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By Christina Towers, PhD.
Misfolded and damaged proteins are degraded by two canonical mechanisms in the cell including the ubiquitin-mediated proteasome system (UPS) and autophagy. Proteins can be targeted for degradation by their N-terminal amino acid which can be modified to become an N-degron. N-degrons allow for selective degradation first through the UPS, however, if the proteasome is not highly functional, these proteins may be degraded via autophagy, a lysosomal mediated form of proteolysis 1. While many studies have shown that these two pathways interact and can compensate for each other, a longstanding question in the field has been how? What are the shared components of these two pathways? How do they signal between each other under stress?
A recent publication by Kim et al entitled “p62/SQSTM1/Sequestosome-1 is an N-recognin of the N-end rule pathway which modulates autophagosome biogenesis” published in 2017 in the journal Nature Communications is the first to shed light on some of these questions2. The authors use chromatographic separation followed by mass spectrometry to identify N-recognins, defined as proteins that bind to N-terminal residues of 11-mer X-peptides (X = Val, Phe, or Arg). Followed by extensive X-protein pull down assays, they identified the autophagy cargo protein, p62, as an N-recognin that can bind Nt-Arg and other degrons via its ZZ domain. Subsequent to ligand binding, p62 forms self-aggregates via disulfide bonds, and interacts with LC3 on autophagosome membranes delivering its cargo for degradation, where p62 itself is also turned over in the process. These findings confirm the existence of an autophagic N-end rule pathway, and identify p62 as the first N-recognin important in this signaling.
Via comprehensive analysis of LC3 puncta and protein expression, the authors also show that ligand binding by p62 induces autophagosomal formation, suggesting that cells can synchronize the process of identifying cargo and autophagosomal biogenesis to temporally coordinate these events. An important final assay also shows that p62 is essential to activate autophagy in the context of proteosomal inhibition, solidifying p62 as the critical link between the two pathways.
Neurodegenerative diseases affect neuronal function in various brain regions. Alzheimer’s affects several brain regions, but the hippocampus is the first region impacted, resulting in short-term memory loss. In Huntington’s disease, the basal ganglia, involved in organizing motor movements, is the brain region first affected. Lastly, Parkinson’s disease affects primarily dopaminergic neurons in the substantia nigra leading to loss of controlled-body movements.
These findings hold clinical relevance, as many neurological disorders including Huntington’s and Alzheimer’s disease are defined by an accumulation of mutant protein aggregates that can evade proteosomal mediated degradation3. Indeed, the authors show that ligand binding to p62 stimulates degradation of mutant Huntington’s protein (mHTT) containing glutamine repeats, or a PolyQ tract, in an autophagy dependent manner. This suggests that stimulation of autophagy with synthetic ligands that bind to the p62 ZZ domain and induce autophagogenesis could be potential therapeutics in neurodegenerative diseases.
Together these studies illuminate one of the age-old questions in the field: how do the two most prominent degradation pathways, ubiquitin-mediated proteosomal degradation and autophagy cross-talk? While previous studies have proposed mechanisms of communication, there has yet to be a consensus in the field, and this publication adds an eloquent buy in for p62 as a key mediator.
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Christina Towers, PhD
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