The mitoribosome-associated factor Mrx9 acts as a negative regulator of the prohibitin/m-AAA complex

Abstract

The synthesis of mitochondrial-encoded polypeptides is an essential process, primarily regulated at the posttranscriptional level. In yeast, many regulatory factors have been described as acting in association with the mitoribosome to promote efficient translation; however, the precise mechanisms by which these components function remain largely unknown. Here, we expand on findings concerning a previously studied mitoribosome interactor, Mrx9, which is found in large expressosome-like assemblies of mitoribosome clusters. Mrx9 was initially linked to mitochondrial translation and was suggested to be necessary for splicing of COX1 and COB transcripts. Previous data obtained by others in proximity labelling assays demonstrating that Mrx9 is in close proximity to both the mitoribosome exit tunnel and the prohibitin (PHB) complex. Our current experimental data show that overexpression of Mrx9 impairs the proteolytic functions of Yta10 and Yta12 within the prohibitin complex, leading to splicing defects, accumulation of aberrant polypeptides, and a noticeable impairment in the processing of the essential mitoribosomal protein bL32m. These findings support a regulatory role for Mrx9 in the PHB/m-AAA complex by modulating the activities of both Yta10 and Yta12.