Human and avian influenza A viruses bind to sialic acid (Sia) receptors on cells as their primary receptors, and this results in endocytic uptake of the virus. While the role of Sia on glycoproteins and/or glycolipids for virus entry is crucial, the roles of the carrier proteins are still not well understood. Furthermore, it is still unclear how receptor binding leads to infection, including whether the receptor plays a structural or other roles beyond being a simple tether. To enable the investigation of the receptor binding and cell entry processes in a more controlled manner, we have designed a protein receptor for pandemic H1 influenza A viruses. The engineered receptor possesses the binding domains of an anti-HA antibody prepared as a single-chain variable fragment (scFv) fused with the stalk, transmembrane, and cytoplasmic sequences of the feline transferrin receptor type-1 (fTfR). When expressed in cells that lack efficient display of Sia due to a knockout of the