A, DR2-MBP-P4* (160 nm) was photocleaved for 10 min on ice in the presence (red line) or absence (blue line) of MBP-488 (220 nm). When peptide was present during photocleavage, the reaction proceeded rapidly (10 min on ice) to ~100 mP, whereas peptide association was slower when the peptide was added following photocleavage (<100 FP after 50 min). Time 0 is defined by the addition of MBP-488 following photocleavage (blue line). B, dose-dependent acceleration of peptide association to empty DR2. DM was present at different concentrations (150 nm to 5 µm) during photocleavage (10 min, 160 nm DR2-MBP-P4*), and MBP-488 was added (330 nm) after photocleavage. Saturation was not observed because of the lower affinity of soluble DM for soluble DR compared with full-length molecules (42).