Cooperation of mitochondrial and ER factors in quality control of tail-anchored proteins

Abstract

Tail-anchored (TA) proteins insert post-translationally into the endoplasmic reticulum (ER), the outer mitochondrial membrane (OMM) and peroxisomes. Whereas the GET pathway controls ER-targeting, no dedicated factors are known for OMM insertion, posing the question of how accuracy is achieved. The mitochondrial AAA-ATPase Msp1 removes mislocalized TA proteins from the OMM, but it is unclear, how Msp1 clients are targeted for degradation. Here we screened for factors involved in degradation of TA proteins mislocalized to mitochondria. We show that the ER-associated degradation (ERAD) E3 ubiquitin ligase Doa10 controls cytoplasmic level of Msp1 clients. Furthermore, we identified the uncharacterized OMM protein Fmp32 and the ectopically expressed subunit of the ER-mitochondria encounter structure (ERMES) complex Gem1 as native clients for Msp1 and Doa10. We propose that productive localization of TA proteins to the OMM is ensured by complex assembly, while orphan subunits are extracted by Msp1 and eventually degraded by Doa10.

Mitochondrial Quality Control Governed by Ubiquitin

“…(Figure 1 and Table 1). This initial observation suggests a role of ERAD in the turnover of OMM proteins (Dederer et al, 2019;Matsumoto et al, 2019). Indeed, Msp1 co-localizes with its substrates at ER-mitochondria contact sites, which however, seem to be dispensable for Doa10/Cdc48 dependent turnover of Msp1 substrates (Matsumoto et al, 2019).…”

“…Subsequently, Cdc48 is recruited together with the cofactors Ufd1 and Npl4 to translocate mistargeted TA-proteins to the 26S proteasome (Matsumoto et al, 2019). Intriguingly, Msp1 appears to extract only monomeric proteins and not multi-complexes, suggesting that the recognition of mistargeted TA-protein is based on the weak interaction with the membrane of a single transmembrane domain (Dederer et al, 2019). Interestingly, Msp1 emerged as an MAD substrate itself, whose degradation depends on the Doa1-Cdc48 −Ufd1−Npl4 complex (Wu et al, 2016).…”

“…Besides the high degree of mechanistic similarities between the MAD and ERAD pathways, mitochondrial quality control is further defined by functional mitochondria-ER interactions. For example, substrates extracted from the OMM by Msp1 are targeted to the ER for subsequent proteasomal degradation (Dederer et al, 2019;Matsumoto et al, 2019). Moreover, mitochondrial precursor proteins have been identified to associate with the ER membrane before being imported.…”

“…In fact, the physical interaction between these tubular organelles supports the transfer of lipids, calcium ions and other metabolites, localizes to DNA nucleoids and regulates mitochondrial dynamics (Helle et al, 2013;Raturi and Simmen, 2013;van Vliet et al, 2014;Phillips and Voeltz, 2016;Moltedo et al, 2019). Recent studies suggest an intricate cooperation between ER and mitochondria in proteostasis (Dederer et al, 2019;Matsumoto et al, 2019), which constitute a newly developing research field promising for the development of therapeutic interventions as proposed for cardiac pathologies (Arrieta et al, 2020).…”

Protein quality control in the endoplasmic reticulum

“…On the other hand, when ER membrane proteins are mislocalised, either to the mitochondria ( Figure 2 ) or the peroxisome, they are extracted and targeted for degradation by the hexametric ATPase Msp1 [ 15 , 16 ]. Recognition of such mislocalised substrates probably involves assessment of oligomeric state [ 17 , 18 ] and detection of characteristic hydrophobic and charged motifs adjacent to the transmembrane domain [ 19 ]. Substrates mistargeted to mitochondria are extracted by Msp1 near ER contact sites before ubiquitination and degradation at the ER membrane in a process involving the traditional ERAD components Doa10 and Cdc48 [ 17 , 20 , 21 ].…”