News - 09/03/2015 - 10:49am

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Previous work has identified roles for the protein kinase PINK1 and the ubiquitin ligase PARKIN in the synthesis of ubiquitin chains on the surface of mitochondria in response to mitochondrial damage. But how these ubiquitin chains target damaged mitochondria to autophagosomes for “mitophagy” and the regulation of this process remained unclear. New work from the Harper Lab, published in Molecular Cell, defines a mechanism in which ubiquitin chains on mitochondria recruit 4 autophagy adaptors (OPTN, NDP52, p62, and TAX1BP1) through their ubiquitin binding domains to specific foci on damaged mitochondria. This recruitment leads to activation of the TBK1 kinase, which associates with OPTN, NDP52, and p62. In a concerted fashion, TBK1 phosphorylates OPTN, NDP52, and p62. Biochemical studies demonstrated that phosphorylation of OPTN by TBK1 further stimulates its intrinsic ubiquitin binding activity. Thus, this mechanism provides a self-reinforcing system that links these ubiquitin binding autophagy receptors with damaged mitochondria. Importantly, recent work has demonstrated that both TBK1 and OPTN are mutated in ALS.  Thus, this work demonstrates that genes mutated in Parkinson’s Disease and ALS function in a common signal transduction pathway to help rid the cell of damaged mitochondria, and suggests that mechanisms for disposal of damaged mitochondria and possibly other toxic cellular components may be commonly targeted in such neurodegenerative diseases.