-FIP200 interaction connects the ALS/FTD-linked C9orf72 complex to autophagy initiation and mitochondrial quality control

Abstract

Hexanucleotide (GGGGCC) repeat expansions in the non-coding region of C9ORF72 are a major genetic cause of ALS/FTD and reduce C9orf72-SMCR8-WDR41 complex levels, but how this contributes to autophagy-lysosome dysfunction and previously reported mitochondrial quality-control defects in C9ORF72-ALS/FTD remains unclear. Here we identify a direct interaction between SMCR8 and the FIP200 subunit of the ULK1/2 autophagy initiation complex, mediated by two FIP200-interacting region (FIR) motifs in a disordered SMCR8 loop. Phosphorylation of these motifs by ULK1/2 or TBK1 strengthens binding and promotes ULK1/2 complex association in cells. Stabilising the SMCR8-FIP200 interaction suppresses Parkin-dependent mitophagy, whereas both stabilisation and weakening impair deferiprone-induced mitophagy, while leaving bulk autophagy, lysophagy and ivermectin-induced mitophagy largely intact. These findings define a regulated C9orf72-ULK1/2 axis and provide a mechanistic framework by which repeat-expansion-associated reduction in C9orf72 complex abundance may contribute to previously observed mitochondrial quality-control defects in C9ORF72-ALS/FTD.