Summer Research Description: Respiratory syncytial virus (RSV) is the primary cause of serious lower respiratory tract infections in high-risk patients, resulting in 160,000 deaths annually worldwide. Currently, no vaccine or anti-viral treatment is available. The RSV matrix (M) and fusion (F) proteins are two viral proteins that have known roles in viral assembly-mediated processes. Our laboratory has previously demonstrated that RSV F cytoplasmic tail (F CT) is critical for recruitment of viral proteins into viral-induced filaments. We have also observed that RSV M interacts with RSV F CT, and that this interaction is necessary for virion assembly in epithelial cells. Our aim is to more precisely identify the region of the M protein to which the F CT binds to facilitate this interaction. Using site-directed mutagenesis, we generated ~40 RSV M protein mutants to assess changes in filament formation using confocal microscopy and viral protein-protein interactions by proximity ligation assay (PLA). We identified target residues on the sides and back of the M protein that alter filament formation and the RSV M and F CT interaction. These data provide critical insight into how RSV M mediates virion assembly within epithelial cells and provide a potential target for the development of novel antiviral therapeutics.