FIG. 3.
DTT converts MMLV particles produced in DIBA-2 from immature to mature morphology. Virus particles were collected onto beads by immunoprecipitation as follows. After incubation in the presence or absence of DTT as for Fig. 2, virus was diluted with PBS and mixed with goat anti-MMLV gp70SU plus protein A-Sepharose beads (Amersham). The mixture was gently rotated overnight at 4°C. The beads were then washed three times with PBS, pelleted, and fixed with 2.5% glutaraldehyde plus 4% paraformaldehyde in PBS. (A) Immature particles produced by transient transfection of 293T cells with a mutant MMLV genome containing the D32L mutation at the active site of PR (6). (B) Wild-type particles, most of which are morphologically mature. (C) Particles produced in DIBA-2, most of which are morphologically immature. (D) Particles produced in DIBA-2 and treated with DTT before attachment to beads. The majority of these particles are morphologically mature. Scale bar = 100 nm.