Significance With growing antibiotic resistance in bacteria, understanding the lytic cycle of bacteriophages can help developing phage therapy. In the double-stranded DNA bacteriophage φ21, the integral membrane protein pinholin S ²¹ 68 triggers lysis by forming small “pinholes.” It has been proposed that the second transmembrane helix (TMD2) is sufficient to form these pores, which we could confirm in a reconstituted model system, showing a pore diameter of about 2 nm. Using NMR, we determined the structure of TMD2 as it is embedded and aligned in the membrane, revealing a right-handed glycine zipper motif in a prime position for homo- and heteromeric helix–helix interactions. Molecular models are derived from these data to illustrate the assembly of the pinholes via inactive and active dimers.