Structure and function of human respiratory sncytial virus M2-­‐1 protein

The M2-­‐1 protein of the important pathogen human respiratory syncytial virus is a transcription antiterminator that is essential for viral gene expression.
We present the X-­‐ray crystal structure of full-­‐length
M2-­‐1 protein in its native tetrameric form at a resolution
of 2.52 Å. The structure reveals M2-­‐1 forms a disk-­‐like
assembly with tetramerisation driven by a long helix forming
a four-­‐helix bundle at its center, further stabilised by
contact between the zinc finger and adjacent protomers. The
tetramerisation helix is linked to a core domain responsible
for RNA binding activity by a flexible loop on which lie
two functionally critical serine residues, 58 and 61, that
are phosphorylated during infection. The identity of these
residues was confirmed by mass spectrometric analysis of
M2-­‐1 protein expressed in baculovirus-­‐ assisted insect
cell culture. The crystal structure of a phosphomimetic M2-­‐1 variant, S58DS61D revealed altered charge density surrounding this flexible loop, although loop position was
unaffected.
Structure guided mutagenesis identified residues that contributed to RNA binding and antitermination activity,
revealing a strong correlation between these two activities,
and further defining the role of phosphorylation in M2-­‐1
antitermination activity. The data presented here identify
surfaces critical for M2-­‐1 function that may be targeted
by anti-­‐viral compounds, and allow us to propose a possible model for M2-­‐1 function during respiratory syncytial virus transcription.