Human P21Ha-ras, a 21-kDa protein with 189 amino acid residues, plays a key role in controlling cellular growth and acts as a molecular switch in signal transduc-tion pathways by cycling between its biologically active ras-GTP and inactive ras-GDP form . The point mutations at several amino acid sites in ras account for 30% of human cancers. Mg2+ is an essential cofactor for the ras superfamily of small GTPases and is necessary for both guanine nucleotide binding and GTP-hydrolysis [35, 36]. We are interested in comparing the binding of apo-ras to GDP and GTP with and without Mg2+. The C-terminal truncated p21Ha-ras (residues 1-166) preserves crucial kinetic and structural properties [37-39] and, thus, is a model for this study. We used PLIMSTEX to investigate the binding of Mg2+ to ras-GDP to form a ternary complex; that is, the ras-GDP binary complex was treated as the ''apo'' protein and Mg2+ was treated as the ligand.
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