Spatial control of mitochondrial ubiquitination by the AMBRA1-RMC1-HUWE1 axis.

Mitochondrial ubiquitination is a central component of mitochondrial quality control. The PINK1 (PTEN induced kinase 1)-PRKN (parkin RBR E3 ubiquitin protein ligase) pathway established how loss of mitochondrial membrane potential can trigger a phospho-ubiquitin feed-forward cascade on the outer mitochondrial membrane (OMM). It remains less clear how mitochondrial ubiquitination is achieved when PRKN is absent or inactivated. In our recent work, we identify a recruitment platform organized by AMBRA1 (autophagy and beclin 1 regulator 1), in which RMC1 (regulator of MON1-CCZ1) positions HUWE1 (HECT, UBA and WWE domain containing E3 ubiquitin protein ligase 1) at mitochondria. This spatial arrangement promotes HUWE1-dependent ubiquitination and turnover of OMM proteins, including MFN2 (mitofusin 2), VDAC1 (voltage-dependent anion channel 1), and VDAC2 (voltage-dependent anion channel 2). Our findings raise the question of how cells select among distinct mitochondrial ubiquitination pathways and whether these pathways function independently, sequentially, or cooperatively.