P21 holin family

The Phage 21 S Family is a member of the Holin Superfamily II.
The Bacteriophage P21 Lysis protein S holin is the prototype for class II holins. Lysis S proteins have two transmembrane segments, with both the N- and C-termini on the cytoplasmic side of the inner membrane. 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. It may function in toxin export as has been proposed for the X. nematophila holin-1.
A representative list of proteins belonging to the P21 holin family can be found in the .