Packaging-competent capsids of a herpes simplex virus temperature-sensitive mutant have properties similar to those of in vitro-assembled procapsids

Newcomb and coworkers (W.W. Newcomb, F. L. Homa, D. R, Thomsen, F. P, Booy, B. L. Trus, A. C. Steven, J. V. Spencer; and J. C. Brown, J, Mol, Biol. 26 3:432-446, 1996; W. W. Newcomb, F. L, Homa, D, R. Thomsen, Z. Ye, and J. C. Brown, J. Virol. 68:6059-6063, 1994) have recently described an in vitro h erpes simplex virus (HSV) capsid assembly product which, because of certain parallels between its properties and those of bacteriophage proheads, they have designated the procapsid. As in their bacteriophage counterparts, the re are marked differences between the structures of the two types of partic le, and conversion from the procapsid to the capsid form requires extensive reconfiguration of the subunits. This reconfiguration occurs spontaneously upon extended in vitro incubation. One of the distinctive features of the HSV procapsids is that, unlike mature capsids, they are unstable and disass emble upon storage at 2 degrees C. Using a mutant of HSV type 1 (ts1201), w hich has a lesion in the protease responsible for maturational cleavage of the scaffolding protein, we have demonstrated that capsids present within c ells infected at nonpermissive temperatures are also cryosensitive and disa ppear if the cells are incubated at 0 degrees C, This suggests that ts1201 capsids may resemble procapsids in structure. However, ts1201 capsids remai n cryosensitive following extended incubation at an elevated temperature an d, therefore, do not appear to undergo the spontaneous reconfiguration seen with in vitro-assembled procapsids. The lesion in ts1201 is reversible, an d capsids formed at the nonpermissive temperature can undergo maturational cleavage and go on to form infectious virions following downshift to permis sive temperatures. The sensitivity of ts1201 capsids to low temperatures is closely correlated with the cleavage status of the scaffolding protein, su ggesting that proteolysis may act to trigger their conversion to the stable form. The experiments described here provide the firmest evidence yet that the procapsid has a biologically relevant role in the virus life cycle.