InterPro domain: IPR045086

The P-loop guanosine triphosphatases (GTPases) control amultitude of biological processes, ranging from cell division, cell cycling,and signal transduction, to ribosome assembly and protein synthesis. GTPasesexert their control by interchanging between an inactive GDP-bound state andan active GTP-bound state, thereby acting as molecular switches. The commondenominator of GTPases is the highly conserved guanine nucleotide-binding (G)domain that is responsible for binding and hydrolysis of guanine nucleotides.

Within the translation factor-related (TRAFAC) class of P-loop GTPases, theOBG family comprises a group of high-molecular mass GTPases conserved frombacteria to eukaryotes. The OBG family consists of [ 1 ]:

• Obg from bacteria and eukaryotes [ 2 , 3 ].
• DRG from eukaryotes and archaea. DRG proteins may regulate fundamental cellular processes through RNA binding [ 4 ].
• Nog1 from eukaryotes and archaea. It is involved in the assembly of the large ribosomal subunit.
• Yyaf/YchF from bacteria and eukaryotes. It consists of a central G domain, flanked by a coiled-coil domain and a TGS (ThrRS, GTPase, SpoT) domain. Members of this subfamily bind and hydolyze ATP more efficiently than GTP [ 5 , 6 ].
• Ygr210 from archaea and fungi.

The OBG-type G domain has a mononucleotide binding fold typical for the P-loopNTPases. A six-stranded mostly parallel beta-sheet is flanked by alpha-heliceson both sides. The OBG-type G domain contains fivecharacteristic sequence motifs, termed G1-G5, involved in nucleotide bindingand hydrolysis. The G1/Walker A motif (GXXXXGK(S/T)), also referred to as P-loop, helps to position the triphosphate moiety of the bound nucleotide. TheG2 (X(T/S)X) and G3/Walker B (hhhDXXG) motifs are involved in the coordinationof a Mg(2)+ ion that is required for nucleotide binding and hydrolysis.Specificity in nucleotide binding is conferred by the G4 motif, which has a(N/T)KXD signature in guanine nucleotide binding P-loop NTPases. The G5 motif((T/G)(C/S)A) supports guanine base recognition [ 7 , 7 , 7 , 7 ].

1. Classification and evolution of P-loop GTPases and related ATPases. J. Mol. Biol. 317, 41-72
2. Structural and biochemical analysis of the Obg GTP binding protein. Structure 10, 1581-92
3. Crystal structure of the GTP-binding protein Obg from Thermus thermophilus HB8. J. Mol. Biol. 337, 761-70
4. Cloning and characterization of Xenopus laevis drg2, a member of the developmentally regulated GTP-binding protein subfamily. Gene 322, 105-12
5. Crystal structure of the YchF protein reveals binding sites for GTP and nucleic acid. J. Bacteriol. 185, 4031-7
6. Human OLA1 defines an ATPase subfamily in the Obg family of GTP-binding proteins. J. Biol. Chem. 282, 19928-37