| Abstrakt: |
A rat liver nuclear protein, unimolecular quadruplex telomere-binding protein 25, (uqTBP25) is described that binds tightly and specifically single-stranded and unimolecular tetraplex forms of the vertebrate telomeric DNA sequence 5′-d(TTAGGG) n-3′. A near homogeneous uqTBP25 was purified by ammonium sulfate precipitation, chromatographic separation from other DNA binding proteins, and three steps of column chromatography. SDS-polyacrylamide gel electrophoresis and Superdex© 200 gel filtration disclosed for uqTBP25 subunit and nativeMrvalues of 25.4 ± 0.5 and 25.0 kDa, respectively. Sequences of uqTBP25 tryptic peptides were closely homologous, but not identical, to heterogeneous nuclear ribonucleoprotein A1, heterogeneous nuclear ribonucleoprotein A2/B1, and single-stranded DNA-binding proteins UP1 and HDP-1. Complexes of uqTBP25 with single-stranded or unimolecular quadruplex 5′-d(TTAGGG)4-3′, respectively, had dissociation constants,Kd, of 2.2 or 13.4 nm. Relative to d(TTAGGG)4, complexes with 5′-r(UUAGGG)4-3′, blunt-ended duplex telomeric DNA, or quadruplex telomeric DNA had >10 to >250-fold higher Kdvalues. Single base alterations within the d(TTAGGG) repeat increased theKdof complexes with uqTBP25 by 9–215-fold. Association with uqTBP25 protected d(TTAGGG)4against nuclease digestion, suggesting a potential role for the protein in telomeric DNA transactions. |
