Ferbitz et al. resolved a 2.7 Å crystal structure of the E. coli trigger protein and of the NH2-terminal domain of the trigger factor bound to the 50S ribosomal subunit of H. marismortui. The tail domain of the trigger factor binds to the ribosome with moderate affinity. The remainder of the protein—the head, back, and arms—loom over the tunnel where polypeptides exit the ribosome, appearing like a dragon shielding its treasure. “We wouldn't have predicted this structure from the linear sequence,” says Ferbitz, especially since the protein is only 48 kD in size.
The chaperone creates a hydrophobic space within which nascent proteins can fold, protected from potentially damaging agents, such as proteases. The team hypothesizes that once a domain in the new protein folds, it loses affinity for the hydrophobic surface of the chaperone. The weak interaction between the ribosome and the trigger factor then gives way, releasing the intact domain. ▪