Epidermal growth factor receptor transmembrane domain:: 2H NMR implications for orientation and motion in a bilayer environment

As part of a study of receptor tyrosine kinase behavior in membranes, we have collected extensive NMR data from three well-defined probe locations within the transmembrane region of the human EGF receptor. Spectra were obtained for selectively deuterated alanine residues in a series of peptides corresponding to the putative transmembrane domain (with short extramembranous extensions). Peptides were incorporated into fluid unsonicated liposomes of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and POPC containing 33 mol % cholesterol to mimic common lipid composition of cell plasma membranes. The peptide concentration was in the range of 1-6 mol % relative to that of phospholipid. Data acquired at 35 degrees C have been analyzed quantitatively to determine their implications to receptor spatial orientation and dynamics. If it is presumed that the single transmembrane portion approximates an a-helix of 3.6 residues per turn, this helix was found to be tilted away from the membrane perpendicular, about which there was rapid axial diffusion. However, rotation about the peptide long axis was static on the NMR time scale of 10(-4) s, and the peptide appeared to have a preferred direction(s) of lean. The results for this peptide, whose hydrophobic length is greater than the membrane hydrophobic thickness, were very similar between membranes of POPC and membranes of POPC containing 33 mol % cholesterol, despite considerable host matrix differences in thickness and order. Allowed values of peptide tilt occupied a narrow range: between 10 and 14 degrees in POPC and between 10 and 12 degrees in POPC/cholesterol. Although the existence of some preferred direction(s) of lean was demanded by the results, the direction of lean was not uniquely determined. We have interpreted these results, which were essentially unchanged at 65 OC, as reflecting the behavior of peptide monomers undergoing rapidly reversible peptide-peptide interactions. For transmembrane monomers, interference with rotation about the peptide long axis might be understood to arise from an energy benefit tin a tilted peptide) to prevention of particular amino acid side chains near the membrane surfaces from moving in and out of hydrophobic or hydrophilic environments. It will be desirable to test the conclusion of preferential lean of a monomeric receptor since such behavior could provide a mechanism for modulating monomer association with other species (i.e., signal transduction).