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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 Lambda Holin S Family is a group of integral membrane transporter proteins belonging to the Holin Superfamily III. Members of this family generally consist of the characteristic three transmembrane segments (TMSs) and are of 110 amino acyl residues (aas) in length, on average. A representative list of members belonging to this family can be found in the Transporter Classification Database.
The Pseudomonas aeruginosa Hol Holin Family is a group of transporters belonging to the Holin Superfamily III.
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 Mycobacterial 4 TMS Phage Holin Family is a group of transporters belonging to Holin superfamily IV. A representative list of members belonging to the MP4 holin family can be found in the Transporter Classification Database.
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 holin LLH family is a group of transporters belonging to the Holin superfamily VI. The Holin LLH family is found in Firmicutes and phage of Firmicutes as well as other bacteria. Members are fairly large, between 100 and 160 amino acyl residues in length, and have an N-terminal transmembrane segment (TMS). Some proteins, such as putative holin of Fusobacterium varium, may exhibit 2 TMSs. A representative list of proteins belonging to the Holin LLH 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 Lactococcus lactis Phage r1t Holin Family is a family of putative pore-forming proteins that typically range in size between about 65 and 95 amino acyl residues (aas) in length, although a few r1t holins have been found to be significantly larger. Phage r1t holins exhibit between 2 and 4 transmembrane segments (TMSs), with the 4 TMS proteins resulting from an intragenic duplication of a 2 TMS region. A representative list of the proteins belonging to the r1t holin family can be found in the Transporter Classification Database.
The Actinobacterial 1 TMS Holin Family consists of proteins found in actinobacteria, their conjugative plasmids and their phage. They are usually between 90 and 140 amino acyl residues (aas) in length and exhibit 1 or sometimes even 2 transmembrane segments despite the families name. Although some are annotated as phage proteins or holins, members of the A-1 family are not yet functionally characterized. A representative list of proteins belonging to the A-1 Holin family can be found in the Transporter Classification Database (TCDB).
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 3-4 TMS Transglycosylase-associated Holin Family is believed to be a group of holins that does not belong to one of the seven holin superfamilies. Homologues include thousands of diverse phage and bacterial proteins between 80 and 140 amino acyl residues (aas) in length that exhibit 3 to 4 transmembrane segments (TMSs). These proteins are holin-like in their size and topology and are designated 'Transglycosylase-associated', 'Putative holin', 'Phage-like transmembrane protein', 'YeaQ protein', etc. in the NCBI protein database. As of early 2016, they remain functionally uncharacterized. They derive from a wide range of bacterial and archaeal phyla including both Gram-negative and Gram-positive bacteria. These proteins are related to the RDD family in the conserved domain database. A representative list of proteins belonging to the T-A Hol family can be found in the Transporter Classification 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 β-proteobacterial holin (BP-Hol) family is a small family that includes members derived from a number of Burkholderia phage as well as a Poloromonas species. As of April 3, 2016, this family belongs to the Holin superfamily II. Members of Saier Bioinformatics Lab at University of California, San Diego found that the BP-Hol family is most closely related to the T7 holin family. These proteins are of 60 to 110 amino acyl residues (aas) in length and exhibit 1 or 2 transmembrane segments (TMSs). Some are annotated as type II hollins and may be related to members of the T7 Holin family, although BP-Hol proteins remain functionally uncharacterized. A representative list of the proteins belonging to the BP-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.
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.
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.
- ↑ Catalão, Maria João; Gil, Filipa; Moniz-Pereira, José; São-José, Carlos; Pimentel, Madalena (2013-07-01). "Diversity in bacterial lysis systems: bacteriophages show the way". FEMS Microbiology Reviews. 37 (4): 554–571. doi: 10.1111/1574-6976.12006 . ISSN 1574-6976. PMID 23043507.
- ↑ Kamilla, Soumya; Jain, Vikas (2016-01-01). "Mycobacteriophage D29 holin C-terminal region functionally assists in holin aggregation and bacterial cell death". The FEBS Journal. 283 (1): 173–190. doi:10.1111/febs.13565. ISSN 1742-4658. PMID 26471254. S2CID 39285978.
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