A CARBOXY-TERMINAL FRAGMENT OF PROTEIN MU-1/MU-1C IS PRESENT IN INFECTIOUS SUBVIRION PARTICLES OF MAMMALIAN REOVIRUSES AND IS PROPOSED TO HAVE A ROLE IN PENETRATION

Penetration of a cell membrane as an early event in infection of cells by mammalian reoviruses appears to require a particular type of viral particle, the infectious subvirion particle (ISVP), which is generated from an intact virion by proteolytic cleavage of the outer capsid proteins sigma3 and mul/mu1C. Characterizations of the structural components and properties of ISVPs are thus relevant to attempts to understand the mechanism of penetration by reoviruses. In this study, a novel, approximately 13-kDa carboxy-terminal fragment (given the name phi was found to be generated from protein mul/mu1C during in vitro treatments of virions with trypsin or chymotrypsin to yield ISVPs. With trypsin treatment, both the carboxy-terminal fragment phi and the amino-terminal fragment mu1delta/delta were shown to be generated and to remain attached to ISVPs in stoichiometric quantities. Sites of protease cleavage were identified in the deduced amino acid sequence of mu1 by determining the amino-terminal sequences of phi proteins: trypsin cleaves between arginine 584 and isoleucine 585, and chymotrypsin cleaves between tyrosine 581 and glycine 582. Findings in this study indicate that sequences in the phi portion of mul/mu1C may participate in the unique functions attributed to ISVPs. Notably, the delta-phi cleavage junction was predicted to be flanked by a pair of long amphipathic alpha-helices. These amphipathic alpha-helices, together with the myristoyl group at the extreme amino terminus of mu1/mu1N, are proposed to interact directly with the lipid bilayer of a cell membrane during penetration by mammalian reoviruses.