Functional changes in the structure of the SRP GTPase on binding GDP and Mg(2+)GDP

Ffh is a component of a bacterial ribonucleoprotein complex homologous to t he signal recognition particle (SRP) of eukaryotes. It comprises three doma ins that mediate both binding to the hydrophobic signal sequence of the nas cent polypeptide and the GTP-dependent interaction of Ffh with a structural ly homologous GTPase of the SRP receptor. The X-ray structures of the two-d omain 'NG' GTPase of Ffh in complex with Mg(2+)GDP and GDP have been determ ined at 2.0 Angstrom resolution. The structures explain the low nucleotide affinity of Ffh and locate two regions of structural mobility at opposite s ides of the nucleotide-binding site, One of these regions includes highly c onserved sequence motifs that presumably contribute to the structural trigg er signaling the GTP-bound state. The other includes the highly conserved i nterface between the N and G domains, and supports the hypothesis that the N domain regulates or signals the nucleotide occupancy of the G domain.