title: The role of Mg2+ cofactor in the guanine nucleotide exchange and GTP hydrolysis reactions of Rho family GTP-binding proteins.
abstract: The biological activities of Rho family GTPases are controlled by their guanine nucleotide binding states in cells. Here we have investigated the role of Mg(2+) cofactor in the guanine nucleotide binding and hydrolysis processes of the Rho family members, Cdc42, Rac1, and RhoA. Differing from Ras and Rab proteins, which require Mg(2+) for GDP and GTP binding, the Rho GTPases bind the nucleotides in the presence or absence of Mg(2+) similarly, with dissociation constants in the submicromolar concentration. The presence of Mg(2+), however, resulted in a marked decrease in the intrinsic dissociation rates of the nucleotides. The catalytic activity of the guanine nucleotide exchange factors (GEFs) appeared to be negatively regulated by free Mg(2+), and GEF binding to Rho GTPase resulted in a 10-fold decrease in affinity for Mg(2+), suggesting that one role of GEF is to displace bound Mg(2+) from the Rho proteins. The GDP dissociation rates of the GTPases could be further stimulated by GEF upon removal of bound Mg(2+), indicating that the GEF-catalyzed nucleotide exchange involves a Mg(2+)-independent as well as a Mg(2+)-dependent mechanism. Although Mg(2+) is not absolutely required for GTP hydrolysis by the Rho GTPases, the divalent ion apparently participates in the GTPase reaction, since the intrinsic GTP hydrolysis rates were enhanced 4-10-fold upon binding to Mg(2+), and k(cat) values of the Rho GTPase-activating protein (RhoGAP)-catalyzed reactions were significantly increased when Mg(2+) was present. Furthermore, the p50RhoGAP specificity for Cdc42 was lost in the absence of Mg(2+) cofactor. These studies directly demonstrate a role of Mg(2+) in regulating the kinetics of nucleotide binding and hydrolysis and in the GEF- and GAP-catalyzed reactions of Rho family GTPases. The results suggest that GEF facilitates nucleotide exchange by destabilizing both bound nucleotide and Mg(2+), whereas RhoGAP utilizes the Mg(2+) cofactor to achieve high catalytic efficiency and specificity.
authors.affiliation: Department of Biochemistry, University of Tennessee, Memphis 38163, USA.
authors.forename: B
authors.initials: B
authors.lastname: Zhang
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.forename: Y
authors.initials: Y
authors.lastname: Zheng
mesh_terms: D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013997:Time Factors; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000378:metabolism; D020741:rho GTP-Binding Proteins / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D020662:Guanine Nucleotide Exchange Factors; C057811:MCF2 protein, human; D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D017346:Protein Serine-Threonine Kinases; C099628:TRIO protein, human; D020764:cdc42 GTP-Binding Protein; D020830:rac1 GTP-Binding Protein; D020741:rho GTP-Binding Proteins; D020742:rhoA GTP-Binding Protein; D008274:Magnesium