Structural analysis of Spi-B DNA-binding Ets domain recognizing 5'-AGAA-3' and 5'-GGAA-3' sequences.

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• Plasmacytoid dendritic cells are key producers of type-I interferon (IFN-I) upon pathogen detection via Toll-like receptors (TLR7 and TLR9).
• The transcription factor Spi-B, along with IFN regulatory factor-7 (IRF-7), cooperatively transactivates the Ifna4 promoter, crucial for IFN-I production.
• Spi-B recognizes the DNA sequence 5'-AGAA-3', distinct from the canonical Ets domain sequence 5'-GGAA-3'.

Purpose of the Study:

• To elucidate the molecular mechanism underlying the cooperative transactivation of the Ifna4 promoter by Spi-B and IRF-7.
• To determine the structural basis for Spi-B's specific DNA binding and its role in facilitating IRF-7 interaction.

Main Methods:

• X-ray structural determination of the Spi-B Ets domain complexed with target DNA sequences (5'-AGAA-3' and 5'-GGAA-3').
• Computational modeling of the Spi-B and IRF-7 complex with the Ifna4 promoter DNA.

Main Results:

• X-ray structures revealed that Spi-B binding induces a DNA kink at the recognition site, with a more pronounced kink observed for the 5'-AGAA-3' sequence.
• The Spi-B-induced kink in the 5'-AGAA-3' DNA sequence creates a conformation favorable for the association of Spi-B and IRF-7.

Conclusions:

• Spi-B's unique DNA binding preference and its ability to induce DNA kinking are critical for the cooperative transcriptional activation of the Ifna4 promoter.
• This structural mechanism facilitates the assembly of the Spi-B-IRF-7 complex, leading to robust type-I interferon production in response to TLR7/9 stimulation.