*1.E.19 The Clostridium difficile* TcdE Holin (TcdE Holin) Family** Toxigenic strains of C. difficile produce two large toxins (TcdA and TcdB) encoded within a pathogenicity locus. tcdE, encoded between tcdA and tcdB, encodes a 166 aa protein which causes death to E. coli when expressed, and the structure of TcdE resembles holins. TcdE acts on the bacterial membrane. Since TcdA and TcdB lack signal peptides, they may be released via TcdE either prior to or subsequent to cell lysis. Other Clostridial species bear homologues of TcdE. This family belongs to the Phage holin 4 superfamily. The transport reaction believed to be catalyzed by TcdE is: Toxin (in) → Toxin (out)
This family belongs to the Holin IV Superfamily .
References:
El Meouche, I. and J. Peltier. (2018). Toxin release mediated by the novel autolysin Cwp19 in. Microb Cell 5: 421-423.
Govind, R. and B. Dupuy. (2012). Secretion of Clostridium difficile toxins A and B requires the holin-like protein TcdE. PLoS Pathog 8: e1002727.
Govind, R., L. Fitzwater, and R. Nichols. (2015). Observations on the Role of TcdE Isoforms in Clostridium difficile Toxin Secretion. J. Bacteriol. 197: 2600-2609.
Jin, M., T. Ye, and X. Zhang. (2013). Roles of bacteriophage GVE2 endolysin in host lysis at high temperatures. Microbiology 159: 1597-1605.
Kong M. and Ryu S. (2015). Bacteriophage PBC1 and its endolysin as an antimicrobial agent against Bacillus cereus. Appl Environ Microbiol. 81(7):2274-83.
Mehner-Breitfeld, D., C. Rathmann, T. Riedel, I. Just, R. Gerhard, J. Overmann, and T. Brüser. (2018). Evidence for an Adaptation of a Phage-Derived Holin/Endolysin System to Toxin Transport in. Front Microbiol 9: 2446.
Sirigi Reddy, A.R., B.P. Girinathan, R. Zapotocny, and R. Govind. (2013). Identification and characterization of Clostridium sordellii toxin gene regulator. J. Bacteriol. 195: 4246-4254.
Takác, M., A. Witte, and U. Bläsi. (2005). Functional analysis of the lysis genes of Staphylococcus aureus phage P68 in Escherichia coli. Microbiology 151: 2331-2342.
Tan, K.S., B.Y. Wee, and K.P. Song. (2001). Evidence for holin function of tcdE gene in the pathogenicity of Clostridium difficile. J. Med. Microbiol. 50: 613-619.
Zimmer, M., N. Vukov, S. Scherer, and M.J. Loessner. (2002). The murein hydrolase of the bacteriophage phi3626 dual lysis system is active against all tested Clostridium perfringens strains. Appl. Environ. Microbiol. 68: 5311-5317.
Examples:
TC#	Name	Organismal Type	Example
1.E.19.1.1	Holin (161 aas)	Actinobacteria	Holin of Mobiluncus mulieris

1.E.19.1.10	Phage-related holin of 142 aas and 4 TMSs	Tenericutes	Holin of Acholeplasma laidlawii

1.E.19.1.11	Holin of 141 aas and 2 TMSs	Chloroflexi	Holin of Dehalococcoides ethenogenes

1.E.19.1.12	*Holin of 137 aas and 2 TMSs (Jin et al.* 2013).**	Firmicutes	*Holin of Geobacillus* phage GBSV1**

1.E.19.1.13	Toxin secretion/holin system; holin of 124 aas	Tenericutes	*Holin of Mycoplasma* sp.**

1.E.19.1.14	Putative holin of 142 aas	Chloroflexi	Putative holin of Herpetosiphon aurantiacus

1.E.19.1.15	Putative holin of 141 aas and 3 TMSs	Bacteroidetes	Putative holin of Alistipes shahii

1.E.19.1.16	Holin of 138 aas and 3 TMSs (Kong and Ryu 2015).	Firmicute phage	*Holin of Bacillus* phage PBC1**

1.E.19.1.17	Holin of 139 aas and 2 TMSs.		Holin of Bacteroides fragilis

1.E.19.1.18	Holin of a tactivirus of 131 aas and 3 TMSs.		Holin of Tectiviridae sp.

1.E.19.1.2	Uncharacterized protein	Firmicutes	Uncharacterized protein of Lachnospiraceae bacterium

1.E.19.1.3	Putative holin of 152 aas and 3 TMSs	Firmicutes	Putative holin of Brevibacillus brevis

1.E.19.1.4	Uncharacterized protein	Firmicute viruses	*Uncharacterized protein of Bacillus* phage IEBH**

1.E.19.1.5	Uncharacterized protein (142 aas)	Firmicutes	Uncharacterized protein of Clostridium bolteae

1.E.19.1.6	Holin	Actinobacteria	Holin of Corynebacterium diphtheriae

1.E.19.1.7	Putative holin of 139 aas nd 3 TMSs	Thermatogae	Putative holin of Petrotoga mobilis

1.E.19.1.8	Prophage lambda Sa04 protein	Firmicutes	Sa04 protein of Megasphaera elsdenii

1.E.19.1.9	Holin (129 aas)	Fusobacteria	Fusobacterium sp. ZP-0

Examples:
TC#	Name	Organismal Type	Example
1.E.19.2.1	Holin of 170 aas and 3 TMSs	Firmicutes	Holin of Macrococcus caseolyticus

1.E.19.2.2	Staphylococcus aureus phage P68 holin, hol15 (Takác et al. 2005).	Firmicute viruses	Hol15 of Staphylococcal phage P68

1.E.19.2.3	phage-like holin, YqxH1 (135 aas)	Firmicutes	YqxH1 of Bacillus amyloliquefaciens

Examples:
TC#	Name	Organismal Type	Example
1.E.19.3.1	The C. difficile holin, TcdE. TcdE acts as a holin-like protein to facilitate the release of C. difficile toxins A and B to the extracellular environment, but, unlike most phage holins, it does not cause the non-specific release of cytosolic contents. TcdE is thus a bacterial holin that releases toxins into the environment by a phage-like system (Tan et al. 2001; Govind and Dupuy 2012). Different isoforms have differing activities in toxin release (Govind et al. 2015). An autolysin, Cwp19 (L7PGA3), may also play a role in toxin release (El Meouche and Peltier 2018). Adaptation of this phage-derived holin/endolysin system to toxin transport has been discussed (Mehner-Breitfeld et al. 2018). Thus, TcdE exists as three isoforms, differing with respect to their N-terminal extensions. The longest TcdE isoform has a moderate effect on cell growth, whereas the shortest isoform strongly induces lysis. The effect of the longest isoform was inhibitory for cell lysis, implying a regulatory function of the N-terminal 24 residues. Mehner-Breitfeld et al. 2018 analyzed the PaLoc sequence of 44 C. difficile isolates and found that four of these encode only the short TcdE isoform, and the most closely related holins from C. difficile phages only possess one of these initiation codons, indicating that the N-terminal extensions of TcdE evolved in C. difficile.	Firmicutes	TcdE of Clostridium difficile

1.E.19.3.2	*Holin of 141 aas and 3 TMSs, TcsE (Sirigi Reddy et al.* 2013).**	Firmicutes	TcsE of Clostridium sordellii

Examples:
TC#	Name	Organismal Type	Example
1.E.19.4.1	Holin of Clostridium phage phi3626. The function has been demonstrated (Zimmer et al. 2002). The charge distribution of this 2 TMS protein suggestions that the β-turn between TMSs 1 and 2, with 7 lys and arg residues, is in the cytoplasm and the N- and C-termini are in the periplasm (Zimmer et al. 2002).	Firmicute viruses	*Holin of Clostridium* phage phi3626**

1.E.19.4.2	Toxin secretion holin of 137 aas and 3 TMSs.	Actinobacteria	Holin of Atopobium minutum

1.E.19.4.3	Putative holin of 158 aas and 3 TMSs.	Actinobacteria	Putative holin of Gardnerella vaginalis

1.E.19.4.4	Uncharacterized holin of 161 aas and 4 TMSs.		Putative holin of Candidatus Bathyarchaeota archaeon

1.E.19.4.5	Phage holin family protein of 146 aas and 3 TM		*Holin of Methanoculleus* sp.**

Examples:
TC#	Name	Organismal Type	Example
1.E.19.5.1	Holin of 162 aas and 3 TMSs	δ-proteobacteria	Putative holin of Desulfovibrio vulgaris

1.E.19.5.2	Holin of 199 aas and 3 TMSs.		Holin of Maridesulfovibrio frigidus

1.E.19.5.3	Phage holin family protein of 166 aas and 3 TMSs.		Holin of Bilophila wadsworthia

Examples:
TC#	Name	Organismal Type	Example
1.E.19.6.1	Putative holin of 178 aas and 4 TMSs	Bacteroidetes	Holin of Pedobacter saltans

1.E.19.6.2	Putative holin of 181 aas and 3 TMSs	Bacteroidetes	Holin of Capnocytophaga sputigena

1.E.19.6.3	Putative holin of 170 aas and 3 TMSs	Bacteroidetes	Holin of Cyclobacterium marinum (Flectobacillus marinus)