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  • Effects of Salt on the Stability of a G-Quadruplex from the Human c-MYC Promoter.

Effects of Salt on the Stability of a G-Quadruplex from the Human c-MYC Promoter.

Biochemistry (2015-05-20)
Byul G Kim, Heather M Evans, David N Dubins, Tigran V Chalikian
ABSTRACT

In an atmosphere of potassium ions, a modified c-MYC NHE III1 sequence with two G-to-T mutations (MYC22-G14T/G23T) forms a highly stable parallel-stranded G-quadruplex. The G-quadruplex exhibits a steady increase in its melting temperature, T(M), with an increase in the concentration of the stabilizing cation K(+). On the other hand, an increase in the concentration of nonstabilizing Cs(+) or TMA(+) cations at a constant concentration of K(+) causes a sharp decline in T(M) followed by a leveling off at ∼200 mM Cs(+) or TMA(+). At 51 °C and 600 μM K(+), an increase in Cs(+) concentration from 0 to 800 mM leads to a complete unfolding of the G-quadruplex. These observations are consistent with the picture in which more counterions accumulate in the vicinity of the unfolded state of MYC22-G14T/G23T (nonspecific ion binding) than in that of the G-quadruplex state. We estimate that the unfolded state condenses one extra counterion compared to the G-quadruplex state. Taken together with our earlier results, our data suggest that sodium or potassium cations sequestered inside the central cavity stabilize the G-quadruplex conformation acting as specifically bound ligands. Nonspecifically bound (condensed) counterions may slightly stabilize, exert no influence (human telomeric G-quadruplexes), or strongly destabilize (MYC22-G14T/G23T) the G-quadruplex conformation. We offer a structural rationalization for the enhanced thermal stability of the MYC22-G14T/G23T G-quadruplex.

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