Biophysical and dynamic characterization of a fine-tuned binding of the human Respiratory Syncytial Virus M2-1 core domain to long RNAs

The human Respiratory Syncytial Virus (hRSV) M2-1 protein functions as a processivity and antitermination factor of the viral polymerase complex. Here it is presented the first evidence that hRSV M2-1 core domain (cdM2-1) alone has an unfolding activity for long RNAs, as well as a biophysical and dynamic characterization of the cdM2-1/RNA complex. The main contact region of cdM2-1 with RNA was the 1-2-5-6 helix bundle, which suffered local conformational changes and promoted the RNA unfolding activity. This activity may be triggered by base-pairing recognition. RNA molecules wrap around the whole cdM2-1, protruding their terminals over the domain. The 2-3 and 3-4 loops of cdM2-1 were marked by an increase in picosecond internal motions upon RNA binding even though they are not directly involved in the interaction. The results revealed that the cdM2-1/RNA complex originates from a fine-tuned binding, contributing to unraveling interaction aspects necessary to M2-1 activity. IMPORTANCEThe main outcome is the molecular description of a fine-tuned binding of the cdM2-1/RNA complex and the evidence that the domain alone has an unfolding activity for long RNAs. This binding mode is essential in the understanding of the function in the full-length protein. Orthopneumovirus, as the human Respiratory Syncytial Virus (hRSV), stands out for the unique role of M2-1 as a transcriptional antitermination factor able to increase the RNA polymerase processivity.