eRF3 is a GTPase associated with eRF1 in a complex that mediates translation termination in eukaryotes. In mammals, two genes encode two distinct forms of eRF3, eRF3a and eRF3b, which differ in their N-terminal domains. Both bind eRF1 and stimulate its release activity in vitro. However, whether both proteins can function as termination factors in vivo has not been determined. In this study, we used short interfering RNAs to examine the effect of eRF3a and eRF3b depletion on translation termination efficiency in human cells. By measuring the readthrough at a premature nonsense codon in a reporter mRNA, we found that eRF3a silencing induced an important increase in readthrough whereas eRF3b silencing had no significant effect. We also found that eRF3a depletion reduced the intracellular level of eRF1 protein by affecting its stability. In addition, we showed that eRF3b overexpression alleviated the effect of eRF3a silencing on readthrough and on eRF1 cellular levels. These results suggest that eRF3a is the major factor acting in translation termination in mammals and clearly demonstrate that eRF3b can substitute for eRF3a in this function. Finally, our data indicate that the expression level of eRF3a controls the formation of the termination complex by modulating eRF1 protein stability.In eukaryotes, two release factors, eRF1 and eRF3, are required to complete protein synthesis. These translation termination factors associate in a complex which binds to the elongating ribosome when a stop codon enters the A site. eRF1 recognizes all three stop codons by direct interaction at the decoding A site (5, 10) and activates the peptidyltransferase center, which triggers the hydrolysis of the peptidyltRNA, generating a free full-sized polypeptide. eRF3 is a GTPase that stimulates eRF1 activity in a GTP-dependent manner (40). eRF3 alone can bind GTP, but its GTPase activity requires the presence of both eRF1 and ribosomes, which may play the role of a composite GTPase-activating protein (11, 12). The eRF3 GTP-bound form interacts with eRF1 in vitro and in vivo to constitute the active translation termination complex (12,36,40). In the yeast Saccharomyces cerevisiae, eRF1 and eRF3 are encoded by the essential genes SUP45 and SUP35, respectively. Mutations in either of these genes cause an omnipotent suppressor phenotype and the ability to enhance nonsense suppression by a weak suppressor tRNA (reviewed in reference 18). Recently, it has been shown in S. cerevisiae that eRF3 GTPase activity facilitates stop codon decoding by eRF1 (33).The C-terminal regions of eRF3 proteins are highly conserved through evolution and carry the four canonical GTPbinding motifs of the GTPase superfamily (2). This domain is essential for translation termination and interaction with eRF1. The N-terminal region varies in both length and sequence among species. In yeast, it is neither essential for cell viability nor required for termination but is responsible for prion-like [PSI ϩ ] factor formation (29, 31). Otherwise, this domain participates in...

Cell polarity is a fundamental property of cells from unicellular to multicellular organisms. Most of the time, it is essential so that the cells can achieve their function. The fission yeast Schizosaccharomyces pombe is a powerful genetic model organism for studying the molecular mechanisms of the cell polarity process. Indeed, S. pombe cells are rod-shaped and cell growth is restricted at the poles. The accurate localization of the cell growth machinery at the cell cortex, which involves the actin cytoskeleton, depends on cell polarity pathways that are temporally and spatially regulated. The importance of interphase microtubules and cell polarity factors acting at the cortex of cell ends in this process has been shown. Here, we review recent advances in knowledge of molecular pathways leading to the establishment of a cellular axis in fission yeast. We also describe the role of cortical proteins and mitotic cytoskeletal rearrangements that control the symmetry of cell division.