Accumulates harmlessly in the cytoplasmic membrane until it reaches a critical concentration that triggers the formation of nanometer-scale pores (pinholes) causing host cell membrane depolarization and endolysin refolding and release into the periplasmic space (PubMed:19861547, PubMed:17827300, PubMed:23671069). Once the pinholin has permeabilized the host cell membrane, the SAR-endolysin is released into the periplasm and breaks down the peptidoglycan layer (PubMed:19861547, PubMed:17827300). Determines the precise timing of host cell lysis (PubMed:23671069). Participates with the SAR-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 (Probable) (PubMed:17827300). Counteracts the aggregation of the holin molecules and thus of pore formation. Homoheptamer; forms small heptameric channels of about 2 nm (pinholes) (PubMed:19861547). Interacts with isoform Antiholin; this interaction blocks the holin homomultimerization and delays host cell lysis (Probable). Classified as a class II holin. The N-terminal transmembrane domain seems to be externalized during the pinhole formation. Antiholin and Holin ratio is about 1:2.