1.E.27 The BhlA Holin (BhlA Holin) Family A 71 aa BhlA bacteriocin-related holin-like peptide with 1 N-terminal TMS is encoded upstream of a gene encoding an 86 aa holin-like peptide, XhlB in the genome of Bacillus pumilus strain WAPB4 (Aunpad and Panbangred 2012). XhlB is composed of two putative transmembrane domains separated by a β-turn and numerous charged residues in the C-terminus. Dual start motifs were found in both BhlA and XhlB. To analyze the effect of BhlA on bacteria cells, its ORF was cloned and expressed in Escherichia coli. Expression of BhlA was toxic, and the site of action was in the cell membrane, causing death by membrane disruption as demonstrated by transmission electron microscopy (Aunpad and Panbangred 2012).
References:
Anthony, T., G.S. Chellappa, T. Rajesh, and P. Gunasekaran. (2010). Functional analysis of a putative holin-like peptide-coding gene in the genome of Bacillus licheniformis AnBa9. Arch. Microbiol. 192: 51-56.
Aunpad, R. and W. Panbangred. (2012). Evidence for two putative holin-like peptides encoding genes of Bacillus pumilus strain WAPB4. Curr. Microbiol. 64: 343-348.
Brüser, T. and D. Mehner-Breitfeld. (2022). Occurrence and potential mechanism of holin-mediated non-lytic protein translocation in bacteria. Microb Cell 9: 159-173.
Examples:
TC#	Name	Organismal Type	Example
1.E.27.1.1	holin BhlA (70 aas). A member of the DUF2762 family. Its holin function has been demonstrated (Aunpad and Panbangred 2012).	Firmicutes	BhlA of Bacillus pumilus

1.E.27.1.2	Bacillus subtilis phage SP beta-holin-like protein (70 aas).	Viruses	Phage SP beta-holin

1.E.27.1.3	Phage-like protein (80 aas).	Firmicutes	Phage-like protein of Clostridium botulinum

1.E.27.1.4	*Holin protein BhlA (Anthony et al.* 2010).**	Firmicutes	BhlA of Bacillus licheniformis

1.E.27.1.5	Bacteriocin UviB of 64 aas. This protein is in the DUF2762 Superfamily.		UviB of Clostridium perfringens

1.E.27.1.6	Uncharacterized protein of 76 aas		UP of Bacillus cereus

1.E.27.1.7	Uncharacterized protein of 78 aas		UP of Bacillus thuringiensis

1.E.27.1.8	Plasmid-encoded UviB-like holin of 65 aas with 1 TMS, TpeE. This protein mediates non-lytic protein translocation (Brüser and Mehner-Breitfeld 2022). Holins generate large membrane lesions that permit the passage of endolysins across the cytoplasmic membrane of prokaryotes, ultimately resulting in cell wall degradation and cell lysis. However, there are examples known for non-lytic holin-dependent secretion of proteins by bacteria, indicating that holins may transport proteins without causing large membrane lesions. Phage-derived holins can be used for a non-lytic endolysin translocation to permeabilize the cell wall for the passage of secreted proteins. In addition, clostridia, which do not possess the Tat pathway for transport of folded proteins, most likely employ non-lytic holin-mediated transport also for secretion of toxins and bacteriocins that are incompatible with the general Sec pathway. The small holin TpeE mediates non-lytic toxin secretion in Clostridium perfringens. TpeE contains only one short transmembrane helix that is followed by an amphipathic helix, which is reminiscent of TatA, the membrane-permeabilizing component of the Tat translocon for folded proteins. Brüser and Mehner-Breitfeld 2022 reviewed the known cases of non-lytic holin-mediated transport and then focus on the structural and functional comparison of TatA and TpeE, resulting in a mechanistic model for holin-mediated transport. This model is strongly supported by a so far not recognized naturally occurring holin-endolysin fusion protein.