September 2006 Molecule of the Month by David Goodsell
Keywords: translation elongation factor activity, transfer RNA, peptide elongation factor Tu, peptide elongation factor Ts, peptide elongation factor G
At first glance, we might think that cells are primarily protein synthesis factories. Over half of the molecular machinery in a typical bacterial cell is dedicated to building new proteins. These include the DNA and messenger RNA, which provide the instructions, transfer RNA, which performs the translation of this information, and ribosomes, which do the major construction work. Protein synthesis also requires a flurry of protein factors to orchestrate each step. These include initiation factors that get it all started, release factors that finish each chain, and elongation factors that assist the many steps between the beginning and the end.
Three types of elongation factors are built, in more-or-less similar form, by all living things. These are termed EF-Tu, EF-Ts, and EF-G. As with much of molecular biology, however, there are also other names for each of these, coined over the many years that researchers have been studying these molecules. So, in eukaryotes, you will see these three factors called EF-1A, EF-1B, and EF-2, often with an extra "e" in front, like eEF-1A, and sometimes with Greek letters. So, as you explore these molecules yourself, don't be surprised if the names change.
EF-Tu, shown here from PDB entry 1ttt, performs the important job of shepherding each transfer RNA to the ribosome, powered by a molecule of GTP. EF-Tu is the most plentiful protein in bacterial cells?typically there will be enough that every tRNA may be matched with one. It binds to a tRNA after the proper amino acid has been attached to it. Then, the complex docks into the active site of the ribosome. When the tRNA anticodon matches up correctly with the mRNA codon, a signal from the ribosome causes EF-Tu to change shape and the molecule of GTP is cleaved. This causes EF-Tu to let go of the tRNA and leave, allowing the tRNA to enter into the reaction.