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Review
. 2012 Dec;22(6):797-803.
doi: 10.1016/j.sbi.2012.08.003. Epub 2012 Sep 6.

Structural aspects of mitochondrial translational apparatus

Rajendra K Agrawal  1 , Manjuli R Sharma
Affiliations
Review

Structural aspects of mitochondrial translational apparatus

Rajendra K Agrawal et al. Curr Opin Struct Biol. 2012 Dec.

Abstract

During the last decade groundbreaking progress has been made towards the understanding of structure and function of cell's translational machinery. Cryo-electron microscopic (cryo-EM) and X-ray crystallographic structures of cytoplasmic ribosomes from several bacterial and eukaryotic species are now available in various ligand-bound states. Significant advances have also been made in structural studies on ribosomes of the cellular organelles, such as those present in the chloroplasts and mitochondria, using cryo-EM techniques. Here we review the progress made in structure determination of the mitochondrial ribosomes, with an emphasis on the mammalian mitochondrial ribosome and one of its translation initiation factors, and discuss challenges that lie ahead in obtaining their high-resolution structures.

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Figures

Fig. 1
Fig. 1. Comparison between structures of bacterial and mitochondrial ribosomes
RNA-protein segmented structures of ribosomes from (A) bacteria (70S, E. coli) (B) mammalian mitochondria (55S, Bos taurus), and (C) protistan mitochondria (50S, Leishmania tarentolae). In panel A, atomic structure [5] has been low-pass filtered to match the resolution of the cryo-EM map of mammalian mitoribosome shown in panel B, whereas the cryo-EM map of the protistan mitoribosome is shown at ~14 Å [15]. Mito-specific MRPs of SSU and LSU are colored yellow and blue, respectively; conserved MRPs of SSU and LSU are colored green and aquamarine, respectively; and rRNAs of SSU and LSU are colored orange and magenta, respectively. Landmarks of the small subunit: h, head; mgt, mRNA gate; sh, shoulder; sp, spur. Landmarks of the large subunit: CP, central protuberance; Sb, Stalk base or protein L11 region.
Fig. 2
Fig. 2. Structure of the mammalian IF2mt as derived by cryo-EM and location of its insertion domain relative to the binding site of bacterial IF1
(A) Domain alignment of E. coli IF2 and bovine IF2mt. Note that the mature IF2mt (i.e., after deletion of the import sequence) starts from aa residue 78. The 37-aa insertion domain in IF2mt is highlighted in red. (B) Fitting of atomic models of the IF2mt and initiator tRNA (P/I stands for P-site tRNA at initiator position) into the corresponding cryo-EM densities (meshwork) extracted from the map of the 70S•IF2mt•GMPPNP•fMet-tRNA complex [35••]. The color codes used for various domains of IF2mt are the same as in panel A. Asterisk (*) point to the region that would correspond to domain III of IF2mt but was not modeled. (C) Binding positions of the insertion domain (red), and (D) IF1 [37] (green), into a common binding pocket on the SSU of the ribosome. Landmarks of the SSU: h44 and h18, 16S rRNA helices; and S12, r-protein S12 (adopted from ref. [35••]).
Fig. 3
Fig. 3. Hypothetical models of the mammalian mitoribosome interaction with the mtIM
(A) The conventional polypeptide-exit site (PES) of the ribosome interacts with the matrix side of the mtIM, which is depicted as a generic lipid bilayer (semitransparent grey). (B) The lower portion of the mitoribosome is partially embedded into the mtIM, such that the conventional PES is exposed in the inter-membrane space side and the polypeptide accessible site (PAS) of the mitoribosome remains exposed on the matrix side of the mtIM. Other labels: NPC, modeled nascent polypeptide chain (red); 28S, SSU (yellow); and 39S, LSU (blue).
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