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 and spanin proteins in the sequential events which lead to the programmed host cell lysis releasing the mature viral particles from the host cell. Homomultimer. Interacts (via C-terminus) with antiholin (via C-terminus); this interaction blocks the holin homomultimerization and delays host cell lysis. Interacts (via N-terminus) with the lysis inhibition accessory protein rIII; this interaction stabilizes the holin-antiholin complex thereby resulting in a robust block of the hole formation. Classified as a class III holin. Disulfide bond is required for functionality. Belongs to the T4likevirus holin family.