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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 Bacillus subtilis φ29 Holin Family is a group of transporters belonging to the Holin Superfamily IV. A representative list of members belonging to the φ29 holin family can be found in the Transporter Classification Database.
The T4 Holin Family is a group of putative pore-forming proteins that does not belong to one of the seven holin superfamilies. T-even phage such as T4 use a holin-endolysin system for host cell lysis. Although the endolysin of phage T4 encoded by the e gene was identified in 1961, the holin was not characterized until 2001. A representative list of proteins belonging to the T4 holin family can be found in the Transporter Classification Database.
The SPP1 Holin Family consists of proteins of between 90 and 160 amino acyl residues (aas) in length that exhibit two transmembrane segments (TMSs). SPP1 is a double-stranded DNA phage that infects the Gram-positive bacteria. Although annotated as holins, members of the SPP1 family are not yet functionally characterized. A representative list of proteins belonging to the SPP1 Holin family can be found in 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 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 putative holin-like toxin (Hol-Tox) family consists of many small proteins, between 34 and 48 amino acyl residues (aas) with a single transmembrane segment (TMSs). Rajesh et al. first identified the gene and designated it tmp1, which coded for a 34 amino acyl peptide that acts as an antibacterial agent on gram-positive bacteria. This peptide exhibits a single transmembrane domain (TMD) that is believed to play a role in facilitating the antibacterial activity. A representative list of proteins belonging to the Hol-Tox family can be found in the Transporter Classification Database.
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 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 in 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.
Gene transfer agent-release holins are holins which are believed to facilitate the lysis-dependent release of a gene transfer agent. Particularly the gene transfer agent of Rhodobacter capsulatus (RcGTA), which is known to be a bacteriophage-like genetic element that induces horizontal gene transfer. The promoter of the RcGTA gene was identified by Westbye et al. in 2013. A representative list of members belonging to the GTA-Hol family can be found in the Transporter Classification Database with homologues found in proteobacteria and caudovirales.
The Actinobacterial Phage Holin (APH) Family is a fairly large family of proteins between 105 and 180 amino acyl residues in length, typically exhibiting a single transmembrane segment (TMS) near the N-terminus. A representative list of proteins belonging to the APH family can be found in the Transporter Classification Database.
The Putative Acholeplasma Phage L2 Holin Family consists of a putative holin and a homologous uncharacterized protein. These proteins are of particular interest because they may show a link between prokaryotic holins and eukaryotic virus viroporins. While functionally uncharacterized, this putative holin comes up in BLAST searches when members of viroporin families TC# 1.A.95 and TC# 1.A.100 are used as query sequences.
The Putative Archaeal 2 TMS Holin (A2-Hol) Family consists of a few putative holins from Thaumarchaeota ranging in size from about 130 to 165 amino acyl residues (aas) and exhibiting 2 transmembrane segments (TMSs). A representative list of proteins belonging to the A2-Hol family can be found in the Transporter Classification Database. The archaeon, Candidatus Nitrosoarchaeum limnia, encodes adjacent genes designated Toxin Secretion/Lysis Holin. The "toxin" gene encodes a soluble protein of 325 aas stated as belonging to the "Glycosyltransferase GBT-type Superfamily". This protein brings up other glycosyltransferases in a NCBI BLAST search. The adjacent gene encodes a small protein of 132 aas and 2 TMSs that could be a holin, based on its size and topology. This protein has the UniProt accession number of S2E3C4. Paralogues are found in this same organism and other closely related species.
The Putative Holin-2 (PH-2) Family is a large family with members from a wide variety of bacteria. As of early 2016, functional data is not available for members of the PH-2 family, but based on their size and topology, it is believed they act as holins to facilitate cell lysis. PH-2 family proteins are of 130 to 210 amino acyl residues (aas) in length and may exhibit 1 or 2 transmembrane segments (TMSs). A representative list of proteins belonging to the PH-2 family can be found in the Transporter Classification 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.
The Firmicute PhageφU53 Holin Family consists of putative holins that range in size from 117 to 124 amino acyl residues (aas) in length and exhibit 3 transmembrane segments (TMSs) found in Firmicute phage. While annotated as holins, it appears as though many members of the φU53 holin family are not yet functionally characterized. A representative list of homologues can be found in the Transporter Classification Database.
The ArpQ Holin Family consists of a single holin-like protein 58 amino acyl residues (aas) in length with 2 transmembrane segments (TMSs). This protein is encoded by the arpQ gene in Enterococcus hirae. While annotated as a holin, it is not functionally characterized.
The Bacillus Spore Morphogenesis and Germination Holin (BSH) Family is a family of proteins named after a holin in Bacillus subtilis described to be involved in spore morphogenesis and germination by Real et al (2005). The gene encoding this holin is ywcE. Mutants lacking this gene or its product have spores that exhibit outer coat defects. These spores lack the characteristic striatal pattern resulting in the failure of the outer coat to attach to the underlying inner coat. Finally, the mutant spores accumulate reduced amounts of dipicolinic acid. BSH proteins average about 90 amino acyl residues in length and exhibit 3 transmembrane segments (TMSs). A representative list of homologous proteins, found only in Bacillus species, is available in the Transporter Classification Database.
References
- ↑ Anthony, Thangamani; Chellappa, Gunasekaran Stalin; Rajesh, Thangamani; Gunasekaran, Paramasamy (2009-12-05). "Functional analysis of a putative holin-like peptide-coding gene in the genome of Bacillus licheniformis AnBa9". Archives of Microbiology. 192 (1): 51–56. doi:10.1007/s00203-009-0530-7. ISSN 0302-8933. PMID 19967339.
- ↑ Aunpad, Ratchaneewan; Panbangred, Watanalai (2012-01-10). "Evidence for Two Putative Holin-Like Peptides Encoding Genes of Bacillus pumilus Strain WAPB4". Current Microbiology. 64 (4): 343–348. doi:10.1007/s00284-011-0074-3. ISSN 0343-8651. PMID 22231453.
- ↑ "1.E.27 The BhlA Holin (BhlA Holin) Family". TCDB. Retrieved 2016-03-30.
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