Interactions among red cell membrane proteins

Interactions between human red band 2.1 with spectrin and depleted inside-out vesicles were studied by fluorescence resonance energy transfer and batch microcalorimetry. The band 2.1-spectrin binding isotherm is consistent with a one to one mole ratio. The association constant of 1.4 X 10(8) M-1 corresponds to the association free energy of -11.1 kcal/mol. Under our experimental conditions, the enthalpy of interaction of band 2.1-spectrin was found to be -10.8 kcal/mol and is independent of the protein mole ratio. The calculated entropic factor (-T delta S = 0.3 kcal/mol) strongly suggests a predominantly enthalpic character of the reaction. In addition, we investigated the role of band 2.1 on the binding of band 4.1 to spectrin [Podgorski, A., & Elbaum, D. (1985) Biochemistry 24, 7871-7876] and concluded that only small, if any, alterations of binding of band 4.1 to spectrin have taken place in the presence or absence of band 2.1. This suggests thermodynamic independence of the binding sites. Although the attachment of the cytoskeletal network to the membrane takes place through, at least, two different interactions, band 2.1-band 3 and 4.1-glycophorin, the relative enthalpy values suggest that band 2.1 contributes significantly more than band 4.1 to the energy of the interaction. In addition, we observed that polymerization of actin is modulated by the cytoskeletons as judged by their effect on the rate of actin polymerization.