The alpha-helical transmembrane (TM) domains of single-pass membrane proteins can engage in specific interactions that are central to the structure and activity of cell-surface receptors governing adhesion and signalling pathways. Detailed atomic structures of the TM complexes underlying these functions exist for only a small number of proteins and derive mostly from solution NMR studies using detergent micelles or small isotropic bicelles to approximate the lipid bilayer. Despite significant advances in crystallographic techniques for polytopic membrane proteins such as GPCRs, no structures of single-pass alpha-helical TM complexes crystallised in a lipid bilayer have previously been reported. In fact, TM peptides are generally considered poor candidates for crystallisation due to their predominantly hydrophobic nature and lack of structured extramembranous domains to make crystal contacts. Here we describe two structures containing only the alpha-helical TM domain of the immunoreceptor signalling component DAP12, which was crystallised using lipidic cubic phase (LCP) media. Previous biochemical^1-3 and solution NMR⁴ studies demonstrated that DAP12 forms a disulfide-linked homodimer in the membrane and assembles with ligand-binding subunits through polar contacts within their TM domains. In a monoolein LCP environment, DAP12-TM peptides crystallised in parallel trimeric and tetrameric arrangements around the key aspartic acid-threonine motifs, yielding structures to 1.77 Å and 2.14 Å resolution, respectively. In place of the positively charged lysine side-chain normally contributed by an associated receptor TM domain, the polar centres in these structures contain coordinated cations obtained from precipitant solutions. We found that these oligomeric structures form in the endoplasmic reticulum of DAP12-expressing cells during natural biosynthesis of the full-length protein and represent either receptor assembly intermediates or a competing folding pathway. Thus we demonstrate for the first time that eukaryotic single-pass TM peptide complexes can be crystallised in LCP membranes to yield high-resolution structures. In this instance, the unexpected oligomeric forms that crystallised predicted the existence of previously unappreciated cellular products, establishing a close parallel between the behaviour of TM peptides in LCP and the native, full-length protein within the confines of a cellular lipid bilayer.