Nucleotide-binding flexibility in ultrahigh-resolution structures of the SRP GTPase Ffh

Crystal structures of the Ffh NG GTPase domain at < 1.24 Å resolution reveal multiple overlapping nucleotide binding modes

Structural Basis for Mobility in the 1.1 A

and present a detailed structural analysis of the packing of the residues, including the critical a4 helix, that comprise the interface. Our data allows us to propose a structural explanation for the functional significance of sequence elements conserved at the N/G interface. q 2002 Elsevier Science Ltd. All rights reserved Keywords: ultrahigh resolution; SRP; Ffh; GTPase; X-ray crystallography *Corresponding author Introduction The signal recognition particle (SRP) is a phylogenetically conserved ribonucleoprotein that mediates co-translational targeting of nascent proteins to the membrane. SRP-mediated targeting is dependent on GTP binding and hydrolysis by two components of the pathway, SRP54 and SR, its membrane receptor. Homologs of these two proteins are found throughout evolution, and in prokaryotes they are termed Ffh and FtsY, respectively. Remarkably, the two proteins, the SRP GTPase and its receptor, each contains a structurally homologous two-domain module, the NG dom