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. Participates in determining 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. Classified as a class II holin although it seems to have only one transmembrane domain. Belongs to the T7likevirus holin family.