Activating receptors in cells of hematopoetic origin include members of two unrelated protein families, the immunoglobulin (Ig) and C type lectins, which differ even in the orientation of the transmembrane (TM) domains. We examined assembly of four receptors with diverse function: the NK receptors KIR2DS and NKG2C/CD94, the Fc receptor for IgA, and the GPVI collagen receptor. For each of the four different receptors studied here, assembly results in the formation of a three-helix interface in the membrane involving two acidic TM residues from the signaling dimer and a basic TM residue from the ligand recognition module, an arrangement remarkably similar to the T cell receptor (TCR)-CD3 complex. The fact that the TM domains of Ig family and C type lectins adopt opposite orientations proves that these receptor families independently evolved toward the same structural arrangement of the interacting TM helices. This assembly mechanism is thus widely utilized by receptors in cells of hematopoetic origin.