Abstract
The crystal structure of the signal sequence binding subunit of the signal recognition particle (SRP) from Thermus aquaticus reveals a deep groove bounded by a flexible loop and lined with side chains of conserved hydrophobic residues. The groove defines a flexible, hydrophobic environment that is likely to contribute to the structural plasticity necessary for SRP to bind signal sequences of different lengths and amino acid sequence. The structure also reveals a helix-turn-helix motif containing an arginine-rich alpha helix that is required for binding to SRP RNA and is implicated in forming the core of an extended RNA binding surface.
Comment
Robert Keenan, then in Bob Stroud's lab at UCSF, determined the crystal structure of the intact SRP GTPase from T. aquaticus. Unfortunately, while he could trace the path of the polypeptide of the three domains N, G & M, he could not establish the linkage between the NG and M domains because it was disordered in the crystal. Because of three-fold symmetric non-crystallographic packing in the crystal, this led to an ambiguity in the arragnement that has bedeviled us to this day.