A Fitting Accommodation

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Science  19 Jul 2002:
Vol. 297, Issue 5580, pp. 303
DOI: 10.1126/science.297.5580.303a

The elongation factor Tu (EF-Tu) delivers amino acids in the form of aminoacyl transfer RNAs (aa-tRNAs) to the ribosome, which couples the amino acids together to form proteins. The choice of aa-tRNA (and thus the choice of amino acid) is dictated by the matching of the codon (on the messenger RNA) and the anticodon (at one end of the tRNA). Proper matching triggers a movement that results in hydrolysis of the EF-Tu-bound guanosine triphosphatase (GTP), releasing the aa-tRNA, which is bound less tightly by the GDP form of EF-Tu.

Valle et al. present a detailed analysis of the conformational changes that occur upon aa-tRNA delivery by fitting the crystal structures of the isolated components into a cryoelectron microscopic map of the entire ribosome captured in a state with the aa-tRNA-EF-Tu complex in the act of delivery. The remarkable result is that the tRNA inserts first the anticodon end into the decoding center on the small subunit of the ribosome and then the amino acid-carrying end into the peptidyl transferase center on the large subunit by swiveling one end and then the other into place. A twist in the middle or elbow region of the L-shaped tRNA apparently serves as the sign that the codon-anticodon match is perfect, telling EF-Tu to hydrolyze GTP and to let go of the other end of the tRNA, which then can slide into position. The proposed flexibility brings to mind previous studies and raises the possibility that the fidelity of interactions at the decoding center is signaled generally by tRNA arm movements. — GJC

EMBO J.21, 3557 (2002).

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