Mitoribosomes: Mitochondria—found in all eukaryotic cells—have transferred most of their genes to the nuclear genome. The nuclear-localized mitochondrial genes are expressed and translated in the cytoplasm and the resulting mitochondrial proteins are imported into the mitochondria. Nevertheless, a few genes remain within mitochondria in the mitochondrial genome, and these genes are translated by mitochondrial ribosomes (mitoribosomes). Amunts et al. (p. 1485 ; see the Perspective by Kühlbrandt ) determined the structure of mitoribosomes from yeast using single-particle cryo–electron microscopy. The mitoribosome is highly diverged from the bacterial and eukaryotic ribosomes with, for example, a distinctive exit tunnel for the newly synthesized peptide, and a membrane facing protuberance that might help to anchor the mitoribosome to the mitochondrial membrane.

The whole mitoribosome at high resolution: Mitochondria are thought to be the descendents of a prokaryotic cell that took up residence in a protoeukaryotic cell. Mitochondria retain a few genes involved in oxidative phosphorylation. To translate these genes, mitochondria contain highly divergent mitochondrial ribosomes, or mitoribosomes. Amunts et al. determined the high-resolution structures of complete mammalian mitoribosomes using cryoelectron microscopy. Mitoribosomes include an unusual mRNA binding channel. The findings elucidate how aminoglycoside antibiotics can inadvertently inhibit mitoribosomes and how mutations in mitoribosomes can lead to disease. Science , this issue p. 95

All higher organisms on Earth receive energy directly or indirectly from oxygenic photosynthesis performed by plants, green algae and cyanobacteria. Photosystem I (PSI) is a supercomplex of a reaction centre and light-harvesting complexes. It generates the most negative redox potential in nature, and thus largely determines the global amount of enthalpy in living systems. We report the structure of plant PSI at 3.4 Å resolution, revealing 17 protein subunits. PsaN was identified in the luminal side of the supercomplex, and most of the amino acids in the reaction centre were traced. The crystal structure of PSI provides a picture at near atomic detail of 11 out of 12 protein subunits of the reaction centre. At this level, 168 chlorophylls (65 assigned with orientations for Q_x and Q_y transition dipole moments), 2 phylloquinones, 3 Fe₄S₄ clusters and 5 carotenoids are described. This structural information extends the understanding of the most efficient nano-photochemical machine in nature.

Making mitochondrial hydrophobic proteins: Mitochondria produce chemical energy for the cell. Human mitochondria have their own specific ribosomes—mitoribosomes, which are distinct from cytoplasmic ribosomes. Mitoribosomes synthesize the mitochondrial membrane proteins that generate the chemical energy. Brown et al. used cryo–electron microscopy to determine the high-resolution structure of the large subunit of the human mitoribosome. The mitoribosome has a number of unique features, including an exit tunnel lined with hydrophobic amino acid residues. Science , this issue p. 718

Photosystem I functions as a sunlight energy converter, catalyzing one of the initial steps in driving oxygenic photosynthesis in cyanobacteria, algae, and higher plants. Functionally, Photosystem I captures sunlight and transfers the excitation energy through an intricate and precisely organized antenna system, consisting of a pigment network, to the center of the molecule, where it is used in the transmembrane electron transfer reaction. Our current understanding of the sophisticated mechanisms underlying these processes has profited greatly from elucidation of the crystal structures of the Photosystem I complex. In this report, we describe the developments that ultimately led to enhanced structural information of plant Photosystem I. In addition, we report an improved crystallographic model at 3.3-A resolution, which allows analysis of the structure in more detail. An improved electron density map yielded identification and tracing of subunit PsaK. The location of an additional ten beta-carotenes as well as five chlorophylls and several loop regions, which were previously uninterpretable, are now modeled. This represents the most complete plant Photosystem I structure obtained thus far, revealing the locations of and interactions among 17 protein subunits and 193 non-covalently bound photochemical cofactors. Using the new crystal structure, we examine the network of contacts among the protein subunits from the structural perspective, which provide the basis for elucidating the functional organization of the complex.

Mitochondria are essential organelles of endosymbiotic origin that are responsible for oxidative phosphorylation within eukaryotic cells. Independent evolution between species has generated mitochondrial genomes that are extremely diverse, with the composition of the vestigial genome determining their translational requirements. Typically, translation within mitochondria is restricted to a few key subunits of the oxidative phosphorylation complexes that are synthesized by dedicated ribosomes (mitoribosomes). The dramatically rearranged mitochondrial genomes, the limited set of transcripts, and the need for the synthesized proteins to coassemble with nuclear-encoded subunits have had substantial consequences for the translation machinery. Recent high-resolution cryo–electron microscopy has revealed the effect of coevolution on the mitoribosome with the mitochondrial genome. In this review, we place the new structural information in the context of the molecular mechanisms of mitochondrial translation and focus on the novel ways protein synthesis is organized and regulated in mitochondria.