CRYSTAL-STRUCTURE OF THE NG DOMAIN FROM THE SIGNAL-RECOGNITION PARTICLE RECEPTOR FTSY

Newly synthesized proteins destined either for secretion or incorporat ion into membranes are targeted to the membrane translocation machiner y by a ubiquitous system consisting of a signal recognition particle ( SRP) and its receptor(1,2). Both the SRP receptor and the protein with in the SRP that binds the signal sequence contain GTPases(3,4). These two proteins, together with the RNA component of the SRP, form a compl ex(5-7) and thereby regulate each other's GTPase activity(8). Here we report the structure of the GTPase-containing portion of FtsY, the fun ctional homologue of the SRP receptor of Escherichia coli(9), at 2.2 A ngstrom resolution without bound nucleotide. This so-called NG domain displays similarities to the Ras-related GTPases, as well as features unique to the SRP-type GTPases(10), such as a separate aminoterminal d omain, an insertion within the p21(ras) (Ras) effector domain(11), and a wide-open GTP-binding region. The structure explains the low affini ty of FtsY for GTP, and suggests rearrangements that may occur on nucl eotide binding. It also identifies regions potentially involved in the transmission of signals between domains and in interactions with regu latory proteins.