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 (By similarity). Determines the precise timing of host cell lysis (By similarity). 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 (By similarity). Isoform Antiholin: Counteracts the aggregation of the holin molecules and thus of pore formation. Homomultimer. Interacts with isoform Antiholin; this interaction blocks the holin homomultimerization and delays host cell lysis. Classified as a class I holin. Isoform Holin has 3 transmembrane regions whereas isoform Antiholin lacks the first transmembrane region.