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 (Probable). Determines the precise timing of host cell lysis (PubMed:32477303). 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 (PubMed:26471254). Homomultimer (PubMed:26701742). Self-associates to form a pore (PubMed:26701742). 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 (PubMed:26701742). The coiled coil region is required for host cell lysis and for cytotoxic activity (PubMed:33627396, PubMed:26471254). The C-terminus determines the size of the hole (PubMed:26471254). Belongs to the Mycobacterium phage D29 holin family.