FIGURE 4.

Tighter folding of the substrate decreases the rate of degradation but not of ATP consumption. A, Ub₅-DHFR (∼40 nm) degradation by WT mouse 26 S proteasomes (1 nm) was measured in the presence of 25 μm folic acid or methotrexate (MTA). B, ATP hydrolysis by the 26 S proteasome was measured under the same conditions as described for A. ATPase activity without the addition of substrate was taken as 100%. C, Ub₅-DHFR degradation (∼40 nm) by WT mouse 26 S proteasomes (1 nm) was measured in the presence of 2 mm ATP with and without the addition of 37.5 μm folic acid or 500 μm ATPγS. TCA-soluble radioactivity was measured after a 30-min incubation.