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Biochemistry. 2016 Jan 26;55(3):543-59. doi: 10.1021/acs.biochem.5b00902. Epub 2016 Jan 14.

Kinetic Mechanism of Formation of Hyperactive Embryonic Ras in Cells.

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Department of Chemistry and Biochemistry, The University of Texas at Arlington , Arlington, Texas 76019, United States.
Department of Life Science, Sogang University , Seoul 121-742, Korea.
Humanzyme , 2201 West Campbell Park Drive, Chicago, Illinois 60612, United States.


Embryonic Ras (ERas)--a new subset of Ras proteins--are characterized by a unique p-loop residue, unique Switch II residues, and an unusual extended N-terminus. When expressed, both murine and human ERas are highly populated in their GTP-bound forms. The expression of murine ERas is linked to the development of murine embryonic cells, and the expression of human ERas is correlated to certain human cancers. Mutation-based kinetic analyses, in combination with assessments of the kinetic parameter-based calculation of the fraction of the GTP-bound active form of ERas proteins, explain the kinetic mechanism that produces the unprecedented hyperactive ERas. The ERas-specific p-loop residue contributes ERas proteins to intrinsically populate their GTP-bound form in cells. Furthermore, the ERas-specific Switch II residues block the catalytic action of p120GAP on ERas proteins. This blockage sustains the previously mentioned GTP-bound ERas proteins. In essence, the combined work of the ERas-specific p-loop and Switch II residues populates the exceedingly high GTP-bound form of ERas in cells. This study also rules out any kinetic function of the unique ERas-specific N-terminus in the production of the hyperactive GTP-bound ERas in cells. The biological role of this N-terminus remains uninvestigated. Intriguingly, the ERas-specific p-loop residue matches the mutated Ser residue of the Costello Syndrome G12S HRas mutant that also intrinsically populates its GTP-bound form in cells. However, because the effector protein of ERas differs from that of G12S HRas, this kinetic similarity does not confer on ERas biological and/or pathophysiological similarity to G12S HRas.

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