The Lambda Holin S (λ Holin) Family
TC# 1.E.2
is a group of integral membrane transporter proteins belonging to the

Holin Superfamily III 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 functionalit ...

. Members of this family generally consist of the characteristic three

transmembrane segment A transmembrane domain (TMD) is a membrane-spanning protein domain. TMDs generally adopt an alpha helix topological conformation, although some TMDs such as those in porins can adopt a different conformation. Because the interior of the lipid bil ...

s (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.

Lambda Holin S

Lambda holin S (Lysis protein S of phage lambda,

holin 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 ...

S105
TC# 1.E.2.1.1
is the prototype for class I holins. It has 3 TMSs with the N-terminus in the

periplasm The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the ''periplasmic space'' in gram-negative bacteria. Using cryo-electron microscopy it has been found that ...

and the C-terminus in the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

. Its 107 codon sequence encodes two proteins with opposing functions, the holin, S105, and the holin inhibitor, S107. The latter protein, S107, is a 2-amino acid extension of the former protein, S105, due to a different translational initiation start site (M1-K2-M3 vs. M3). A cationic amino acid at position 2 is largely responsible for the inhibiting effect of S107. The ratio of S105 to S107 influences the timing of phage lambda-induced

cell lysis Lysis ( ) is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ''lysate''. In molecular bio ...

. The highly hydrophilic C-terminal domains of holins (e.g., lambda S105) have been shown to be localized cytoplasmically and serve as regulatory domains. Like the N-terminal 2 amino acid extension in S107, they influence the timing of lysis by a charge dependent mechanism.

Mechanism

Expression of holin S at a precisely scheduled time after phage infection terminates respiration and allows release of a muralytic enzyme, endolysin, that

hydrolyzes Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

the cell wall. Point mutations in the S gene that prevent lethality alter TMSs 1 and 2 and the connecting loop. TMS 2 is particularly important for function. A three-step mechanism (monomer → dimer → oligomeric pore) has been proposed for assembly of the pore. S105 (holin) and S107 (inhibitor) form an abortive dimer. Only when S105 production exceeds that of S107 (which occurs at a specific developmental time), do functional holes appear in the bacterial cell membrane. For holin S105, the helix-turn-helix motif in transmembrane domain 3 provides the driving force of dimerization. Holins regulate the length of the infection cycle of tailed

phages A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacterio ...

(

caudovirales ''Caudovirales'' is an order of viruses known as the tailed bacteriophages (''cauda'' is Latin for "tail"). Under the Baltimore classification scheme, the ''Caudovirales'' are group I viruses as they have double stranded DNA (dsDNA) genomes, ...

) by oligomerizing to form lethal holes in the cytoplasmic membrane at a time dictated by their primary structures. Savva et al. (2008) used

electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...

and single-particle analysis to characterize structures formed by the

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...

lambda holin (S105) in vitro. In non-ionic or mild

zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. : With amino acids, for example, in solution a chemical equilibrium wil ...

ic detergents, purified S105, but not the lysis-defective variant S105A52V, formed rings of at least two size classes, the most common having inner and outer diameters of 8.5 and 23 nm respectively, and containing approximately 72 S105 monomers. The height of these rings, 4 nm, closely matches the thickness of the

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...

. The central channel is of unprecedented size for channels formed by integral membrane proteins, consistent with the non-specific nature of holin-mediated membrane permeabilization. S105, present in detergent-solubilized rings and in inverted membrane vesicles, showed similar sensitivities to proteolysis and cysteine-specific modification, suggesting that the rings are representative of the lethal holes formed by S105 to terminate the infection cycle and initiate lysis.

Homologues

A homologue of λ holin S from the lysogenic ''Xenorhabdus nematophila'', hol-1
TC #1.E.2.1.4
, has been shown to be a functional holin. When cloned into wild-type ''E. coli'', it causes hemolysis due to the release of the SheA hemolysin. Another holin (phage H-19B holin) is encoded by a gene associated with the Shiga-like toxin I gene of '' E. coli.'' Thus, it appears that holins can export various toxins as well as

autolysin Autolysins are endogenous lytic enzymes that break down the peptidoglycan components of biological cells which enables the separation of daughter cells following cell division. They are involved in cell growth, cell wall metabolism, cell division a ...

s. The holes caused by S105 have an average diameter of 340 nm, and some exceeding 1 micron. Most cells exhibit only one irregular hole, randomly positioned in the membrane, irrespective of its size. During λ infection, S105 accumulates harmlessly in the membrane until it forms a single irregular hole, releasing the endolysin from the cytoplasm, resulting in lysis within seconds. Using a functional S105-GFP fusion, it was demonstrated that the protein accumulates uniformly in the membrane, and then within 1 minute, it forms aggregates at the time of lethality. Thus, like

bacteriorhodopsin Bacteriorhodopsin is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting ...

, the protein accumulates until it reaches a critical concentration for nucleation.

References

{{Dual, source=Transporter Classification Database (TCDB), sourcepath=http://www.tcdb.org/search/result.php?tc=1.E.2, sourcearticle=1.E.2 The Lambda Holin S (λ Holin) Family, date=10 March 2016 Holins Protein families

Lambda Holin S

Mechanism

Homologues

See also

Further reading

References