Different DR2-peptide complexes (150 nm) were left unilluminated (−UV) or exposed to UV light (+UV) in the presence of MBP-488 (30 nm) for 2 min. Association of MBP-488 was followed by FP after photocleavage. A, rapid association of MBP-488 occurs upon UV irradiation of DR2-MBP-P4*, but not in control reactions. B and C, high affinity MBP85–99 peptide could not be displaced (B), and slow binding to DR2 molecules with the lower affinity CLIP peptide (C) was observed. D, the rate of MBP-488 association was compared for reactions with and without DM. DM was present during photocleavage at an equimolar concentration to DR2, and MBP-488 was added after photocleavage.