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 (PubMed:18788120, PubMed:21187415). Determines the precise timing of host cell lysis (PubMed:21187415). 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 (PubMed:21187415). Counteracts the aggregation of the holin molecules and thus of pore formation. Homomultimer (PubMed:11029428). Interacts with isoform Antiholin; this interaction blocks the holin homomultimerization and delays host cell lysis (PubMed:11029427). Classified as a class I holin. Expressed in the late phase of the viral replicative cycle. Has 3 transmembrane regions whereas isoform Antiholin lacks the first transmembrane region. The C-terminus acts as a cytoplasmic regulatory region. Belongs to the lambdavirus holin family.