Influenza neuraminidase is delivered directly to the apical surface of MDCK cell monolayers

The aim of this study was to investigate whether influenza neuraminidase travels directly from the Golgi complex to the apical domain of the plasma membrane in virally infected epithelial (MDCK) cell monolayers, or whether it passes transiently through the basolateral domain. Using a new assay for the delivery of neuraminidase to the plasma membrane, we found that the time course of transport of this protein from the Golgi complex to the apical surface of MDCK cell monolayers was very similar to that for influenza haemagglutinin, which is known to be delivered directly to its destination. In addition, a similar time course of neuraminidase transport was found in BHK cells, which are not asymmetric and in which delivery must therefore be direct. Finally, basolateral exposure of MDCK cell monolayers grown on nitrocellulose filters to an anti-neuraminidase antibody was shown to have no effect on the delivery of active neuraminidase to the apical surface. We conclude from these results that neuraminidase, like haemagglutinin, is delivered directly to the apical surface.