Oligomerization of the EGF receptor transmembrane domain: a 2H NMR study in lipid bilayers

During the course of a previous study by wideline 2H NMR, we noted spectral features suggesting the possibility of monitoring homodimer/oligomer interactions between transmembrane domains of the EGF receptor in lipid bilayers [Rigby, A. R., Shaw, G. S., Barber, K. R., & Grant, C. W. M. (1996) Biochemistry 35, 12591-12601]. In the present work this possibility was explored using the 34-residue peptide EGFRtm. The peptide sequence included the 23 amino acid hydrophobic stretch thought to span the membrane (Ile622-Met644 of the EGF receptor), plus the first 10 amino acids of the receptor's cytoplasmic domain (Arg645-Thr654). Selective deuteration was carried out at sites corresponding to Ala623, Met644, and Val650. Samples were studied from 12 to 65 degrees C by 2H NMR in fluid membranes having low peptide concentration (1 mol %) or high peptide concentration (6 mol %). Methyl groups proved to be technically particularly attractive probe locations. Reversible homodimer/oligomer interactions were detected in membranes of the common natural phospholipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), without cholesterol. Effects on the EGF receptor transmembrane domain included alterations in peptide backbone motional order and/or conformation at the site of Ala623 within the membrane, and alterations in motional properties of the Val650 side chain in the cytoplasmic domain. There was little spectral evidence of stable oligomer formation except at the lowest temperature studied. Addition of 33% cholesterol to these membranes was accompanied by spectral changes consistent with the formation of more stable peptide oligomers, and by evidence that peptide-peptide interactions were sensed at all three probe locations. Peptide-peptide interactions remained easily reversible, particularly at higher temperatures. Freeze-fracture electron microscopy of the NMR samples demonstrated peptide-related intramembranous particles traversing the membranes. To our knowledge, this is the first electron microscopy description of receptor tyrosine kinases or their fragments in model membranes. In the presence of cholesterol, the peptide-related particles were generally larger, more sharply demarcated, and showed a tendency to cluster. These observations relate to models of receptor lateral association as an aspect of signal transduction, and to forces that may determine protein sorting and organization in cell membranes. We suggest that the cholesterol effects reflect a general phenomenon rather than one specific to the EGF receptor.