Simian Rotavirus VP4 (Outer Capsid protein VP4) (Hemagglutinin) functions as a spike-forming protein that mediates virion attachment to the host epithelial cell receptors and plays a major role in cell penetration, determination of host range restriction and virulence. Rotavirus entry into the host cell probably involves multiple sequential contacts between the outer capsid proteins VP4 and VP7, and the cell receptors. According to the considered strain, VP4 seems to essentially target sialic acid and/or the integrin heterodimer ITGA2/ITGB1. VP4 is a homotrimer and adopts a dimeric appearance above the capsid surface, while forming a trimeric base anchored inside the capsid layer. The priming trypsin cleavage triggers its rearrangement into rigid spikes with approximate two-fold symmetry of their protruding parts. After an unknown second triggering event, cleaved VP4 may undergo another rearrangement, in which two VP5* subunits fold back on themselves and join a third subunit to form a tightly associated trimer, shaped like a folded umbrella. VP4 interacts with host ITGA2 (via ITAG2 I-domain); this interaction occurs when ITGA2 is part of the integrin heterodimer ITGA2/ITGB1. VP4 interacts with host integrin heterodimer TGA4/ITGB1 and ITGA4/ITGB7. Proteolytic cleavage by trypsin results in activation of VP4 functions and greatly increases infectivity. The penetration into the host cell is dependent on trypsin treatment of VP4. It produces two peptides, VP5* and VP8* that remain associated with the virion.