Protease Cleavage of Reovirus Capsid Protein µ1/µ1C Is Blocked by Alkyl Sulfate Detergents, Yielding a New Type of Infectious Subvirion Particle

ABSTRACTMammalian reovirus virions undergo partial disassembly of the outer capsid upon exposure to proteases in vitro, producing infectious subvirion particles (ISVPs) that lack protein ?3 and contain protein µ1/µ1C as endoprotease-generated fragments µ1d/d and f. ISVPs are thought to be required for two early steps in reovirus infection: membrane penetration and activation of the particle-bound viral transcriptase complexes. Genetic and biochemical evidence implicates outer-capsid protein µ1 in both these steps. To determine whether the cleavage of µ1/µ1C is relevant to the unique properties of ISVPs, we analyzed the properties of novel subvirion particles that lacked ?3 yet retained µ1/µ1C in an uncleaved but cleavable form. These detergent-plus-protease subvirion particles (dpSVPs) were produced by treating virions with chymotrypsin in the presence of micelle-forming concentrations of alkyl sulfate detergents. Infections with dpSVPs in murine L or canine MDCK cells provided evidence that the cleavage of µ1/µ1C during viral entry into these cells is dispensable for reovirus infection. Additionally, dpSVPs behaved like ISVPs in their capacity to permeabilize lipid bilayers and to undergo transcriptase activation in vitro, supporting the conclusion that cleavage of µ1/µ1C to µ1d/d and f during viral entry is not required for either membrane penetration or transcriptase activation in cells. The capacity of alkyl sulfate detergents to inhibit the cleavage of µ1/µ1C in a reversible fashion suggests a specific association between virus particle and detergent micelles that may mimic virus particle-phospholipid membrane interactions during reovirus entry into cells.