Insight into the Thermodynamics and Equilibrium Kinetics of the Interaction between Transmembrane α-Helices in the Membrane Domain of ErbB4

Abstract

Interactions between the transmembrane (TM) α-helices in the membrane domains (MD) of integral membrane proteins are believed to determine their spatial structure and functionality. Nevertheless, the basic principles underlying such interactions still need to be elucidated. In the present work, the interaction between the TM segments in two-helical MD of the receptor tyrosine kinase ErbB4 was investigated by means of solution NMR spectroscopy in lipidic bicelles. According to the obtained data, the α-helical TM domains of the receptor associate via the double motif A655GxxGG660, forming the parallel dimer, stabilized by the polar contacts. The slow character of dimer-monomer transition of the ErbB4 TM domains permitted to access the thermodynamics and equilibrium kinetics of the dimerization. Lipidic bicelles appeared to be an ideal solvent in terms of dimer-monomer equilibrium of the ErbB4 TM domains, which allowed to measure the free energy of their dimerization equal to −1.4 kcal/mol. Noteworthy, the dimerization constant began to increase dramatically when more than one peptide on average were induced to reside in one bicelle (“saturated bicelles”). Enthalpy, entropy and heat capacity of the dimerization were obtained from the temperature dependence of the dimerization constant in “saturated bicelles”. As was shown, the dimerization of TM domains of ErbB4 is an endothermic process, going with the substantial growth of entropy and heat capacity of the system, suggesting the important role of lipids in the TM helix-helix interactions. The temperature dependence of linewidths of NMR signals was interpreted in terms of the reaction rates and the transition state theory to derive the free energy, entropy and enthalpy of the transition state and to estimate the contribution from the lipid-protein and protein-protein interactions into the free energy of dimerization.