Mutation of the PEBP-like domain of the mitoribosomal MrpL35/mL38 protein results in production of nascent chains with impaired capacity to assemble into OXPHOS complexes.

Located in the central protuberance region of the mitoribosome and mitospecific mL38 proteins display homology to PEBP (Phosphatidylethanolamine Binding Protein) proteins, a diverse family of proteins reported to bind anionic substrates/ligands and implicated in cellular signaling and differentiation pathways. In this study, we have performed a mutational analysis of the yeast mitoribosomal protein MrpL35/mL38 and demonstrate that mutation of the PEBP-invariant ligand binding residues Asp(D)232 and Arg(R)288 impacted MrpL35/mL38's ability to support OXPHOS-based growth of the cell. Furthermore, our data indicate these residues exist in a functionally important charged microenvironment, which also includes Asp(D)167 of MrpL35/mL38 and Arg(R)127 of the neighboring Mrp7/bL27m protein. We report that mutation of each of these charged residues resulted in a strong reduction in OXPHOS complex levels that was not attributed to a corresponding inhibition of the mitochondrial translation process. Rather, our findings indicate that a disconnect exists in these mutants between the processes of mitochondrial protein translation and the events required to ensure the competency and/or availability of the newly synthesized proteins to assemble into OXPHOS enzymes. Based on our findings, we postulate that the PEBP-homology domain of MrpL35/mL38, together with its partner Mrp7/bL27m, form a key regulatory region of the mitoribosome.