Ribosome recycling factor (RRF)
Thermotoga maritima (thermophile, bacteria)
Protein synthesis, i.e. translation of mRNA, is a fundamental process in life and involves three steps; initiation, elongation and termination. There is moreover a fourth step, ribosome recycling or disassembly of the posttermination complex. This stop is required for ribosome to be used for another cycle of protein synthesis. In bacteria, it is catalyzed by ribosome recycling factor (RRF). In the termination step, when the release factors such as RF1 or RF2 recognize the stop codons of mRNA, peptidyl-tRNA is hydrolyzed and the peptide is released from ribosomes. RF1 and RF2 are then released from ribosomes by RF3. As the result, the posttermination complex constructed of a 70S ribosome with mRNA, an empty A-site to which amino-acyl tRNA binds, and a deacylated tRNA bound to the P-site to which peptidyl-tRNA binds, is formed. RRF and elongation factor G (EF-G) disassemble this complex by using energy of the GTP hydrolysis, leading to the ribosome recycling. In the absent of RRF, because mRNA remains bound to ribosomes, an unexpected translation may initiate without any initiation signals. Thus RRF is an essential protein to prevent this false translation.
The structure of RRF from Thermotoga maritima has been determined. The RRF consists of two domains. Domain I is a three-helix bundle which is tightly packed and slightly right-handed twisted, and is different from three-helix bundles observed in other proteins. Domain II is a three-layer beta/alpha/beta sandwich inserted into domain I. Surprisingly, the shape of RRF is very similar to that of tRNA-Phe from yeast, so the following mechanism of the ribosome recycling is proposed. RRF may bind to the empty A-site of posttermination complex, due to the structural similarity between RRF and tRNA. It is then transferred to the P-site by EF-G, forcing the tRNA in the P-site to be released from the ribosome. Because there is then no tRNA left in the complex which stabilizes the biding of the mRNA to the ribosome, it results in the release of ribosomes from mRNA. Eventually the EF-G and RRF dissociate from the ribosome so that the posttermination complex is disassembled and the ribosome is recycled.
Protein Data Bank (PDB)
author: Yuko Tsuchiya