Bussiere et al. describe how new ribosomes check that they're ready to start synthesizing proteins.
The large and small subunits of eukaryotic ribosomes are preassembled in the nucleus, but, after their export into the cytoplasm, they undergo a series of maturation steps before they start translating mRNAs. In one of the final maturation events, the large subunit sheds an inhibitory protein called Tif6 so that it can associate with the small subunit and initiate protein synthesis. Bussiere et al. found that Tif6 wasn't released from yeast large subunits lacking the ribosomal protein Rpl10.
Rpl10 is located next to the P-site—the catalytic heart of ribosomes where amino acid–carrying tRNAs are coupled to the growing polypeptide chain. Mutating a region of Rpl10 that contacts the P-site tRNA also blocked Tif6 release, thereby inhibiting yeast cell growth. Maturation and growth were restored by mutations in Tif6 that weaken the inhibitory protein's association with the large subunit. Rpl10 mutants were also rescued by mutants of Efl1, a GTPase that promotes Tif6 release. The mutations in Efl1 appear to facilitate a conformational change analogous to changes observed in the translocation factor eEF2 during translation.
The results suggest that Efl1 normally removes Tif6 from large subunits only if the P-site is correctly assembled, ensuring that new ribosomes don't become active if their catalytic sites are nonfunctional. Senior author Arlen Johnson now wants to examine the role of Sdo1, another protein required for Tif6 release that is shaped somewhat like a tRNA and that may therefore help Efl1 to determine whether or not the P-site is able to support protein synthesis.
