Fiber-like molecule that attaches the virion to the host cell membrane by binding to the primary receptor F11R/JAM-A and to sialic acid containing proteins (coreceptor). The interaction of sigma-1 with F11R is required for NF-kB activation and apoptosis. Binding to both sialic acid and F11R is required to induce maximal levels of apoptosis. Homotrimer. Interacts (via the head region) with human F11R. Found in the outer capsid (36 copies). Undergoes dramatic conformational rearrangements during viral disassembly in the endocytic pathway. Belongs to the orthoreovirus sigma-1 protein family.