The non-structural NS1 protein is a main player for immune evasion by respiratory syncytial virus (RSV). RSV NS1 is a small protein, comprising a globular α/β domain and a C-terminal α3-helix. It is a multifunctional protein, which contributes, together with RSV NS2, to inhibit type I/III interferon production and signaling. NS1 also acts on host transcription, via an interaction with the MED25 subunit of the Mediator. Proteomic screens, performed by several groups, have identified numerous interaction partners for RSV NS1, underscoring its multifunctionality. Only one complex structure has been reported to date, for the NS1-MED25 ACID complex. Structure predictions for other RSV NS1 complexes often involve NS1 α3 but remain of low confidence. Biophysical characterization of the NS1-MED25 ACID interaction has demonstrated that, although a peptide mimicking α3 binds on its own to MED25 ACID, the presence of the α/β domain is necessary to achieve strong binding and to correctly position α3 at the surface of MED25 ACID. Binding of both NS1 subdomains is cooperative, and involves two binding sites on MED25 ACID that were previously reported to be allosterically coupled. Investigation of the NS1-MED25 ACID interaction in solution by NMR was hampered by dramatic line broadening of NS1 signals. Removal of α3 in the NS1Δα3 deletion mutant allowed to investigate the NS1 α/β domain at the residue scale by NMR. It revealed a region with large conformational fluctuations, which seems to be linked to a propensity for self-assembly. The α3 peptide bound to two sites on the α/β domain, confirming its stickiness. Taken together, our results show that RSV NS1 is a globular protein that displays conformational plasticity and partial disorder, providing a rationale for the high number of multiple binding partners of this small protein.
Dong et al. (Sun,) studied this question.