Abstract
The processes of association and dissociation of ribosomal subunits are of great importance for the protein biosynthesis. The mechanistic details of these processes, however, are not well known. In bacteria, upon translation termination, the ribosome dissociates into subunits which is necessary for its further involvement into new initiation step. The dissociated state of the ribosome is maintained by initiation factor 3 (IF3) which binds to free small subunits and prevents their premature association with large subunits. In this work, we have exchanged IF3 in Escherichia coli cells by its ortholog from Saccharomyces cerevisiae mitochondria (Aim23p) and showed that yeast protein cannot functionally substitute the bacterial one and is even slightly toxic for bacterial cells. Our in vitro experiments have demonstrated that Aim23p does not split E. coli ribosomes into subunits. Instead, it fixes a state of ribosomes characterized by sedimentation coefficient about 60S which is not a stable structure but rather reflects a shift of dynamic equilibrium between associated and dissociated states of the ribosome. Mitochondria-specific terminal extensions of Aim23p are necessary for “60S state” formation, and molecular modeling results point out that these extensions might stabilize the position of the protein on the bacterial ribosome.
Highlights
Upon termination of protein biosynthesis in bacteria, 70S ribosome dissociates into small (30S) and large (50S) subunits
Full-length Aim23p is undesirable for E. coli cells due to its terminal extensions As it has been already mentioned in “Introduction,” mammalian mitochondrial IF3 (mtIF3) possesses some functional activity in E. coli cells (Ayyub et al, 2018)
The main result of this work is the detection of a state of E. coli ribosomes (“60S state”) which is formed as a result of interaction with S. cerevisiae mitochondrial translation initiation factor 3 (IF3), Aim23p
Summary
Upon termination of protein biosynthesis in bacteria, 70S ribosome dissociates into small (30S) and large (50S) subunits. It is known that two proteins, namely RRF and EF-G, are responsible for bacterial ribosomes dissociation into subunits after termination of protein biosynthesis. Once free 30S and 50S subunits appear, initiation factor 3 (IF3) binds the small subunit in order to keep it apart from the large one (Zavialov, Hauryliuk & Ehrenberg, 2005). This stage is, the very first stage of the translation initiation process; 30SIF3 complex becomes the basis for the full-size initiatory complex formation which includes Shine-Dalgarno sequence of mRNA, initiator tRNA, and initiation factors 1 and 2. It is worth mentioning that anti-association activity of IF3 is definitely of passive mode: it does not promote dissociation of the ribosome into subunits but instead binds to free small subunit and prevents its re-association with the large one (Gualerzi, Risuleo & Pon, 1977; Gottleib, Davis & Thompson, 1975)
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