Requirements for the formation of membrane pores by the reovirus myristoylated μ1N peptide

The outer capsid of the nonenveloped mammalian reovirus contains 200 trimers of the μ1 protein, each complexed with three copies of the protector protein σ3. Conformational changes in μ1 following the proteolytic removal of σ3 lead to release of the myristoylated N-terminal cleavage fragment μ1N and ultimately to membrane penetration. The μ1N fragment forms pores in red blood cell (RBC) membranes. In this report, we describe the interaction of recombinant μ1 trimers and synthetic μ1N peptides with both RBCs and liposomes. The μ1 trimer mediates hemolysis and liposome disruption under conditions that promote the μ1 conformational change, and mutations that inhibit μ1 conformational change in the context of intact virus particles also prevent liposome disruption by particle-free μ1 trimer. Autolytic cleavage to form μ1N is required for hemolysis but not for liposome disruption. Pretreatment of RBCs with proteases rescues hemolysis activity, suggesting that μ1N cleavage is not required when steric barriers are removed. Synthetic myristoylated μ1N peptide forms size-selective pores in liposomes, as measured by fluorescence dequenching of labeled dextrans of different sizes. Addition of a C-terminal solubility tag to the peptide does not affect activity, but sequence substitution V13N or L36D reduces liposome disruption. These substitutions are in regions of alternating hydrophobic residues. Their locations, the presence of an N-terminal myristoyl group, and the full activity of a C-terminally extended peptide, along with circular dichroism data that indicate prevalence of β-strand secondary structure, suggest a model in which μ1N β-hairpins assemble in the membrane to form a β-barrel pore.