Membrane-spanning epidermal, growth factor receptor ErbB2 is of key importance in cell division, in which a dimeric complex of the protein is responsible for tyrosine kinase activation following ligand binding. The rat homologue of this receptor (Neu) is prone to a valine to glutamic acid mutation in the transmembrane domain (TM), resulting in permanent activation and oncogenesis. In this study, the TM domains of Neu and the corresponding oncogenic mutant Neu*, which contains a V to E mutation at position 664 in the TM domain, have been analyzed to improve our understanding of the structural effects of the oncogenic V664E mutation. Building on previous work, we have focused here on understanding the sequence dependence of TM helix-helix interactions and any differences in behavior upon introduction of the V664E mutation. Using a variety of biochemical and biophysical methods, we find that the rat Neu TM domain forms strong oligomers and, similar to previous observations for the human ErbB2 TM domain, the oncogenic mutation results in a reduced level of self-association. Our data also strongly indicate that the proto-oncogenic Neu TM domain, can adopt multiple (at least two) oligomeric conformations in the membrane, possibly corresponding to the active and inactive forms of the receptor, and can "switch" between the two. Further, the oncogenic Neu* mutant appears to inhibit this "conformational switching" of TM dimers, as we observe that dimerization of the Neu* TM domain in the Escherichia coli inner membrane strongly favors a single conformation stabilized by an IXXXV motif (I 659-XXX-V663) originally identified by site-specific infrared spectroscopic studies.