A Role for the Mitochondrial Protein Mrpl44 in Maintaining OXPHOS Capacity.

Janet H C Yeo,Ruth M Kluck,Mark M W Chong,Jian-Guo Zhang,Jarrod P J Skinner,Luke E Formosa,Gabrielle T Belz,Matthew J Bird,Robert Lightowlers

doi:10.1371/journal.pone.0134326

Abstract

We identified Mrpl44 in a search for mammalian proteins that contain RNase III domains. This protein was previously found in association with the mitochondrial ribosome of bovine liver extracts. However, the precise Mrpl44 localization had been unclear. Here, we show by immunofluorescence microscopy and subcellular fractionation that Mrpl44 is localized to the matrix of the mitochondria. We found that it can form multimers, and confirm that it is part of the large subunit of the mitochondrial ribosome. By manipulating its expression, we show that Mrpl44 may be important for regulating the expression of mtDNA-encoded genes. This was at the level of RNA expression and protein translation. This ultimately impacted ATP synthesis capability and respiratory capacity of cells. These findings indicate that Mrpl44 plays an important role in the regulation of the mitochondrial OXPHOS capacity.

Highlights

Almost all eukaryotic cells contain mitochondria, double membrane-enclosed structures that are crucial for the production of energy, in the form of ATP, by oxidative phosphorylation (OXPHOS)
In an attempt to identify additional RNase III enzymes in animals, we performed a search for proteins with sequence homology to the RNase III domain and double-stranded RNA-binding motif of murine Drosha (S1 Fig)
Mrpl44 is a protein that was previously identified as part of the large subunit of the mitoribosome in bovine mitochondrial extracts [8]

Summary

Introduction

Almost all eukaryotic cells contain mitochondria, double membrane-enclosed structures that are crucial for the production of energy, in the form of ATP, by oxidative phosphorylation (OXPHOS). The OXPHOS system, encoded by genes within both the mitochondrial and nuclear genomes (mtDNA and nDNA), is comprised of 88 protein subunits that assemble into five multi-subunit complexes (Complex I—V), to form the electron transport chain at the inner membrane of the mitochondrion [1]. The system functions to link the oxidation of reduced coenzymes (NADH at Complex I, and FADH2 at Complex II), with proton pumping (at Complex I, III and IV) to generate the mitochondrial membrane potential (ΔCm). Complex V utilizes the ΔCm by mechanically coupling the flow of protons through the complex down their electrochemical gradient, driving ATP synthesis from ADP and inorganic phosphate.

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Conclusion