Canonical and alternate mechanisms that regulate ubiquitylation by the E3 ligase parkin

Parkin, a Ring-InBetweenRING-Rcat E3 ubiquitin ligase, plays a vital role in the clearance of damaged mitochondria (mitophagy) by ubiquitylating a broad spectrum of mitochondrial proteins. Mutations in the PRKN gene alter parkin ubiquitylation activity and are a leading cause of early-onset Parkinsonism, underlining its critical function in maintaining mitochondrial homeostasis. The structures, substrates, and ubiquitylation mechanisms used by parkin in mitophagy are well established. Yet, early studies as well as more recent proteomics studies identify alternative substrates that reside in the cytosol or other cellular compartments, suggesting potential roles for parkin beyond mitophagy. In addition to its well-documented activation via S65 phosphorylation, numerous other post-translational modifications (PTMs) have been identified in parkin. Some of these modifications have the potential to serve key regulatory mechanisms, perhaps fine-tuning parkin activity or potentially signaling the involvement in alternative cellular pathways beyond mitochondrial quality control. This review examines the canonical mechanism of parkin-mediated ubiquitylation while also exploring alternative regulatory influences that may modulate its enzyme activity. By analyzing emerging evidence on PTMs including phosphorylation, acetylation, ubiquitylation, oxidation, and interaction with alternative activating molecules, we highlight the broader functional landscape of parkin and its implications for cellular stress response.