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Holins are a diverse group of small proteins produced by dsDNA bacteriophages in order to trigger and control the degradation of the host's cell wall at the end of the lytic cycle. Holins form pores in the host's cell membrane, allowing lysins to reach and degrade peptidoglycan, a component of bacterial cell walls. Holins have been shown to regulate the timing of lysis with great precision. Over 50 unrelated gene families encode holins, making them the most diverse group of proteins with common function. Together with lysins, holins are being studied for their potential use as antibacterial agents.
The Holin Superfamily III is a superfamily of integral membrane transport proteins. It is one of the seven different holin superfamilies in total. In general, these proteins are thought to play a role in regulated cell death, although functionality varies between families and individual members.
The Holin superfamily VII is a superfamily of integral membrane transport proteins. It is one of the seven different holin superfamilies in total. In general, these proteins are thought to play a role in regulated cell death, although functionality varies between families and individual members.
The Listeria Phage A118 Holin (Hol118) Family is a group of transporters belonging to the Holin Superfamily V. A representative list of proteins belonging to the Hol118 family can be found in the Transporter Classification Database.
The 2 or 3 TMS Putative Holin Family consists of many proteobacterial proteins ranging in size from about 70 to 120 amino acyl residues (aas) in length that exhibit 2 or 3 predicted transmembrane segments (TMSs). Although annotated as holins, these proteins are not yet functionally characterized. A representative list of proteins belonging to the 2/3 Holin family can be found in the Transporter Classification Database.
The Mycobacterial 1 TMS Phage Holin Family was identified and recognized by Catalao et al. (2012). Members of this family are found in mycobacterial phage, exhibit a single transmembrane segment (TMSs), and are about 75 to 95 amino acyl residues in length. Although annotated as holins, members of this family are not yet functionally characterized. A representative list of proteins belonging to this family can be found in the Transporter Classification Database.
The Phage T1 Holin Family is represented in enterobacterial phages T1, RTP and F20, Klebsiella phage KP36, and Escherichia phage ADB-2. All of these possess a putative holin that share a high level of identity. Additionally, Gp9 of E. coli phage phiE49 is similar in sequence. These proteins are short, 55 to 71 amino acyl residues (aas) in length, and exhibit a single transmembrane segment (TMS). A representative list of proteins belonging to the T1 Holin family can be found in the Transporter Classification Database.
The Staphylococcusphage P68 Putative Holin Family consists of a single putative holin from Staphylococcus aureus phage P68 that is 92 amino acyl residues (aas) in length and exhibits 2 transmembrane segments (TMSs). While annotated as a holin, this protein has not been functionally characterized.[2]
The Mycobacterial Phage PBI1 Gp36 Holin Family consists of a single protein, Gp36 of Mycobacterial phage PBI1 identified by Castalao et al. (2012). Gp36 is 116 amino acyl residues (aas) in length and exhibits 2 transmembrane segments (TMSs). While annotated as a holin, this protein has not been functionally characterized.
The Putative Lactococcus lactis Holin (LLHol) Family consists of just a few proteins from Lactococcus lactis species and their phage. These proteins are small, between 61 and 78 amino acyl residues (aas) in length, and exhibit one or two transmembrane segments (TMSs). As of March 2016, LLHol proteins remain functionally uncharacterized. They are not demonstrably homologous to members of other holin families and thus do not belong to one of the seven holin superfamilies. A representative list of proteins belonging to the LLHol family can be found in the Transporter Classification Database.
The XanthomonasPhage Holin (XanPHol) Family consists of a single protein of 64 amino acyl residues (aas) in length with 2 transmembrane segments (TMSs). It is a putative uncharacterized protein from Xanthomonas phage Xp15. This protein corresponds to sequence 68 from patent US 7919601. As of March 2016, this protein does not show appreciable sequence similarity to any other proteins in the NCBI protein database.
The CaulobacterPhage Holin (CauHol) Family consists of several putative holins of 157 to 159 amino acyl residues (aas) in length that exhibit 2 transmembrane segments (TMSs). They derive from phage specific for Caulobacter species. These proteins are not functionally characterized. A representative list of proteins belonging to the CauHol family can be found in the Transporter Classification Database.
The Putative Treponema 4 TMS Holin (Tre4Hol) Family consists of several proteins from Treponema species. They range in size from 100 to 110 amino acyl residues (aas) in length and exhibit 4 transmembrane segments (TMSs). A fragment has been sequenced from Treponema phage Phi td1 (D2ECI8) and was designated a putative holin.
The Putative Listeria Phage Holin (LP-Hol) Family consists of several small proteins of 41 amino acyl residues (aas) and 1 transmembrane segment (TMS). They can be found in several Listeria phage as well as in Listeria monocytogenes. While annotated as holins, these proteins remain functionally uncharacterized. A representative list of proteins belonging to the LP-Hol family can be found in the Transporter Classification Database.
The Flp/Fap Pilin Putative Holin (FFPP-Hol) Family is a large diverse family with members from many bacterial phyla. Some members are annotated as Flp or Fap pilin subunits; others are identified as Holin BlyA family members. They range in size of 50 to 80 amino acyl residues (aas) with a single N-terminal transmembrane segment (TMS) although one member has 99 aas and 2 TMSs. Flp homologues are included in TCDB under TC# 3.A.7.13.1 (pXO1-63) and TC# 3.A.7.15.1 (Flp-1). As on March 2016, their precise functions appear to be unknown. A representative list of proteins belonging to the FFPP-Hol family can be found in the Transporter Classification Database.
The Putative 3 TMS Holin (3-Hol) Family is large, consisting of many members derived from proteobacteria and their phage, all of small size and usually with 3 transmembrane segments (TMSs). A representative list of the proteins belonging to this family can be found in the Transporter Classification Database. While many of the proteins belonging to this family are annotated in holins, they remain functionally uncharacterized.
The ErwiniaPhage Phi-Ea1h Holin (EPPE-Hol) Family consists of a single protein, holin of Erwinia Phage Phi-Ea1h, which is 119 amino acyl residues in length and exhibits a single transmembrane segment (TMS). Out of three open reading frames sequenced from bacteriophage Phi-Ea1h, the second ORF encodes this holin. Kim and Geider found that no signal sequence was observed at the N-terminus of the enzyme and suggested that the holin possibly facilities the export of an which may export a lysozyme and EPS depolymerase that carries out extracellular polysaccharide (EPS)-degrading activity.
The Prophage Hp1 Hol (Hp1Hol) Family consists of a single putative holin of 69 amino acyl residues in length, exhibiting what appears to be a single transmembrane segment (TMS). It is derived from the firmicute, Clostridium hathewayi DSM 13479. This protein is functionally uncharacterized and does not appear to be homologous to other holins. It does, however, show 31% identity to a heavy metal transporter from Dethiosulfovibrio peptidovorans.
The Enterobacterial Holin (EBHol) Family consists of many closely related proteins of 100 to 120 amino acyl residues (aas) in length with a single C-terminal transmembrane segment (TMS). They derive from γ-proteobacteria of many genera: Salmonella, Escherichia, Klebsiella and Photorhabdus, and their phage. As of March 2016, these proteins have not been functionally characterized. A representative list of proteins belonging to the EBHol family can be found in the Transporter Classififcation Database.
The Putative Bacterial Archaeal Holin(BAH) Family consists of several uncharacterized proteins. However, these proteins were retrieved when functionally characterized holins from the T-A Hol family were BLASTED against the NCBI database, and thus may be related to the T-A Hol family. Most BAH proteins are between 125 and 140 amino acyl residues (aas) in length and exhibit 4 transmembrane segments (TMSs), although at least one putative holin is almost 260 aas long. A representative list of proteins belonging to the BAH family can be found in the Transporter Classification Database.
References
- ↑ Reddy, Bhaskara L.; Saier Jr., Milton H. (2013-11-01). "Topological and phylogenetic analyses of bacterial holin families and superfamilies". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1828 (11): 2654–2671. doi:10.1016/j.bbamem.2013.07.004. PMC 3788059 . PMID 23856191.
- 1 2 "1.E.43 Putative 3-4 TMS Transglycosylase-associated Holin (T-A Hol) Family". TCDB. Retrieved 2016-03-28.
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