The downstream prion-like Doppel (Dpl) protein is a homologue related to the prion protein (PrP). Dpl is expressed in the brains of mice that do not express PrP, and Dpl is known to be toxic to neurons. One mode of toxicity has been suggested to involve direct membrane interactions. PrP under certain conditions of cell trafficking retains an uncleaved signal peptide, which may also hold for the much less studied Dpl. For a peptide with a sequence derived from the N-terminal part (1-30) of mouse Dpl (mDpl(1-30)) CD spectroscopy shows about 40% alpha-helical structure in DHPC and SDS micelles. In aqueous solution it is mostly a random coil. The three-dimensional solution structure was determined by NMR for mDpl(1-30) associated with DHPC micelles. 2D 1H NMR spectra of the peptide in q = 0.25 DMPC/DHPC bicelles only showed signals from the unstructured termini, indicating that the structured part of the peptide resides within the lipid bilayer. Together with 2H2O exchange data in the DHPC micelle solvent, these results show an alpha-helix protected from solvent exchange between residues 7 and 19, and suggest that the alpha-helical segment can adopt a transmembrane localization also in a membrane. Leakage studies with entrapped calcein in large unilamellar phospholipid vesicles showed that the peptide is almost as membrane perturbing as melittin, known to form pores in membranes. The results suggest a possible channel formation mechanism for the unprocessed Dpl protein, which may be related to toxicity through direct cell membrane interaction and damage.