Spatial structure and dimer--monomer equilibrium of the ErbB3 transmembrane domain in DPC micelles

In present work the interaction of two TM α-helices of the ErbB3 receptor tyrosine kinase from the ErbB or HER family (residues 639-670) was studied by means of NMR spectroscopy in a membrane-mimicking environment provided by the DPC micelles. The ErbB3 TM segment appeared to form a parallel symmetric dimer in a left-handed orientation. The interaction between TM spans is accomplished via the non-standard motif and is supported by apolar contacts of bulky side chains and by stacking of aromatic rings together with π-cation interactions of Phe and Arg side chains. The investigation of the dimer--monomer equilibrium revealed thermodynamic properties of the assembly and the presence of two distinct regimes of the dimerization at low and at high peptide/detergent ratio. It was found that the detergent in case of ErbB3 behaves not as an ideal solvent, thus affecting the dimer--monomer equilibrium. Such behavior may account for the problems occurring with the refolding and stability of multispan helical membrane proteins in detergent solutions. The example of ErbB3 allows us to conclude that the thermodynamic parameters of dimerization, measured in micelles for two different helical pairs, cannot be compared without the investigation of their dependence on detergent concentration.