Accumulates harmlessly in the cytoplasmic membrane until it reaches a critical concentration that triggers the formation of micron-scale pores (holes) causing host cell membrane disruption and endolysin escape into the periplasmic space. Determines the precise timing of host cell lysis. Participates with the endolysin protein in the sequential events which lead to the programmed host cell lysis releasing the mature viral particles from the host cell. Homomultimer. Self-associates to form a pore. The first transmembrane region undergoes a helix to beta-hairpin conformational change, which is responsible for its self-association and pore formation in the host membrane. The coiled coil region is required for host cell lysis and for cytotoxic activity. The C-terminus determines the size of the hole. Belongs to the Mycobacterium phage D29 holin family.