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1.E.1 The P21 Holin S (P21 Holin) Family

Phage 21 S holin is the prototype for class II holins. It has two TMSs with both the N- and C-termini on the cytoplasmic side of the inner membrane of E. coli (Gründling et al. 2000).  As for other holins, it functions in the export of an endolysin, but the holin channel also allows release of small ions and metabolites (Park et al. 2006).  TMS1 may be dispensable for function.  

A homologue of the P21 holin is the holin of bacteriophage H-19B. The gene encoding it has been associated with the Shiga-like Toxin I gene in E. coli (Neely and Friedman, 1998). It may function in toxin export as has been proposed for the X. nematophila holin-1 (Brillard et al., 2003; TC #1.E.2.1.4).

This family belongs to the: Holin Superfamily II.

References associated with 1.E.1 family:

Brillard, J., M.-H. Boyer-Giglio, N. Boemare, and A. Givaudan. (2003). Holin locus characterisation from lysogenic Xenorhabdus nematophila and its involvement in Escherichia coli SheA haemolytic phenotype. FEMS Microbiol. Lett. 218: 107-113. 12583905
Govorunova, E.G., O.A. Sineshchekov, R. Hemmati, R. Janz, O. Morelle, M. Melkonian, G.K. Wong, and J.L. Spudich. (2018). Extending the Time Domain of Silencing with Cryptophyte Anion Channelrhodopsins. eNeuro 5:. 30027111
Gründling, A., U. Bläsi, and R. Young. (2000). Biochemical and genetic evidence for three transmembrane domains in the class I holin, lambda S. J. Biol. Chem. 275: 769-776. 10625606
Neely, M.N. and D.I. Friedman. (1998). Functional and genetic analysis of regulatory regions of coliphage H-19B: location of shiga-like toxin and lysis genes suggest a role for phage functions in toxin release. Mol. Microbiol. 28: 1255-1267. 9680214
Pang, T., C.G. Savva, K.G. Fleming, D.K. Struck, and R. Young. (2009). Structure of the lethal phage pinhole. Proc. Natl. Acad. Sci. USA 106: 18966-18971. 19861547
Park, T., D.K. Struck, C.A. Dankenbring, and R. Young. (2007). The pinholin of lambdoid phage 21: control of lysis by membrane depolarization. J. Bacteriol. 189(24):9135-9139. 17827300
Park, T., D.K. Struck, J.F. Deaton, and R. Young. (2006). Topological dynamics of holins in programmed bacterial lysis. Proc. Natl. Acad. Sci. USA 103: 19713-19718. 17172454
Srividhya, K.V. and S. Krishnaswamy. (2007). Subclassification and targeted characterization of prophage-encoded two-component cell lysis cassette. J Biosci 32: 979-990. 17914239