Kinetics of interaction of nucleotides with nucleotide-free H-ras p21

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Abstract
A method is described for the convenient preparation of substantial guantities of nucleotide-free p21 or of 1:1 complexes with nucleotides other than GDP. The nucleotide-free protein has been used kinetic studies of the binding of GDP and GTP, making use of the fluorescent analogues 3''-(methylanthraniloyl)-2''deoxy-GDP AND -GTP. Stopped-flow studies have led to the formulation of a two-step binding mechanism for both GDP and GTP, involving initial rapid but weak binding of the nucleotide followed by a relatively slow (10-20 s-1 at 25.degree.C; 3-5 s-1 at 5.degree.C) quasi-irreversible isomerization reaction. By use of a nonequilibrium competition method, guanosine and GMP have been shown to interact weakly but significantly with p21 (dissocation constants of 153 and 29 .mu.M, respectively). The presence of guanosine or GMP at the active site of p21 leads to a marked stabilization of p21 against spontaneous denaturation when compared with the nucleotide- and nucleoside-free protein.