Ubiquitin-mediated regulation of the E3 ligase GP78 by MGRN1 in trans affects mitochondrial homeostasis.

Cellular quality control provides an efficient surveillance system to regulate mitochondrial turnover. This study elucidates a new interaction between the cytosolic E3 ligase mahogunin RING finger 1 (MGRN1) and the endoplasmic reticulum (ER) ubiquitin E3 ligase GP78 (also known as AMFR). Loss of Mgrn1 function has been implicated in late-onset spongiform neurodegeneration and congenital heart defects, among several developmental defects. Here, we show that MGRN1 ubiquitylates GP78 in trans through non-canonical K11 linkages. This helps maintain constitutively low levels of GP78 in healthy cells, in turn downregulating mitophagy. GP78, however, does not regulate MGRN1. When mitochondria are stressed, cytosolic Ca(2+) increases. This leads to a reduced interaction between MGRN1 and GP78 and its compromised ubiquitylation. Chelating Ca(2+) restores association between the two ligases and the in trans ubiquitylation. Catalytic inactivation of MGRN1 results in elevated levels of GP78 and a consequential increase in the initiation of mitophagy. This is important because functional depletion of MGRN1 by the membrane-associated disease-causing prion protein (Ctm)PrP affects polyubiquitylation and degradation of GP78, also leading to an increase in mitophagy events. This suggests that MGRN1 participates in mitochondrial quality control and could contribute to neurodegeneration in a subset of (Ctm)PrP-mediated prion diseases.