Nonfunctional coq10 mutants maintain the ERMES complex and reveal true phenotypes associated with the loss of the coenzyme Q chaperone protein Coq10
Coenzyme Q (CoQ) is a redox-active lipid molecule that acts as an electron carrier in the mitochondrial electron transport chain. In Saccharomyces cerevisiae, CoQ is synthesized in the mitochondrial matrix by a multi-subunit protein-lipid complex termed the CoQ synthome, the spatial positioning of which is coordinated by the Endoplasmic Reticulum-Mitochondria Encounter Structure (ERMES). The MDM12 gene encoding the cytosolic subunit of ERMES, MDM12, is co-expressed with COQ10, which encodes the putative CoQ chaperone Coq10, via a shared bidirectional promoter. Deletion of COQ10 results in respiratory deficiency, impaired CoQ biosynthesis, and reduced spatial coordination between ERMES and the CoQ Synthome. While Coq10 protein content is maintained upon deletion of MDM12, we show that deletion of COQ10 by replacement with a HIS3 marker results in diminished Mdm12 protein content. Since deletion of individual ERMES subunits prevents ERMES formation, we asked whether some or all of the phenotypes associated with COQ10 deletion result from ERMES dysfunction. To identify the phenotypes resulting solely due to the loss of Coq10, we constructed strains expressing a functionally impaired (coq10-L96S) or truncated (coq10-R147*) Coq10 isoform using CRISPR-Cas9. We show that both coq10 mutants preserve Mdm12 protein content and exhibit impaired respiratory capacity like the coq10? mutant, indicating that Coq10's function is vital for respiration regardless of ERMES integrity. Moreover, the maintenance of CoQ synthome stability and efficient CoQ biosynthesis observed for the coq10-R147* mutant suggests these deleterious phenotypes in the coq10? mutant result from ERMES disruption. Overall, this study clarifies the role of Coq10 in modulating CoQ biosynthesis. Overall design: The coq10 point mutations,  coq10-R147* and coq10-L96S were introduced chromosomally using CRISPR-Cas9. For the Coq10 knockout the COQ10 ORF was replaced with the HIS3 gene. Cells were grown in YPGal in biological triplicates. Differential gene expression analysis was performed on on these strains and their triplicates.