Figure 4.

Viral antigens on enveloped viruses. Viral glycoproteins embedded in the viral membrane are responsible for entry of viruses into host cells. These envelope proteins contain receptor-binding sites and membrane fusion activities or are involved in progeny release and cell egress (influenza neuraminidase). Many of these viral antigens are homotrimers (e.g., influenza HA (A), Ebola virus GP (C), HIV-1 (D), RSV F (F), MERS (G), and SARS-CoV 1 and 2 spike proteins (H and I)), whereas Influenza NA (B) is a homotetramer. The HepC E1E2 glycoprotein is thought to exist as a heterodimer, but only the structure for the monomeric E2 core has been determined (E). These antigens are usually heavily glycosylated to shield themselves from antibodies in the immune system. In all panels, the individual subunits of each glycoprotein are colored red, white, or blue and top (looking down the trimer or tetramer axis) and side views where the viral cell membrane would be on the bottom are shown for each. Carbohydrate is shown in black.