# Solution structure of the C-terminal domain of the measles virus V protein in its free form and mechanistic analysis of STAT2 targeting

## TL;DR

Researchers determined the structure of a key part of the measles virus protein that helps it avoid the immune system and found it can bind to different host proteins.

## Contribution

The study presents the first solution structure of the measles virus V protein's C-terminal domain in its unbound state.

## Key findings

- The structure reveals a zinc-finger motif and distinct proline cis conformers in the measles virus V protein's C-terminal domain.
- Key residues for STAT2 binding were identified, with conformational flexibility enabling interactions with multiple host proteins.
- The binding site for STAT2 overlaps with that of IRF9, explaining how the virus disrupts the interferon response.

## Abstract

Viruses commonly evade the host antiviral interferon (IFN) response by targeting key components of the Janus kinase–signal transducer and activator of transcription (JAK–STAT) pathway, typically STAT1 and STAT2. Among the well-characterized viral IFN antagonists, measles virus (MeV), a member of the Morbillivirus genus, encodes a multifunctional V protein (MeV-V) that directly interacts with STAT proteins. The C-terminal domain (CTD) of MeV-V selectively binds to STAT2, disrupting the formation of the IFN-stimulated gene factor 3 (ISGF3) complex by inhibiting the STAT2–interferon regulatory factor 9 (IRF9) association. Here, we report a solution structure covering the MeV-VCTD in its unbound form, as determined by nuclear magnetic resonance spectroscopy. While the overall architecture, including a distinctive zinc-finger motif, conforms to previously predicted features, our analysis reveals unexpected features, including distinct proline cis conformers that may have functional relevance. Molecular mapping analysis, combined with relaxation measurements, identified key residues implicated in STAT2 recognition and revealed substantial conformational flexibility within the domain. These findings suggest that MeV-VCTD employs a shared binding surface for STAT2 binding as for melanoma differentiation-associated protein 5 (MDA5) interaction, underscoring its structural adaptability. As V proteins across Morbillivirus species engage diverse host pathways, including immune signaling, cell cycle regulation, and apoptosis, by targeting multiple proteins, we propose that the dynamic yet folded nature of the VCTD underlies its ability to serve as a versatile interaction module in host–pathogen interplay.

The measles virus V protein, encoded by the P gene, orchestrates the broad modulation of host responses, including immune evasion, by interacting with multiple host factors. With regard to structural studies of VCTD, to date, only one protein from parainfluenza virus 5 has been crystallographically analyzed in complex with host targets. Despite the conserved nature of the VCTD among paramyxoviruses, structural information on the unbound state of this domain is lacking, and current insights largely rely on computational predictions based on the structure of the bound form. Our nuclear magnetic resonance work provides the first structure of the VCTD from paramyxoviruses in its free form. In accordance with our previously presented data, we further confirmed that the MeV-V binding site of STAT2 overlaps that of IRF9. The conformational flexibility observed within the folded CTD provides the structural basis for its ability to engage with multiple host targets with high specificity.