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SUMMARYEvidence is presented supporting the hypothesis that reovirus intermediate subviral particles (ISVP), which show increased infectivity relative to intact virions, can gain entry into host L cells by two alternative pathways. One pathway is by the process of viropexis, involving phagocytic vacuoles. A second entry pathway is via direct penetration of the plasma membrane of the cell, without involvement of a phagocytic vacuole. Using electron microscopy, a kinetic analysis of the uptake process was carried out. Results indicate that at 37 °C ISVP gain entry into host cells primarily by direct entry, although viropexis also occurs, while intact virions gain entry by viropexis almost exclusively. A second line of experimental evidence consistent with the idea that ISVP can 'melt' their way through the plasma membrane is provided by studies on the release of pre-loaded radioactive 51Cr from host cells following infection. 51Cr release data demonstrate that infection with ISVP leads to an immediate increased leakiness of the cell plasma membrane, whereas no such increase takes place following infection with an equivalent number of intact virions. This demonstrates that ISVP can interact with the plasma membrane of the cell in a manner which is qualitatively different from the interaction between intact virions and the plasma membrane.The ability of ISVP to directly penetrate the plasma membrane of the host cell, which intact virions apparently cannot do, could explain the decreased duration of the eclipse phase, as well as the increased infectivity of ISVP, relative to that observed for infection with intact virions.

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Reovirus virions, grown in suspension cultures of L cells and extensively purified by density gradient and velocity gradient centrifugation after their release from cell debris by fluorocarbon extraction, are characterized by a mean particle diameter of 73 nm and a density in CsCl of 1.36 to 1.37 g/cm 3 . Treatment of intact virions by chymotrypsin (CHT) digestion in vitro converts them to subviral particles (SVP) having characteristics which are determined by the species of monovalent cation present during the digestion. In the presence of Cs + ions, CHT converts the virions to SVP of mean diameter 51 nm and density 1.43 to 1.44 g/cm 3 . In the presence of K + ions, the conversion is to SVP of diameter 51 nm and density 1.39 to 1.40 g/cm 3 . The SVP made in the presence of either Cs + or K + possess an extremely active RNA polymerase and nucleoside triphosphate phosphohydrolase (NTPase) activity in vitro and are resistant to further digestion by CHT. Treatment of intact virions with CHT in the presence of Na + or Li + ions results in their conversion to SVP of mean diameter 64 nm and density 1.37 to 1.38 g/cm 3 . Such SVP are not active in in vitro RNA synthesis or NTP hydrolysis and are resistant to further digestion by CHT even during prolonged exposure to high concentrations of enzyme. Addition of Cs + or K + ions to the digestion mixture allows conversion of the 64-nm diameter SVP to 51-nm diameter SVP in which the RNA polymerase and NTPase are active in vitro. Analysis of the proteins present in intact virions and in the different SVP reveals clear differences which indicate that the conversions are accomplished by removal or cleavage of particular species of polypeptides.