Structures of the Ribosome in Intermediate States of Ratcheting

Wen Zhang and Jack Dunkle in the Cate lab have a nice report out in the August 21, 2009 issue of Science describing their latest crystal structures of the E. coli ribosome with and without tRNA mimcs. These new structure shed light on the rachet-like action of the intact ribosome as it interacts with tRNA in the A, P, and E sites.

Protein biosynthesis on the ribosome requires repeated cycles of ratcheting, which couples rotation of the two ribosomal subunits with respect to each other, and swiveling of the head domain of the small subunit. However, the molecular basis for how the two ribosomal subunits rearrange contacts with each other during ratcheting while remaining stably associated is not known. Here, we describe x-ray crystal structures of the intact Escherichia coli ribosome, either in the apo-form (3.5 angstrom resolution) or with one (4.0 angstrom resolution) or two (4.0 angstrom resolution) anticodon stem-loop tRNA mimics bound, that reveal intermediate states of intersubunit rotation. In the structures, the interface between the small and large ribosomal subunits rearranges in discrete steps along the ratcheting pathway. Positioning of the head domain of the small subunit is controlled by interactions with the large subunit and with the tRNA bound in the peptidyl-tRNA site. The intermediates observed here provide insight into how tRNAs move into the hybrid state of binding that precedes the final steps of mRNA and tRNA translocation.

Zhang W, Dunkle JA, Cate JHD. "Structures of the Ribosome in Intermediate States of Ratcheting" Science 21 August 2009: Vol. 325. no. 5943, pp. 1014 - 1017.
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