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 (By similarity). 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.