Newly synthesized proteins destined either for secretion or incorporation into membranes are targeted to the membrane translocation machinery by a ubiquitous system consisting of a signal-recognition particle (SRP) and its receptor. Both the SRP receptor and the protein within the SRP that binds the signal sequence contain GTPases. These two proteins, together with the RNA component of the SRP, form a complex and thereby regulate each other's GTPase activity. Here we report the structure of the GTPase-containing portion of FtsY, the functional homologue of the SRP receptor of Escherichia coli, at 2.2 A 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, such as a separate amino-terminal domain, an insertion within the p21ras (Ras) effector domain, and a wide-open GTP-binding region. The structure explains the low affinity of FtsY for GTP, and suggests rearrangements that may occur on nucleotide binding. It also identifies regions potentially involved in the transmission of signals between domains and in interactions with regulatory proteins.