title: Binding of guanine nucleotides and Mg2+ to tubulin with a nucleotide-depleted exchangeable site.
abstract: Binding of GTP and GDP to tubulin in the presence or absence of Mg2+ was measured following depletion of the exchangeable site--(E-site) nucleotide. The E-site nucleotide was displaced with a large molar excess of the nonhydrolyzable GTP analogue, GMPPCP, followed by the removal of the analogue. Using a micropartition assay, the equilibrium constant measured in 0.1 M 1.4-piperazinediethanesulfonic acid (Pipes), pH 6.9, 1 mM ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid, 1 mM dithiothreitol, and 1 mM MgSO4 at 4 degrees C was 9.1 x 10(6) M-1 for GTP and 4.4 x 10(6) M-1 for GDP. Removal of Mg2+ reduced the binding affinity of GTP by 160-fold while the affinity of GDP remained essentially unchanged. Similar values were obtained if 0.1 M Tris, pH 7.0, was used instead of Pipes. Binding of Mg2+ to tubulin containing GTP, GDP, or no nucleotide at the E-site was also examined by the micropartition method. Tubulin-GTP contained one high affinity Mg2+ site (K alpha = 1.2 x 10(6) M-1) in addition to the one occupied by Mg2+ as tubulin is isolated, while only weak Mg2+ binding to tubulin-GDP and to tubulin with a vacant E-site (K alpha = 10(3) M-1) was observed. It is suggested that Mg2+ binds to the beta and gamma phosphates of GTP, and only to the beta phosphate of GDP, as shown for the H. ras p21 protein.
authors.affiliation: Department of Biochemistry, University of Kansas, Lawrence 66045-2106.
authors.forename: M R
authors.initials: MR
authors.lastname: Mejillano
authors.forename: R H
authors.initials: RH
authors.lastname: Himes
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D006150:Guanine Nucleotides / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D011487:Protein Conformation; D014404:Tubulin / Q000378:metabolism*
chemical_list: D006150:Guanine Nucleotides; D014404:Tubulin; D006153:Guanosine Diphosphate; C004805:5'-guanylylmethylenebisphosphonate; D006160:Guanosine Triphosphate; D008274:Magnesium