Positive cofactor 4 (PC4)  -  DNA

Biological function
The human positive cofactor 4 (PC4) recruits general transcription factors and stimulates RNA polymerase II (RNAP II). PC4 has also been shown to repress transcription under specific conditions.

Domain organization/sequence features
The activity of PC4 is regulated by a disordered NTD (1-60 AA), which consists of a Ser- and acidic- rich region and a Lys-rich region. The NTD alone lacks considerable affinity for either ssDNA or dsDNA. Acetylation of two lysine residues by the cofactor p300 increases the dsDNA-binding affinity by 40%.

Structural evidence
The moderate NOE signal intensities, in combination with the chemical-shift data, indicate that NTD regions are predominantly unstructured in nature, which suggests either a pliable structure or exchange between the random-coil and more ordered structures. NMR data show that the disordered NTD region establishes transient, dynamic interactions with the structured CTD and these compete with the ssDNA binding sites. The interaction between both domains is relatively weak and can only be observed when covalently linked. Formation of a rigid tertiary structure of PC4 NTD is not observed when PC4 binds to DNA.

Biochemical evidence
Owing to the presence and transient interactions of the fuzzy NTD, the ssDNA binding affinity and the DNA unwinding activity of PC4 are both reduced. DNA-binding and DNA-unwinding activity are increased by both deletion of the whole amino terminus and mutation of specific positive-charged residues.

Transient interaction between both NTD and CTD domains might reduce the ssDNA-binding affinity and the unwinding activity because of competitive binding and/or steric hindrance. Progressive phosphorylation of eight Ser residues in the disordered PC4 NTD shields the neighboring Lys-rich region, which mediates communication between the disordered N-terminal and the ordered C-terminal part of PC4. Hence, DNA binding of the structured region is affected indirectly via an ID segment and not directly via phosphoserine contacts.

Mechanism category
competitive binding

Posttranslational modification
Phosphorylation of PC4 gradually decreases its binding affinity for dsDN.

The fuzzy NTD region enables dynamic functional regulation of the PC4 cofactor by specific interactions with the activator and through modulation and/or shielding of the interaction surface in the structured core of PC4 CTD.