Paramyxoviruses are a leading cause of childhood illness worldwide. A recently discovered paramyxovirus, human metapneumovirus (hMPV), has been studied by our group in order to determine the structural relevance of its fusion (F) protein to other well-characterized viruses utilizing type I integral membrane proteins as fusion aids. Sequence analysis and homology models suggested the presence of requisite heptad repeat (HR) regions. Synthetic peptides from HR regions 1 and 2 (HR-1 and -2, respectively) were induced to form a thermostable (melting temperature, approximately 90 degrees C) helical structure consistent in mass with a hexameric coiled coil. Inhibitory studies of hMPV HR-1 and -2 indicated that the synthetic HR-1 peptide was a significant fusion inhibitor with a 50% inhibitory concentration and a 50% effective concentration of approximately 50 nM. Many viral fusion proteins are type I integral membrane proteins utilizing the formation of a hexameric coiled coil of HR peptides as a major driving force for fusion. Our studies provide evidence that hMPV also uses a coiled-coil structure as a major player in the fusion process. Additionally, viral HR-1 peptide sequences may need further investigation as potent fusion inhibitors.