FC0051
Facilitates chromatin transcripton (FACT)  -  DNA

Biological function
Facilitates chromatin transcription (FACT) is involved in RNAP II-mediated transcription by displacing histone H2A–H2B dimers from nucleosomes.

Domain organization/sequence features
FACT interacts with DNA via a structured high-mobility group (HMG) domain, which is flanked by two charged ID regions of opposite character.

Structural evidence
The acidic region and the basic segment are entirely unstructured, as convincingly demonstrated by the high-speed AFM and CD. NMR titration experiments indicate interactions between the basic and acidic ID domain as well as

Biochemical evidence
Phosphorylated Form of FACT in Sf9 Cells Inhibits the Nucleosomal DNA Binding Activity. The dephosphorylated dFACT more strongly interacts with nucleosomes that contain linker DNA, in agreement with the finding that HMG-box proteins bind to linker DNA.

Structure/Mechanism
The negatively charged segment establishes intramolecular interactions with the positive residues of the HMG domain as well as the ID region and competes for DNA. Multiple phosphorylation of this negative ID region increases the inhibitory effect by masking the nucleotide-binding residues. Indeed, FACT was shown to be dephosphorylated in early embryogenesis.

Mechanism category
competitive binding

Posttranslational modification
Multiple phosphorylation of the acidic ID segment increases its negative charge and thus strengthens the interactions with the basic regions. This masks the nucleotide-binding elements, and results in inhibition of DNA binding. Phosphorylation, however, does not induce folding of the ID regions.

Significance
The underlying structural properties of the fuzzy region and the embedded clustered phosphorylation sites enable a gradual control on DNA binding by shifting the intermolecular interactions to intramolecular interactions between the ID segments of FACT. Phosphorylation states of the fuzzy region can accommodate the rapid changes in chromatin transactions in vivo.

Submitted by
Shin-Ichi Tate   tate@hiroshima-u.ac.jp