Phage_holin_2_1 is a family of small hydrophobic holin proteins with one or more transmembrane domains. Members of this family fall into the holin superfamily II, and Phage 21 S holin is the prototype for this superfamily. It has two transmembrane segments with both the N- and C-termini on the cytoplasmic side of the inner membrane in E. coli. Holins are a diverse family of proteins that cause bacterial membrane lysis during late-protein synthesis. It is thought that the temporal precision of holin-mediated lysis may occur through the build up of a holin oligomer which causes the lysis [1,2].

1: Non-specific hole formation in the Escherichia coli inner membrane by lambda S proteins in independent of cellular secY and secA functions and of the proportion of membrane acidic phospholipids. Rietsch A, Blasi U; FEMS Microbiol Lett 1993;107:101-105. PMID:8467992

2: The pinholin of lambdoid phage 21: control of lysis by membrane depolarization. Park T, Struck DK, Dankenbring CA, Young R; J Bacteriol. 2007;189:9135-9139. PMID:17827300