 The activities of the receptor binding hemagglutinin-neuraminidase protein is solely responsible for inducing close interaction between the virus envelope and the cellular membrane.However, it is the F protein (one of many membrane fusion proteins) that, when triggered by local dehydration and a conformational change in the bound HN protein, actively inserts into the cellular membrane, which causes the envelope and the membrane to merge, followed shortly by virion entry.When the HN and F protein are manufactured by the cell and expressed on the surface, the same process may occur between adjacent cells, causing extensive membrane fusion and resulting in the formation of a syncytium.