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Structure-function studies with mammalian reoviruses have been limited by the lack of a reverse-genetic system for engineering mutations into the viral genome. To circumvent this limitation in a partial way for the major outer-capsid protein ς3, we obtained in vitro assembly of large numbers of virion-like particles by binding baculovirus-expressed ς3 protein to infectious subvirion particles (ISVPs) that lack ς3. A level of ς3 binding approaching 100% of that in native virions was routinely achieved. The ς3 coat in these recoated ISVPs (rcISVPs) appeared very similar to that in virions by electron microscopy and three-dimensional image reconstruction. rcISVPs retained full infectivity in murine L cells, allowing their use to study ς3 functions in virus entry. Upon infection, rcISVPs behaved identically to virions in showing an extended lag phase prior to exponential growth and in being inhibited from entering cells by either the weak base NH4Cl or the cysteine proteinase inhibitor E-64. rcISVPs also mimicked virions in being incapable of in vitro activation to mediate lysis of erythrocytes and transcription of the viral mRNAs. Last, rcISVPs behaved like virions in showing minor loss of infectivity at 52°C. Since rcISVPs contain virion-like levels of ς3 but contain outer-capsid protein μ1/μ1C mostly cleaved at the δ-φ junction as in ISVPs, the fact that rcISVPs behaved like virions (and not ISVPs) in all of the assays that we performed suggests that ς3, and not the δ-φ cleavage of μ1/μ1C, determines the observed differences in behavior between virions and ISVPs. To demonstrate the applicability of rcISVPs for genetic studies of protein functions in reovirus entry (an approach that we call recoating genetics), we used chimeric ς3 proteins to localize the primary determinants of a strain-dependent difference in ς3 cleavage rate to a carboxy-terminal region of the ISVP-bound protein.

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