Fig. 1: The crystal structures of eRF1 and eRF3.
Reproduced, with permission, from Ref. 1 © 2009 Cold Spring Harbor Laboratory Press
An international team of molecular biologists has unraveled much of the molecular process by which eukaryotic or nucleated cells recognize when to stop construction of protein chains and release them. This process is fundamental to the synthesis of proteins, and hence the operation of all cells.
Proteins chains are generated in small organelles known as ribosomes. Lengths of messenger-RNA, printed from DNA, are fed through the ribosome and used as templates for joining together specific sequences of amino acids to form protein chains. Particular codes in the mRNA sequence signal where the chain should be terminated and released.
Work by other researchers showed that in eukaryotic cells, the task of recognizing these mRNA stop codons and releasing the newly formed protein chain involves two mutually interdependent protein release factors—eRF1 and eRF3. Efficient termination and release of protein chains requires a complex of both release factors, together with the energy transfer molecule guanosine triphosphate (GTP).
The crystal structure of eRF1, determined in earlier work at Oxford University by Haiwei Song of A*STAR’s Institute of Molecular and Cell Biology, Singapore, showed that it had three active regions or domains—N, involved in stop codon recognition; M, required for chain release; and C, involved in binding to eRF3. Now, the international team led by Song has determined the crystal structure of the complex formed between eRF1 and a truncated version of eRF3, and has also analyzed it in solution using small-angle X-ray scattering (Fig. 1).
The researchers found that eRF1 changes shape to resemble a molecule of transfer-RNA when it binds to eRF3. Transfer-RNA (tRNA) molecules recognize mRNA code and carry appropriate amino acids to add to the growing protein chain. The shape change of the eRF1 molecule not only allows its N domain to ‘read’ stop codons, but also brings the M domain into a position where it can interact with an active region of eRF3.
The researchers propose a mechanism whereby the eRF1–eRF3 complex opens the way to bind GTP, which leads eRF1 to change shape to mimic tRNA. This allows eRF1 to link to a ribosome and to recognize stop codons. The act of binding to the ribosome also transfers energy from GTP to drive protein chain release.
“We now want to determine the structure of eRF1 in complex with the whole eRF3 molecule, to understand fully how the two proteins act cooperatively,” Song says.
The A*STAR-affiliated authors in this highlight are from the Institute of Molecular and Cell Biology.
