Abstract
The 57-kDa hepatic nuclear protein QUAD binds tightly and specifically a parallel tetrahelical form of the IgG switch region DNA (Weisman-Shomer, P. and Fry, M. (1993) J. Biol. Chem.268, 3306-3312). Here we show that QUAD is a heat-stable protein, maintaining ∼90% of its tetrahelix binding activity after 10 min at 100°C and becoming fully inactivated only after 30 min at 100°C. To demonstrate that QUAD protects bound quadruplex DNA, naked and QUAD-bound tetrahelices were boiled, the protein residue in the complex was digested with trypsin and quadruplex and single-strand forms of the DNA component were resolved by electrophoresis. Whereas naked quadruplex DNA became fully denatured after 2 min at 100°C, 55% of the QUAD-bound DNA was conserved as a tetrahelix after 6 min at 100°C. These findings support the proposal that QUAD may act in vivo to stabilize tetrahelical DNA.
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