Structural basis for viral 5'-PPP-RNA recognition by human IFIT proteins

Area of Science:

• Innate immunity
• Molecular virology
• Structural biology

Background:

• Interferon-induced proteins with tetratricopeptide repeats (IFITs) are innate immune molecules.
• IFITs were thought to function by disrupting host translation machinery.
• Recent findings suggest IFITs directly recognize viral RNA signatures.

Purpose of the Study:

• To elucidate the structural basis of viral RNA recognition by IFIT proteins.
• To understand how IFITs distinguish viral RNA from host RNA.
• To investigate the mechanism of IFIT-mediated antiviral defense.

Main Methods:

• X-ray crystallography of human IFIT5 and IFIT1 fragments.
• Complex formation studies with 5'-triphosphate RNA (PPP-RNA).
• Mutational analysis, proteolysis, and gel-shift assays.

Main Results:

• Crystal structures revealed a novel helical domain with a positively charged cavity in IFIT5.
• This cavity specifically binds single-stranded PPP-RNA, differentiating it from RIG-I.
• PPP-RNA binding is non-sequence-specific, requiring a 5'-overhang of ~3 nucleotides.
• Disruption of PPP-RNA binding impaired antiviral responses in human embryonic kidney cells.

Conclusions:

• IFIT proteins selectively recognize viral RNA via a dedicated structural motif.
• This recognition mechanism is distinct from other known innate immune sensors.
• The findings provide insight into the molecular basis of IFIT-mediated antiviral immunity.