title: Kinetic analysis of the hydrolysis of GTP by p21N-ras. The basal GTPase mechanism.
abstract: The rate constants have been determined for elementary steps in the basal GTPase mechanism of normal p21N-ras (Gly-12) and an oncogenic mutant (Asp-12): namely GTP binding, hydrolysis, phosphate release, and GDP release. By extrapolation from data at lower temperatures, the GTP association rate constant at 37 degrees C is 1.4 x 10(8) M-1 s-1 for the normal protein and 4.8 x 10(8) M-1 s-1 for the mutant. Other rate constants were measured directly at 37 degrees C, and three processes have similar slow values. GTP dissociation is at 1.0 x 10(-4) s-1 (normal) and 5.0 x 10(-4) s-1 (mutant). The hydrolysis step is at 3.4 x 10(-4) s-1 (normal) and 1.5 x 10(-4) s-1 (mutant). GDP dissociates at 4.2 x 10(-4) s-1 (normal) and 2.0 x 10(-4) s-1 (mutant). GDP association rate constants are similar to those for GTP, 0.5 x 10(8) M-1 s-1 for normal and 0.7 x 10(8) M-1 s-1 for mutant. Both hydrolysis and GDP release therefore contribute to rate limitation of the basal GTPase activity. There are distinct differences (up to 5-fold) between rate constants for the normal and mutant proteins at a number of steps. The values are consistent with the reduced GTPase activity for this mutant and suggest little difference between normal and mutant proteins in the relative steady-state concentrations of GTP and GDP complexes that may represent active and inactive states. The results are discussed in terms of the likely role of p21ras in transmembrane signalling.
authors.affiliation: Division of Physical Biochemistry, National Institute for Medical Research, London, United Kingdom.
authors.forename: S E
authors.initials: SE
authors.lastname: Neal
authors.forename: J F
authors.initials: JF
authors.lastname: Eccleston
authors.forename: A
authors.initials: A
authors.lastname: Hall
authors.forename: M R
authors.initials: MR
authors.lastname: Webb
mesh_terms: D004926:Escherichia coli / Q000235:genetics; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006868:Hydrolysis; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism*; D009154:Mutation; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D016283:Proto-Oncogene Proteins p21(ras); D011994:Recombinant Proteins / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D006160:Guanosine Triphosphate; D010744:Phosphoric Monoester Hydrolases; D020558:GTP Phosphohydrolases; D016283:Proto-Oncogene Proteins p21(ras)