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

Area of Science:

• Mitochondrial biology
• Protein quality control
• Cellular trafficking

Background:

• Mitochondria require precise protein composition for optimal function.
• Mitochondrial outer membrane (OM) proteins, particularly tail-anchored (TA) proteins, must be correctly localized.
• Efficient removal of mistargeted proteins is crucial for cellular health.

Purpose of the Study:

• To elucidate the mechanism by which Msp1, an AAA-ATPase, facilitates the degradation of mistargeted TA proteins.
• To identify the cooperating factors involved in the Msp1-mediated clearance pathway.
• To understand the sequential steps from Msp1 recognition to final proteasomal degradation.

Main Methods:

• Investigated the interaction of Msp1 with substrate TA proteins.
• Utilized biochemical assays to track protein transfer between organelles.
• Employed ubiquitination assays with specific E2 enzymes (Ubc6, Ubc7).
• Studied the role of AAA-ATPases (Cdc48) and associated factors (Ufd1, Npl4) in substrate extraction.

Main Results:

• Msp1 recognizes and facilitates the transfer of mistargeted TA proteins from the mitochondrial OM to the endoplasmic reticulum (ER).
• The ER-resident E3 ligase Doa10 ubiquitinates these transferred proteins using Ubc6 and Ubc7.
• Cytosolic AAA-ATPase Cdc48, along with Ufd1 and Npl4, extracts ubiquitinated proteins from the ER membrane.
• Extracted proteins are subsequently degraded by the proteasome in the cytosol.

Conclusions:

• Msp1 acts as an extractase, initiating the clearance of mistargeted TA proteins.
• The study delineates a novel pathway involving Msp1, ER-associated degradation (ERAD) machinery, and Cdc48 for mitochondrial protein quality control.
• This mechanism ensures the removal of aberrant proteins, maintaining mitochondrial homeostasis.