Fig. 10. A model for HN tetramer rearrangement upon cell-surface receptor binding.  The HN tetramer is primarily stabilized by the N-terminal stalk region and can interact with F protein. Sialic acid receptors are displayed at the cell surface, where binding of the individual HN NA domains could perturb the NA tetramer arrangement, consistent with the weak interactions between NA domains. Changes in the HN NA domain tetramer could affect F interactions and stimulate membrane fusion. (From Yuan, P., Thompson, T., Wurzburg, B.A., Paterson, R.G., Lamb, R.A., and Jardtezky, T.S. (2005). Structure 13, 803-815. Copyright 2005 by Elsevier Ltd.)