Figure 2.

Varying dissociation rates of the class I peptides do not affect the β₂m exchange rate. Either purified K^b/PolyI complex (A and B) or K^b/PolyG complex (C and D) were incubated with exogenous ¹²⁵I-β₂m (complex β₂m/¹²⁵I-β₂m; 1:1) and with K6 competitor peptide (complex peptide/competitor; 1:>200) in PBS at 37°C. Aliquots were withdrawn at the specified times and analyzed by IEF gel chromatography and visualized by Coomassie staining and by phosphoimage detection of the same gel. (A) The densities of the Coomassie-stained bands for K^b/PolyI complex (▪) and the K^b/K6 complex (•) illustrate K6 peptide exchange over the incubation time; and the t_1/2 for the dissociation of PolyI peptide from K^b/PolyI complex is indicated to be 6 hr. (B) The phosphoimage intensities for ¹²⁵I-β₂m, associated either to K^b/PolyI complex (□) or to K^b/K6 complex (○) are compared. The t_1/2 for the β₂m exchange onto the total amount of K^b/peptide complex is indicated to be ≈30 min (dotted line). (C) The densities of the Coomassie-stained bands for the K^b/PolyG complex (▪) and the K^b/K6 complex (•) illustrate K6 peptide exchange over the incubation time. The t_1/2 for the exchange of K6 onto K^b/PolyG complex could not be measured because of the instantaneous dissociation of the prebound PolyG peptide. (D) The phosphoimage intensities for ¹²⁵I-β₂m, associated either to K^b/PolyG complex (□) or to K^b/K6 (○) complex are compared. As the K^b/PolyG complex was instantaneously exchanged into K^b/K6 complex, the data reflect the exchange of labeled β₂m onto K^b/K6 complex with a t_1/2 of ≈30 min.