The mitochondrial ribosome is responsible for the biosynthesis of protein components crucial to the generation of ATP in the eukaryotic cell. Because the protein:RNA ratio in the mitochondrial ribosome (approximately 69:approximately 31) is the inverse of that of its prokaryotic counterpart (approximately 33:approximately 67), it was thought that the additional and/or larger proteins of the mitochondrial ribosome must compensate for the shortened rRNAs. Here, we present a three-dimensional cryo-electron microscopic map of the mammalian mitochondrial 55S ribosome carrying a tRNA at its P site, and we find that instead, many of the proteins occupy new positions in the ribosome. Furthermore, unlike cytoplasmic ribosomes, the mitochondrial ribosome possesses intersubunit bridges composed largely of proteins; it has a gatelike structure at its mRNA entrance, perhaps involved in recruiting unique mitochondrial mRNAs; and it has a polypeptide exit tunnel that allows access to the solvent before the exit site, suggesting a unique nascent-polypeptide exit mechanism.
After the termination step of protein synthesis, a deacylated tRNA and mRNA remain associated with the ribosome. The ribosomerecycling factor (RRF), together with elongation factor G (EF-G), disassembles this posttermination complex into mRNA, tRNA, and the ribosome. We have obtained a three-dimensional cryo-electron microscopic map of a complex of the Escherichia coli 70S ribosome and RRF. We find that RRF interacts mainly with the segments of the large ribosomal subunit's (50S) rRNA helices that are involved in the formation of two central intersubunit bridges, B2a and B3. The binding of RRF induces considerable conformational changes in some of the functional domains of the ribosome. As compared to its binding position derived previously by hydroxyl radical probing study, we find that RRF binds further inside the intersubunit space of the ribosome such that the tip of its domain I is shifted (by Ϸ13 Å) toward protein L5 within the central protuberance of the 50S subunit, and domain II is oriented more toward the small ribosomal subunit (30S). Overlapping binding sites of RRF, EF-G, and the P-site tRNA suggest that the binding of EF-G would trigger the removal of deacylated tRNA from the P site by moving RRF toward the ribosomal E site, and subsequent removal of mRNA may be induced by a shift in the position of 16S rRNA helix 44, which harbors part of the mRNA. R ibosomes are responsible for translating genetic information carried by mRNAs into specific sequences of amino acids. Translation on the ribosome comprises of four main steps: (i) initiation, (ii) elongation, (iii) termination, and (iv) recycling. Recent advancements in structural studies of the translational machinery have helped elucidate binding positions and functions of various translational factors involved in various stages of initiation (1, 2), elongation (3, 4), and termination (5-7). The fourth step of translation requires binding of a dedicated protein factor, the ribosome-recycling factor (RRF), which in conjunction with elongation factor G (EF-G) helps removing the mRNA and last deacylated tRNA from the ribosome (see ref. 8).Atomic structures of RRF determined from five different species, including Escherichia coli, show that it is comprised of two structural domains: domain I, consisting of three long ␣-helix bundles, and the smaller domain II, which is an ␣͞ motif (9-13). Different orientations of domain II in these structures have been attributed to interdomain flexibility, which is thought to be necessary for RRF to function on the ribosome (12).The overall match in dimensions between RRF and tRNA (9) prompted the proposal of structural and functional molecular mimicry between the two molecules (9). In a recent study (14) using the hydroxyl radical probing (HRP) method, the orientation of RRF on the ribosome was derived. This study did not support the idea of direct molecular mimicry of tRNA by RRF, because the derived binding position of RRF was quite different from that one would expect based on structural mimicry. The inferred bin...
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