The Diversity and Complexity of Parkin Function
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Parkin is an E3 ubiquitin ligase that mediates mitochondrial autophagy, or mitophagy, in multiple cell types. Although discovered in the context of Parkinson disease, Parkin is also an important regulator of mitophagy in the heart. In addition, while most current work is focused on the role of Parkin in mitophagy, accumulating evidence suggests that Parkin also impacts cellular physiology and function through additional processes. This Viewpoint discusses current controversies on the functional role of Parkin-mediated mitophagy and emerging evidence that Parkin regulates several additional pathways.
Ubiquitylation is a post-translational modification that involves covalently attaching ubiquitin to target proteins.1 The process of ubquitylation requires a series of events including ubiquitin activation, conjugation, and ligation, catalyzed by E1, E2, and E3 enzymes, respectively. Parkin is one of >500 E3 ubiquitin ligases encoded by the human genome that selectively bestows ubiquitin molecules onto particular proteins. One of Parkin’s most widely appreciated roles is the ubiquitylation of proteins on the membrane of damaged mitochondria to designate them for clearance via mitophagy. Mitophagy is a selective form of macroautophagy that traffics damaged mitochondria to lysosomes for degradation and serves as a critical mechanism in maintaining the health of the mitochondrial collective in a cell.
Current Model of Parkin in Parkinson Disease
Although it is clear that mutations that impair Parkin’s E3 ligase function contribute to the attrition of dopaminergic neurons through cell death in Parkinson disease,2 the underlying mechanisms are still not fully understood and there is no direct evidence that a defect in mitophagy is the underlying cause of Parkinson disease. The assumption that faulty mitophagy is responsible for Parkinson disease stems from observations that defects in mitochondrial morphology and function are key features of Parkinson disease2 and that loss of Parkin in Drosophila leads to mitochondrial dysfunction and muscle degeneration.3 When combined with the seminal work …