Msp1 Clears Mistargeted Proteins by Facilitating Their Transfer from Mitochondria to the ER

Highlights

• Msp1 extracts mistargeted tail-anchored (TA) proteins from the mitochondrial membrane

• TA proteins extracted by Msp1 can be reinserted into the ER membrane

• TA proteins moved to the ER can be ubiquitinated by Doa10 and extracted by Cdc48

• TA proteins extracted from the ER membrane can be degraded by proteasome

Summary

Normal mitochondrial functions rely on optimized composition of their resident proteins, and proteins mistargeted to mitochondria need to be efficiently removed. Msp1, an AAA-ATPase in the mitochondrial outer membrane (OM), facilitates degradation of tail-anchored (TA) proteins mistargeted to the OM, yet how Msp1 cooperates with other factors to conduct this process was unclear. Here, we show that Msp1 recognizes substrate TA proteins and facilitates their transfer to the endoplasmic reticulum (ER). Doa10 in the ER membrane then ubiquitinates them with Ubc6 and Ubc7. Ubiquitinated substrates are extracted from the ER membrane by another AAA-ATPase in the cytosol, Cdc48, with Ufd1 and Npl4 for proteasomal degradation in the cytosol. Thus, Msp1 functions as an extractase that mediates clearance of mistargeted TA proteins by facilitating their transfer to the ER for protein quality control.