Mdm38 is a 14‐3‐3‐Like Receptor and Associates with the Protein Synthesis Machinery at the Inner Mitochondrial Membrane

Abstract

and Peter Rehling, peter.rehling@medizin.uni-goettingen.de † These authors contributed equally to this work.Mitochondrial ribosomes synthesize core subunits of the inner membrane respiratory chain complexes. In mitochondria, translation is regulated by mRNA-specific activator proteins and occurs on membrane-associated ribosomes. Mdm38/Letm1 is a conserved membrane receptor for mitochondrial ribosomes and specifically involved in respiratory chain biogenesis. In addition, Mdm38 and its higher eukaryotic homolog Letm1, function as K + /H + or Ca 2+ /H + antiporters in the inner membrane. Here, we identify the conserved ribosome-binding domain (RBD) of Mdm38 and determine the crystal structure at 2.1Å resolution. Surprisingly, Mdm38 RBD displays a 14-3-3-like fold despite any similarity to 14-3-3-proteins at the primary sequence level and thus represents the first 14-3-3-like protein in mitochondria. The 14-3-3-like domain is critical for respiratory chain assembly through regulation of Cox1 and Cytb translation. We show that this function can be spatially separated from the ion transport activity of the membrane integrated portion of Mdm38. On the basis of the phenotypes observed for mdm38 as compared to Mdm38 lacking the RBD, we suggest a model that combining ion transport and translational regulation into one molecule allows for direct coupling of ion flux across the inner membrane, and serves as a signal for the translation of mitochondrial membrane proteins via its direct association with the protein synthesis machinery.

Trypanosome Letm1 Protein Is Essential for Mitochondrial Potassium Homeostasis

“…In T. brucei, Letm1 does not appear to play a direct role in mitochondrial translation as has been suggested for the yeast ortholog (4,16,17). Although this process is indeed compromised in the Letm1 knockdowns, it persists when these cells are treated with nigericin.…”

“…Yet another role that has been attributed to Letm1 in S. cerevisiae is the anchoring of mt ribosomes to the inner membrane, into which it facilitates the incorporation of hydrophobic de novo translated subunits of the respiratory chain (4,16,17). This path of inquiry began with an observed reduction of the steady-state levels of a subset of mitochondrially encoded proteins in Letm1 knockouts (4).…”

MPV17L2 is required for ribosome assembly in mitochondria

“…MPV17L2 may be assisted in this by LETM1 (mdm38), with which it shares a number of features. Both are inner membrane proteins that are associated with mitochondrial ribosomes (43,46,51) and the morphological abnormalities observed in MPV17L2 downregulation are similar to LETM1 deficient mitochondria (52,53). These similarities strengthen the view that MPV17L2 plays a role in the interlinked processes of translation and assembly of OXPHOS complexes (40,41).…”

Mitochondrial Protein Synthesis, Import, and Assembly

“…Similarly, Mdm38 promotes export of Cob and Atp6 domains (Frazier et al 2006). Interestingly, Mdm38 is a bifunctional protein that also participates in K + /H + exchange across the inner membrane (Nowikovsky et al 2004;Froschauer et al 2005;Zotova et al 2010;Lupo et al 2011).…”

“…To date, three inner membrane proteins are known to interact with large mitochondrial ribosomal subunits and are thought to tether them to the membrane independently of nascent chains: they are Oxa1 (Jia et al 2003(Jia et al , 2009Szyrach et al 2003), Mba1 (Ott et al 2006;Gruschke et al 2010), and Mdm38 (Frazier et al 2006;Lupo et al 2011). Oxa1 functions as a translocase/ insertase-promoting export of mitochondrially coded hydrophilic domains, particularly those of Cox2, to the intermembrane space and insertion of transmembrane domains (He and Fox 1997;Hell et al 1997Hell et al , 2001).…”