Identification of the single-stranded DNA binding surface of the transcriptional coactivator PC4 by NMR

The C-terminal domain of the eukaryotic transcriptional cofactor PC4 (PC4(CTD)) is known to bind with nanomolar affinity to single-stranded (ss)DNA. Here, NMR is used to study DNA binding by this domain in more detail. Amide resonance shifts that were observed in a1H15N-HSQC- monitored titration of15N-labeled protein with the oligonucleotide dT18indicate that binding of the nucleic acid occurs by means of two anti- parallel channels that were previously identified in the PC4(CTD) crystal structure. The β-sheets and loops that make up these channels exhibit above average flexibility in the absence of ssDNA, which is reflected in higher values of T(1ρ), reduced heteronuclear nuclear Overhauser effects and faster deuterium exchange rates for the amides in this region. Upon ssDNA binding, this excess flexibility is significantly reduced. The binding of ssDNA by symmetry-related channels reported here provides a structural rationale for the preference of PC4(CTD) for juxtaposed single-stranded regions (e.g. in heteroduplexes) observed in earlier work.