Eukaryotic translation termination results from the complex functional interplay between two eukaryotic release factors, eRF1 and eRF3, and the ribosome, in which GTP hydrolysis by eRF3 couples codon recognition with peptidyl-tRNA hydrolysis by eRF1. Here, using cryo-electron microscopy (cryo-EM) and flexible fitting, we determined the structure of eRF1-eRF3-guanosine 5′- [β,γ-imido] triphosphate (GMPPNP)-bound ribosomal pretermination complex (pre-TC), which corresponds to the initial, pre-GTP hydrolysis stage of factor attachment. Our results show that eukaryotic translation termination involves a network of interactions between the two release factors and the ribosome. Our structure provides mechanistic insight into the coordination between GTP hydrolysis by eRF3 and subsequent peptide release by eRF1.T ermination of translation occurs when a ribosome reaches the end of the coding region and a stop codon (UAA, UAG, or UGA) enters the aminoacyl tRNA binding site (A site), leaving peptidyl-tRNA in the peptidyl tRNA binding site (P site). It entails stop codon recognition by specialized release factors followed by hydrolysis of peptidyl-tRNA. In eukaryotes, termination is mediated by the concerted action of two directly interacting release factors, eRF1 and eRF3. eRF1 is responsible for stop codon recognition and inducing hydrolysis of peptidyl-tRNA, whereas eRF3, a ribosome-dependent GTPase, strongly stimulates peptide release by eRF1 in a GTP-dependent manner (for review, see ref. 1).eRF1 also participates in ribosome recycling: after peptide release it remains associated with the ribosome and together with ATP-binding cassette sub-family E member 1 (ABCE1) promotes splitting the ribosome into free 60S and tRNA/mRNA-associated 40S subunits (2). eRF1 and eRF3 have paralogs, Dom34 (yeast)/ Pelota (mammals) and Hbs1, respectively, which do not participate in termination but instead play a key role in No-go and nonstop decay surveillance mechanisms (e.g., see refs. 3, 4). Dom34/Pelota and Hbs1 do not induce peptide release in a mechanism similar to eRFs. Instead, they cooperate with ABCE1 to promote dissociation of stalled elongation complexes, which is accompanied by peptidyltRNA drop off (5-7). eRF1 comprises N-terminal (N), middle (M), and C-terminal (C) domains (8). The N domain is responsible for stop codon recognition, which is achieved through a 3D network of conserved residues that include apical TASNIKS (amino acid sequence: threonine-alanine-serine-asparagine-isoleucine-lysine-serine) and YxCxxxF motifs (e.g., refs. 9-12). Domain M contains the universally conserved GGQ motif, which is critical for triggering peptide release: as shown for prokaryotes, its placement into the peptidyl transferase center (PTC) causes rRNA rearrangement, allowing a water molecule to enter (for review, see ref. 13). The rigid core of domain C forms an α-β sandwich (8) that deviates from the standard form by the presence of a small insertion, which forms a minidomain (14). eRF3 consists of an N-terminal sequence (residues 1-...

Plasmodium falciparum, the mosquito-transmitted Apicomplexan parasite, causes the most severe form of human malaria. In the asexual blood-stage, the parasite resides within erythrocytes where it proliferates, multiplies and finally spreads to new erythrocytes. Development of drugs targeting the ribosome, the site of protein synthesis, requires specific knowledge of its structure and work cycle, and, critically, the ways they differ from those in the human host. Here, we present five cryo-electron microscopy (cryo-EM) reconstructions of ribosomes purified from P. falciparum blood-stage schizonts at sub-nanometer resolution. Atomic models were built from these density maps by flexible fitting. Significantly, our study has taken advantage of new capabilities of cryo-EM, in visualizing several structures co-existing in the sample at once, at a resolution sufficient for building atomic models. We have discovered structural and dynamic features that differentiate the ribosomes of P. falciparum from those of mammalian system. Prompted by the absence of RACK1 on the ribosome in our and an earlier study we confirmed that RACK1 does not specifically co-purify with the 80S fraction in schizonts. More extensive studies, using cryo-EM methodology, of translation in the parasite will provide structural knowledge that may lead to development of novel anti-malarials.

The SecA nanomotor promotes protein translocation in eubacteria by binding both protein cargo and the protein-conducting channel and by undergoing ATP-driven conformation cycles that drive this process. There are conflicting reports about whether SecA functions as a monomer or dimer during this dynamic process. Here we reexamined the roles of the amino and carboxyl termini of SecA in promoting its dimerization and functional state by examining three secA mutants and the corresponding proteins: SecA⌬8 lacking residues 2 to 8, SecA⌬11 lacking residues 2 to 11, and SecA⌬11/N95 lacking both residues 2 to 11 and the carboxylterminal 70 residues. We demonstrated that whether SecA⌬11 or SecA⌬11/N95 was functional for promoting cell growth depended solely on the vivo level of the protein, which appeared to govern residual dimerization. All three SecA mutant proteins were defective for promoting cell growth unless they were highly overproduced. Cell fractionation revealed that SecA⌬11 and SecA⌬11/N95 were proficient in membrane association, although the formation of integral membrane SecA was reduced. The presence of a modestly higher level of SecA⌬11/N95 in the membrane and the ability of this protein to form dimers, as detected by chemical cross-linking, were consistent with the higher level of secA expression and better growth of the SecA⌬11/N95 mutant than of the SecA⌬11 mutant. Biochemical studies showed that SecA⌬11 and SecA⌬11/N95 had identical dimerization defects, while SecA⌬8 was intermediate between these proteins and wild-type SecA in terms of dimer formation. Furthermore, both SecA⌬11 and SecA⌬11/N95 were equally defective in translocation ATPase specific activity. Our studies showed that the nonessential carboxyl-terminal 70 residues of SecA play no role in its dimerization, while increasing the truncation of the amino-terminal region of SecA from 8 to 11 residues results in increased defects in SecA dimerization and poor in vivo function unless the protein is highly overexpressed.

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Highlights d Structure-function insights into actin as revealed by cryo-EM and light scattering d D-loop dynamics and its effect on filament assembly and stability d Phalloidin rescues the assembly of defective crosslinked Gactin D-loop mutant d Phalloidin stabilize the actin assembly by allowing interprotomer D-loop