A crevice adjoining the ribosome tunnel: Hints for cotranslational folding.

RNA protection expts. and the crystal structure of a complex of the large ribosomal subunit from the eubacterium Deinococcus radiodurans with rapamycin, a polyketide compd. resembling macrolides and ketolides, showed that rapamycin binds to a crevice located at the boundaries of the nascent protein exit tunnel, near its entrance. At this location rapamycin cannot occlude the ribosome exit tunnel, consistent with its failure to act as a ribosomal antibiotic drug. In accord with recent biochem. data, this crevice may play a role in facilitating local cotranslational folding of nascent chains, in particular for transmembrane proteins.