Bacteriophage Mu has been shown to have only one lysis gene, the endolysin gp22. This is surprising because lysis by Caudovirales phages usually requires proteins which disrupt all three layers of the cell envelope, a holin, an endolysin, and a spanin targeting the cytoplasmic membrane, peptidoglycan (PG), and outer membrane (OM), respectively, with the holin determining the timing of lysis initiation. Chamblee et al. 2022 found that gp22 is a signal-anchor-release (SAR) endolysin and identified gp23 and gp23.1 as two-component spanin subunits (see TC family 1.M.7). However, Mu lacks a typical holin and instead encodes a membrane-tethered cytoplasmic protein, gp25 (DUF2730), which is required for the release of the SAR endolysin. Mutational analysis showed that this dependence on gp25 is conferred by lysine residues at positions 6 and 7 of the short cytoplasmic domain of gp22. gp25, (a releasin), also facilitates the release of SAR endolysins from other phages. Moreover, the entire length of gp25, including its N-terminal transmembrane domain, belongs to a protein family, DUF2730, found in many Mu-like phages, including those with cytoplasmic endolysins. Mechanism of releasin function as a rationale for Mu lysis without holin control was discussed (Chamblee et al. 2022 ).

Host cell lysis is the terminal event of the bacteriophage infection cycle. In Gram-negative hosts, lysis requires proteins that disrupt each of the three cell envelope components, only one of which has been identified in Mu: the endolysin gp22. gp22 is a SAR endolysin, a muralytic enzyme that activates upon release from the membrane to degrade the cell wall. Genes 23 and 23.1 are spanin subunits used for outer membrane disruption. But Mu is the first characterized Caudovirales phage to lack a holin, a protein that disrupts the inner membrane and is traditionally known to release endolysins. Instead, a lysis protein, termed the 'releasin', is used by Mu for SAR endolysin release. This is an example of a system where the dynamic membrane localization of one protein is controlled by a secondary protein (Chamblee et al. 2022).

1.E.66. The Phage Mu Holin-like Releasin (Releasin) Family Bacteriophage Mu has been shown to have only one lysis gene, the endolysin gp22. This is surprising because lysis by Caudovirales phages usually requires proteins which disrupt all three layers of the cell envelope, a holin, an endolysin, and a spanin targeting the cytoplasmic membrane, peptidoglycan (PG), and outer membrane (OM), respectively, with the holin determining the timing of lysis initiation. Chamblee et al. 2022 found that gp22 is a signal-anchor-release (SAR) endolysin and identified gp23 and gp23.1 as two-component spanin subunits (see TC family 1.M.7). However, Mu lacks a typical holin and instead encodes a membrane-tethered cytoplasmic protein, gp25 (DUF2730), which is required for the release of the SAR endolysin. Mutational analysis showed that this dependence on gp25 is conferred by lysine residues at positions 6 and 7 of the short cytoplasmic domain of gp22. gp25, (a releasin), also facilitates the release of SAR endolysins from other phages. Moreover, the entire length of gp25, including its N-terminal transmembrane domain, belongs to a protein family, DUF2730, found in many Mu-like phages, including those with cytoplasmic endolysins. Mechanism of releasin function as a rationale for Mu lysis without holin control was discussed (Chamblee et al. 2022 ). Host cell lysis is the terminal event of the bacteriophage infection cycle. In Gram-negative hosts, lysis requires proteins that disrupt each of the three cell envelope components, only one of which has been identified in Mu: the endolysin gp22. gp22 is a SAR endolysin, a muralytic enzyme that activates upon release from the membrane to degrade the cell wall. Genes 23 and 23.1 are spanin subunits used for outer membrane disruption. But Mu is the first characterized Caudovirales phage to lack a holin, a protein that disrupts the inner membrane and is traditionally known to release endolysins. Instead, a lysis protein, termed the 'releasin', is used by Mu for SAR endolysin release. This is an example of a system where the dynamic membrane localization of one protein is controlled by a secondary protein (Chamblee et al. 2022).
References:
Chamblee, J.S., J. Ramsey, Y. Chen, L.T. Maddox, C. Ross, K.H. To, J.L. Cahill, and R. Young. (2022). Endolysin Regulation in Phage Mu Lysis. mBio 13: e0081322.
Examples:
TC#	Name	Organismal Type	Example
1.E.66.1.1	*Gp25 protein of E. coli phage Mu of 99 aas. It functions as a holin but has been called a releasin (Chamblee et al.* 2022). Its gene is Mup25.**		Gp25 of phage Mu

1.E.66.1.10	DUF2730 family protein of 111 aas and 1 TMS		DUF2730 protein of Rhizobium pusense

1.E.66.1.11	DUF2730 family protein of 131 aas and 1 TM		*DUF2730 protein of Microvirga* sp.**

1.E.66.1.12	Uncharacterized protein of 103 aas and 1 TMS.		UP of Neptunomonas japonica

1.E.66.1.13	DUF2730 family protein of 113 aas and 1 TM		DUF2730 protein of Halomonas profundi

1.E.66.1.14	DUF2730 family proteinof 116 aas and 1 TM		DUF2730 protein of Pseudoalteromonas flavipulchra

1.E.66.1.15	Uncharacterized protein of 140 aas and 1 TMS		UP of Alteromonadaceae bacterium

1.E.66.1.16	DUF2730 family protein of 144 aas and 1 TM		DUF2730 protein of Roseospira marina

1.E.66.1.17	Uncharacterized protein of 136 aas		UP of Inquilinus limosus

1.E.66.1.2	Uncharacterized DUF2730 protein of 102 aas and 1 N-terminal TMS.		UP of Myoviridae sp.

1.E.66.1.3	Mu-like phage gp25 protein of 80 aas and 1 N-terminal TMS.		Gp25 of Glaesserella parasuis

1.E.66.1.4	Putative Mu-like phage gp25 protein of 107 aas and 1 N-terminal TMS.		gp25-like protein of Avibacterium paragallinarum

1.E.66.1.5	Uncharacterized protein of 112 aas and 1 N-terminal TMS.		UP of Pleomorphomonas diazotrophica

1.E.66.1.6	Uncharacterized protein of 153 aas and 2 N-terminal TMSs. This protein shows sequence similarity with other members of this family in the C-terminal domain, not in the first TMS.		*UP of Salipiger* sp.**

1.E.66.1.7	DUF2730 family protein of 109 aas and 1 N-terminal TMS.		DUF2730 of Grimontia hollisae

1.E.66.1.8	DUF2730 family protein of 104 aas and 1 N-terminal TMS.		DUF2730 protein of Rhodobiaceae bacterium

1.E.66.1.9	DUF2730 protein of 100 aas and 1 TMS		*DUF2730 protein of Ignatzschineria* sp.**

Examples:
TC#	Name	Organismal Type	Example
1.E.66.2.1	DUF2730 family protein of 141 aas and 1 N-terminal TMS.		DUF2730 protein of Azospirillum thiophilum

1.E.66.2.2	Uncharacterized protein of 134 aas and 1 TMS.		*UP of Thalassolituus* sp.**

1.E.66.2.3	Uncharacterized protein of 136 aas and 1 TMS		UP of Caenispirillum bisanense

Examples:
TC#	Name	Organismal Type	Example
1.E.66.3.1	Uncharacterized protein of 127 aas and 1 TMS		UP of Bacteriophage sp.

1.E.66.3.2	Uncharacterized protein of 122 aas		UP of Siphoviridae sp

1.E.66.3.3	Uncharacterized protein of 127 aas		UP of Desulfovibrio fairfieldensis

1.E.66.3.4	Uncharacterized protein of 128 aas		UP of Deltaproteobacteria bacterium

1.E.66.3.5	DUF2730 family protein of 127 aa		DUF2730 protein of Methylocystaceae bacterium

1.E.66.3.6	DUF2730 family protein of 128 aa		DUF2730 protein of Desulfovibrio mexicanus

Examples:
TC#	Name	Organismal Type	Example
1.E.66.4.1	Uncharacterized protein of 117 aas and 1 TMS		UP of Holophagales bacterium

1.E.66.4.2	Uncharacterized protein of 124 aas and 1 TMS		UP of Halomonas salina

1.E.66.4.3	DUF2730 family protein of 115 aas and 1 TMS		DUF2730 protein of Acidihalobacter prosperus

1.E.66.4.4	Uncharacterized protein of 116 aas		UP of Myoviridae sp.

1.E.66.4.5	Uncharacterized protein of 116 aas		UP of Desulfoluna spongiiphila