Structure and function of the Toscana virus cap-snatching endonuclease

Area of Science:

• Virology
• Structural Biology
• Biochemistry

Background:

• Toscana virus (TOSV) is an arthropod-borne pathogen causing fever and meningoencephalitis in the Mediterranean.
• TOSV belongs to the order Bunyavirales, which includes significant human pathogens like Rift Valley fever virus (RVFV).
• The viral polymerase's N-terminal endonuclease domain is crucial for initiating transcription by generating capped primers.

Purpose of the Study:

• To structurally and functionally characterize the TOSV cap-snatching endonuclease.
• To elucidate the enzyme's substrate binding, active site dynamics, and unique structural features.
• To provide insights for developing targeted antiviral therapies.

Main Methods:

• X-ray crystallography was used to determine the structures of the TOSV endonuclease in apo, inhibitor-bound, and inhibitor-release states.
• Biochemical assays were performed to assess enzyme activity and dependence on conserved residues.
• Comparative structural analysis with other cap-snatching endonucleases was conducted.

Main Results:

• Novel crystal structures revealed significant folding rearrangements, including N-terminal relocation and new structural motifs, distinct from other known cap-snatching endonucleases.
• The TOSV endonuclease exhibits high catalytic activity, comparable to other His+ enzymes.
• Enzyme activity was confirmed to be dependent on conserved residues critical for metal ion and substrate binding.

Conclusions:

• The unique structural features of the TOSV endonuclease play vital roles in its transcription and replication functions.
• Understanding these structural and functional aspects is key to designing specific and broad-spectrum antiviral agents against TOSV.
• This work deepens the knowledge of cap-snatching endonuclease mechanisms within the Bunyavirales order.