The yeast non-Mendelian factor [psi+] has been suggested to be a self-modified protein analogous to mammalian prions. Here it is reported that an intermediate amount of the chaperone protein Hsp104 was required for the propagation of the [psi+] factor. Over-production or inactivation of Hsp104 caused the loss of [psi+]. These results suggest that chaperone proteins play a role in prion-like phenomena, and that a certain level of chaperone expression can cure cells of prions without affecting viability. This may lead to antiprion treatments that involve the alteration of chaperone amounts or activity.

Termination of translation in higher organisms is a GTP‐dependent process. However, in the structure of the single polypeptide chain release factor known so far (eRF1) there are no GTP binding motifs. Moreover, in prokaryotes, a GTP binding protein, RF3, stimulates translation termination. From these observations we proposed that a second eRF should exist, conferring GTP dependence for translation termination. Here, we have shown that the newly sequenced GTP binding Sup35‐like protein from Xenopus laevis, termed eRF3, exhibits in vitro three important functional properties: (i) although being inactive as an eRF on its own, it greatly stimulates eRF1 activity in the presence of GTP and low concentrations of stop codons, resembling the properties of prokaryotic RF3; (ii) it binds and probably hydrolyses GTP; and (iii) it binds to eRF1. The structure of the C‐domain of the X.laevis eRF3 protein is highly conserved with other Sup35‐like proteins, as was also shown earlier for the eRF1 protein family. From these and our previous data, we propose that yeast Sup45 and Sup35 proteins belonging to eRF1 and eRF3 protein families respectively are also yeast termination factors. The absence of structural resemblance of eRF1 and eRF3 to prokaryotic RF1/2 and RF3 respectively, may point to the different evolutionary origin of the translation termination machinery in eukaryotes and prokaryotes. It is proposed that a quaternary complex composed of eRF1, eRF3, GTP and a stop codon of the mRNA is involved in termination of polypeptide synthesis in ribosomes.

SUP35 is an omnipotent suppressor gene of Saccharomyces cerevisiae coding for a protein consisting of a C-terminal part similar to the elongation factor EF-1 alpha and a unique N-terminal sequence of 253 amino acids. Twelve truncated versions of the SUP35 gene were generated by the deletion of fragments internal to the coding sequence. Functional studies of these deletion mutants showed that: (i) only the EF-1 alpha-like C-terminal part of the Sup35 protein is essential for the cell viability; (ii) overexpression of either the N-terminal part of the Sup35 protein or the full-length Sup35 protein decreases translational fidelity, resulting in omnipotent suppression and reduced growth of [psi+] strains; (iii) expression of the C-terminal part of the Sup35 protein generates an antisuppressor phenotype; and (iv) both the N- or C-terminal segments of the Sup35 protein can bind to 80S ribosomes. Thus, the data obtained define two domains within the Sup35 protein which are responsible for different functions.

Previously, we have shown that plasmid-mediated multiplication of Saccharomyces cerevisiae wild-type SUP35 gene leads to omnipotent suppression and is incompatible with psi-factor, which is an endogenous extrachromosomal suppressor. Here, we describe a frequent de-novo appearance of psi-like factors in mitotic progeny of yeast transformants containing multicopy SUP35 gene.

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Recent studies of translational control suggest that translation termination may not be simply the end of synthesizing a protein but rather be involved in modulating both the translation efficiency and stability of a given transcript. Using recombinant eukaryotic release factor 3 (eRF3) and cellular extracts, we have shown for Saccharomyces cerevisiae that yeast eRF3 and Pab1p can interact. This interaction, mediated by the N؉M domain of eRF3 and amino acids 473 to 577 of Pab1p, was demonstrated to be direct by the two-hybrid approach. We confirmed that a genetic interaction exists between eRF3 and Pab1p and showed that Pab1p overexpression enhances the efficiency of termination in SUP35 ( In general, termination of protein synthesis occurs when the ribosome elongation machinery encounters an in-frame termination codon on the mRNA. In eukaryotes two release factors have been identified, eukaryotic release factor 1 (eRF1), which recognizes all three stop codons, and eRF3, a GTPase that binds to eRF1 and stimulates its release activity in vitro (35,109). The eRF1 protein has a structure mimicking that of a tRNA molecule. It recognizes the stop codon in the A site of the ribosome and catalyzes the hydrolysis of the peptidyltRNA bond (88).In Saccharomyces cerevisiae eRF1 and eRF3 termination factors are encoded by the essential genes SUP45 and SUP35, respectively (10,43,53,60,105). Mutations in either of these genes give the same pleiotropic phenotypes which were selected as omnipotent nonsense suppressors (for a review see reference 49). Moreover, overexpression of both eRF1 and eRF3 is required to enhance the efficiency of termination in yeast (90). In higher eukaryotes, overproduction of eRF1 alone is sufficient to compete with a suppressor tRNA (62). Either Xenopus laevis or human eRF1 alone was also shown previously to have an antisuppressor effect against a suppressor tRNA in the reticulocyte lysate translation system (28). In vitro the eRF1 of higher eukaryotes has a release activity and does not need any other factor (28, 35), and eRF3 by itself binds GTP, but GTPase activity requires the presence of both eRF1 and ribosomes (36). It has been shown previously that eRF1 and eRF3 interact, suggesting that they form a functional complex (70,90,99,109 . Yeast eRF3 consists of an N-terminal prionforming domain (PrD), a charged M (middle) domain of unknown function, and a C-terminal domain that provides the essential translation termination activity (96,97,105). Recently, the minimum length of PrD was defined as amino acids (aa) 1 to 97 (76). It was shown previously that Hsp104 protein is required for formation and maintenance of [PSI ϩ ] aggregates of eRF3: its overproduction or inactivation cures cells of [PSI ϩ ] (14). The eRF3 family includes proteins from yeasts, humans, X. laevis, and other species that are strongly conserved in the C-terminal region, which has a significant homology with the translation elongation factor eEF1A (47,60,109). In contrast to the C-terminal part, the N-terminal region of eRF3 is ...

In the present review, we describe the history of the identification of the eukaryotic translation termination factors eRF1 and eRF3. As in the case of several proteins involved in general and essential processes in all cells (e.g., DNA replication, gene expression regulation...) the strategies and methodologies used to identify these release factors were first established in prokaryotes. The genetic investigations in Saccharomyces cerevisiae have made a major contribution in the field. A large amount of data have been produced, from which it was concluded that the SUP45 and SUP35 genes were controlling translation termination but were also involved in other functions important for the cell organization and the cell cycle accomplishment. This does not seem to be restricted to yeast but is also probably the case in eukaryotes in general. The biochemical studies of the proteins encoded by the higher eukaryote homologs of SUP45 and SUP35 were efficient and permitted the identification of eRF1 as being the key protein in the termination process, eRF3 having a stimulating role. Around 25 years were needed after the identification of sup45 and sup35 mutants for the characterization of their gene products as eRF1 and eRF3, respectively. It also has to be pointed out that if the results came first from bacteria, the identification of RF3 and eRF3 was made practically at the same time. Moreover, eRF1 was the first crystal structure obtained for a class-1 release factor, the bacterial RF2 structure came later. The goal is now to understand at the molecular level the roles of both eRF1 and eRF3 in addition to their translation termination functions.