FIGURE 5.

Inhibition of the proteasome does not stabilize ricin B chain. A, protoplasts were transfected with plasmids encoding RTB or RTA and subjected to pulse-chase analysis as under “Experimental Procedures” but with immunoprecipitation from combined cell and medium samples. Where indicated, protoplasts were incubated with 80 μm clasto-lactacystin β-lactone during the pulse. Immunoprecipitates were subjected to analysis by SDS-PAGE and fluorography. Graphs show densitometric quantification of the RTB or RTA IPs made in the presence or absence of clasto-lactacystin β-lactone, expressed as percentages of the total RTB or RTA present at the end of the pulse, and are the average values from three independent experiments. Error bars indicate standard deviation. B, densitometric quantification of immunoprecipitates from cells treated with DMSO, 80 μm clasto-lactacystin β-lactone, 80 μm epoxomycin, or a combination of these. Data are expressed as percentages of the total RTB or RTA present at the end of the pulse. These show the average values from three independent experiments. Error bars indicate standard deviation.