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Red Blood Cell Invasion by Plasmodium vivax: Structural Basis for DBP Engagement of DARC

Figure 3

Isothermal titration calorimetry reveals step-wise binding of DARC to DBP-RII in solution.

(A) A biphasic binding profile is observed indicating the formation of the heterotrimer at a molar ratio of 0.5 (n1 = 0.44±0.02, Kd1 = 2183±125 nM, ΔH1 = −2663±69 cal/mol) and heterotetramer at a molar ratio of 1 (n2 = 0.50±0.02, Kd2 = 88.5±26.6 nM, ΔH2 = −3338±23 cal/mol). The fit to the two independent site binding model is shown as a red line. Molar ratios are expressed as monomers of DBP-RII. Open circles denote unbound DBP, closed circles denote bound DBP. Titration of (B) PBS into DBP and (C) DARC into PBS reveals no observable profiles demonstrating the biphasic profile is due to DARC binding to DBP. In all cases, the top panel contains raw binding data, and the bottom panel changes in enthalpy associated with binding.

Figure 3