Functional interplay of p62 and TBK1- implications for autophagy in ALS

Autophagy is a mechanism of intracellular degradation which targets both protein aggregates and organelles for lysosomal based degradation. Although initially thought to be a basal mechanism of bulk catabolism, more recently autophagy has been revealed to be a highly selective process. This selectivity is controlled by autophagy receptors which link specific cargo to the autophagic machinery for their degradation. Dysfunctional autophagy occurs in a number of neurodegenerative diseases including Amytrophic Lateral Sclerosis (ALS). Genetic studies have revealed increased mutations in the genes encoding both the autophagy receptor SQSTM1/p62 and its kinase TBK1 in some patients with ALS.
We have applied a proteomic approach to clarify mechanistically how TBK1 regulates SQSTM1/p62 function. Phosphoproteomic analyses confirmed that TBK1 phosphorylates SQSTM1/p62 within its C-terminal UBA domain, at residue S403. This modification results in increased affinity for its ligand ubiquitin (used as the 'tag' on autophagic cargo). Our analyses also revealed additional hitherto unreported TBK1 phosphorylation sites within critical functional domains of SQSTM1/p62, which could act to further modify its function as an autophagy regulator.
Future work will: (1) use MS-based polyubiquitin linkage analyses to determine the binding selectivity of TBK1-modified SQSTM1/p62; (2) provide molecular details of the TBK1-SQSTM1/p62 protein-protein interaction; and (3) explore the impact of ALS-linked mutations on TBK1-mediated regulation of SQSTM1/p62.