STABILIZATION OF TETRAHELICAL DNA BY THE QUADRUPLEX DNA-BINDING PROTEIN QUAD

The 57-kDa hepatic nuclear protein QUAD binds tightly and specifically a parallel tetrahelical form of the IgG switch region DNA (Weisman-Sh omer, P. and Fry, M. (1993) J; Biol. Chem. 268, 3306-3312). Here we sh ow that QUAD is a heat-stable protein, maintaining similar to 90% of i ts tetrahelix binding activity after 10 min at 100 degrees C and becom ing fully inactivated only after 30 min at 100 degrees C. To demonstra te that QUAD protects bound quadruplex DNA, naked and QUAD-bound tetra helices were boiled, the protein residue in the complex was digested w ith trypsin and quadruplex and single-strand forms of the DNA componen t were resolved by electrophoresis. Whereas naked quadruplex DNA becam e fully denatured after 2 min at 100 degrees C, 55% of the QUAD-bound DNA was conserved as a tetrahelix after 6 min at 100 degrees C. These findings support the proposal that QUAD may act in vivo to stabilize t etrahelical DNA. (C) 1994 Academic Press, Inc.