Distinct mechanisms controlling rough and smooth endoplasmic reticulum-mitochondria contacts

Abstract

Gp78, an ERAD-associated E3 ubiquitin ligase, localizes to mitochondria-associated ER and targets the mitofusin (Mfn1/Mfn2) mitochondrial fusion proteins for degradation. Gp78 is also the cell surface receptor for autocrine motility factor (AMF) that prevents Gp78-dependent mitofusin degradation. Gp78 ubiquitin ligase activity promotes ER-mitochondria association and ER-mitochondria calcium coupling, processes that are reversed by AMF. Electron microscopy of HT-1080 fibrosarcoma cancer cells identified both smooth (∼8 nm) and wider (∼50-60 nm) rough ER-mitochondria contacts. Gp78 shRNA knockdown and AMF treatment selectively reduced the extent of rough ER-mitochondria contacts without impacting smooth ER-mitochondria contacts. Concomitant siRNA knockdown of Mfn1 increased smooth ER-mitochondria contacts in both control and shGp78 cells while knockdown of Mfn2 increased rough ER-mitochondria contacts selectively in shGp78 HT-1080 cells. The mitofusins therefore inhibit ER-mitochondria interaction. Regulation of close ER-mitochondria contacts by Mfn1 and of rough ER-mitochondria contacts by AMF-sensitive Gp78 degradation of Mfn2 define novel mechanisms that regulate ER-mitochondria interactions.