Major outer capsid protein involved in host cell membrane penetration. In the endocytic compartment, outer-capsid protein sigma-3 is removed by cathepsin proteases, which exposes the viral membrane-penetration protein mu-1. Both myristoylated peptides mu-1N and phi are released during infectious subvirion particles (ISVP) formation in the endosome. They associate with host membranes and mu-1N induces permeabilization and delivery of transcriptionally active viral particles into the host cell cytoplasm. Seems to induce apoptosis in the host cell. The viral outer shell polypeptides, of which mu-1 is one, impose structural constraints that prevent elongation of nascent transcripts by the RNA-dependent RNA polymerase lambda-3. Heterohexamer of three sigma-3 and three Mu-1 proteins. Found in the outer capsid. Seems to associate with cell membranes. This association is enhanced by myristoylation (By similarity). The C-terminal region, phi, determines both targeting to intracellular membranes and induction of apoptosis. Cleaved during the endosomal proteolytic disassembly of the outer capsid. Mu-1 is proteolytically cleaved into mu-1N and mu-1C during the maturation step to generate the ISVP. Cleavage of mu-1 to mu-1C is dependent on myristoylation and binding to sigma-3 protein. Mu-1C is further cleaved into delta (59 kDa), and phi (13 kDa) segments during entry into the host cell cytoplasm. Mu-1 and mu-1N are N-terminally myristoylated. This acylation is essential for the membrane fusion activity (By similarity). Belongs to the orthoreovirus mu-1 protein family.