A 50S ribosomal subunit assembly protein with GTPase activity, required for 50S subunit assembly at low temperatures, may also play a role in translation. Binds GTP and analogs. Binds the 70S ribosome between the 30S and 50S subunits, in a similar position as ribosome-bound EF-G; it contacts a number of ribosomal proteins, both rRNAs and the A-site tRNA (By similarity). A ribosome-stimulated GTPase, GTPase activity increases 4 fold in the presence of 70S ribosomes. Bind to 70S ribosomes in the presence of GTP or its non-hydrolyzable analog GMPPNP; in the presence of ppGpp or under stress conditions it binds to 30S ribosomal subunits (PubMed:18621905, PubMed:19803466). GTP + H2O = GDP + H(+) + phosphate Ribosome-associated GTPase is not affected by low levels of ppGpp, >40 uM ppGpp and >50 uM GDP inhibit GTPase (PubMed:18621905). The C-terminus (residues 387-607 or 481-607) inhibits GTPase activity, in its absence kcat increases, but GTPase is no longer stimulated by 70S ribosome or 30S or 50S subunits (PubMed:19803466). Monomer. During exponential growth the GTP-bound state associates 1:1 with 70S ribosomes. Under stress conditions (induction of the stringent response, growth at 42 or 16 degrees Celsius) associates with 30S ribosomal subunits only. Induced 7-fold when cells are treated with human bactericidal permeability-increasing protein (BPI, AC P17213) (at protein level). Has 5 domains; domain I (residues 1-202, also called the G domain), II (203-305, also called the beta barrel domain), III (306-386) V (387-479) and the C-terminal domain (CTD 480-607) which is BipA-specific. Domains I-V are homologous to domains in EF-G and LepA; although the domains are similar, their relative arrangement among these proteins is different. Domains I and II are not required for ribosome binding, although in their absence no 30S binding under stress is seen, whereas domains III, V and the CTD are required to bind both 70S and 30S ribosomes. The data suggests interdomain communication modulates GTPase and ribosome binding. Belongs to the TRAFAC class translation factor GTPase superfamily. Classic translation factor GTPase family. BipA subfamily.