title: The Caulobacter crescentus CgtA protein displays unusual guanine nucleotide binding and exchange properties.
abstract: The Caulobacter crescentus CgtA protein is a member of the Obg-GTP1 subfamily of monomeric GTP-binding proteins. In vitro, CgtA specifically bound GTP and GDP but not GMP or ATP. CgtA bound GTP and GDP with moderate affinity at 30 degrees C and displayed equilibrium binding constants of 1.2 and 0.5 microM, respectively, in the presence of Mg(2+). In the absence of Mg(2+), the affinity of CgtA for GTP and GDP was reduced 59- and 6-fold, respectively. N-Methyl-3'-O-anthranoyl (mant)-guanine nucleotide analogs were used to quantify GDP and GTP exchange. Spontaneous dissociation of both GDP and GTP in the presence of 5 to 12 mM Mg(2+) was extremely rapid (k(d) = 1.4 and 1.5 s(-1), respectively), 10(3)- to 10(5)-fold faster than that of the well-characterized eukaryotic Ras-like GTP-binding proteins. The dissociation rate constant of GDP increased sevenfold in the absence of Mg(2+). Finally, there was a low inherent GTPase activity with a single-turnover rate constant of 5.0 x 10(-4) s(-1) corresponding to a half-life of hydrolysis of 23 min. These data clearly demonstrate that the guanine nucleotide binding and exchange properties of CgtA are different from those of the well-characterized Ras-like GTP-binding proteins. Furthermore, these data are consistent with a model whereby the nucleotide occupancy of CgtA is controlled by the intracellular levels of guanine nucleotides.
authors.affiliation: Department of Biology, University of Michigan, Ann Arbor, Michigan 48109-1048, USA.
authors.forename: B
authors.initials: B
authors.lastname: Lin
authors.forename: K L
authors.initials: KL
authors.lastname: Covalle
authors.forename: J R
authors.initials: JR
authors.lastname: Maddock
mesh_terms: D001426:Bacterial Proteins; D016935:Caulobacter crescentus / Q000378:metabolism*; D003001:Cloning, Molecular; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D020559:Monomeric GTP-Binding Proteins; D016133:Polymerase Chain Reaction / Q000379:methods; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D011994:Recombinant Proteins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; C108621:CgtA protein, bacteria; D019204:GTP-Binding Proteins; D020559:Monomeric GTP-Binding Proteins; D008274:Magnesium