The structural models show that the glycoproteins are not close-packed. The strong crystalline order of the Udorn matrix layer does not appear to extend to the glycoproteins. However, the glycoprotein distribution in Udorn is more ordered than X-31 which points toward translational restriction of the HA and supports the idea learn more of interactions with the matrix layer. Higher

resolution analysis by tomography or biophysical measurement will be required to see whether there is any rotational ordering to the glycoproteins. Our model for the influenza glycoprotein distribution defines several structural parameters that may be important for understanding the virus life cycle as well as preventing infections with drugs and vaccines. The structural Navitoclax solubility dmso models of the envelope glycoprotein on the virus surface suggest geometric constraints on receptor binding determined by the glycoprotein spacing and radius of curvature of the virus membrane. In vitro experiments indicate a weak millimolar binding constant of the HA glycoprotein for sialic acid receptors. Furthermore, influenza host specificity is dependent on very small affinity differences for sialic acid receptors with different glycosidic linkages [18] and [19]. Infection therefore depends on multivalent binding. The number of HAs that can simultaneously participate in binding will be a key determinant in virus entry. The

curvature of the virus surface and spacing of glycoproteins determines the number of adjacent glycoproteins that can simultaneously engage receptors on a planar surface such as those used in in vitro binding studies. The flexibility, length, and density of lipids or proteins bearing sialic acid receptors

on cells will influence the number of HAs engaged with receptors as will the rigidity and contour of the host membrane and its ability to wrap around the curved surface of influenza virus. The three-dimensional structural models of the glycoprotein on the surface of influenza virions describe important structural parameters that govern antibody recognition of the HA including the density and accessibility of epitopes. The average glycoprotein spacing observed Cell press (∼100 Å) is short enough for bivalent IgGs, which possess flexibly linked antigen binding sites that can extend 150 Å apart, to cross-link adjacent HAs [20] and [21]. The off-rate for IgG binding decreases due to avidity, making viral escape from neutralizing antibodies through mutation more difficult [22]. Most neutralizing antibodies recognize sites on the sequence variable globular head domain of HA that are likely to be accessible on the virus surface and block cell attachment by preventing receptor binding [23]. There has been recent interest in broadly neutralizing antibodies that bind to conserved features on the HA [7], [24] and [25].