Holin

Key takeaways

Holin belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Holin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Holin from memory before moving on to harder problems.

Reference excerpt

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. While canonical holins act by forming large pores, pinholins such as the S protein of lambdoid phage 21 act by forming heptameric channels that depolarize the bacterial membrane. They are associated with SAR endolysins, which remain inactive in the periplasm prior to the depolarization of the membrane. Viruses that infect eukaryotic cells may use similar channel-forming proteins called viroporins.

Classification

Structure According to their structure there are three main classes of holins.

Class I holins Class I holins have three transmembrane domains (TMDs) with the N-terminus in the periplasm and the C-terminus in the cytoplasm. They generally have over 95 residues. Examples of class I holins include the bacteriophage λ S protein (λ holin) and the Staphylococcus aureus phage P68 hol15 protein.

Class II holins Class II holins have two TMDs, with both the N- and the C-terminus in the cytoplasm. Their number of residues usually falls between 65 and 95. Examples include the S protein from lambdoid phage 21 and the Hol3626 protein from Clostridium perfringens bacteriophage Ф3626.

Class III holins Unlike class I and class II holins, which are composed of hydrophobic transmembrane helices, class III holins form a single highly hydrophilic TMD, with the N-terminus in the cytoplasm and the C-terminus in the periplasm. The first class III holin to be characterized was the bacteriophage T4-encoded t protein (T4 holin). Other examples include the holins of the ФCP39O and ФCP26F phage.

Gene families According to the Transporter Classification Database, there are a total of seven holin superfamilies.

Holin superfamily I Holin superfamily II Holin superfamily III Holin superfamily IV Holin superfamily V Holin superfamily VI Holin superfamily VII There are also several holin families that do not fall into the superfamilies designated above. These families include:

1.E.8 - The T4 Holin (T4 Holin) Family 1.E.13 - The Firmicute phage φU53 Holin (φU53 Holin) Family 1.E.14 - The CidA/LrgA Holin (CidA/LrgA Holin) Family 1.E.15 - The ArpQ Holin (ArpQ Holin) Family 1.E.17 - The BlyA Holin (BlyA Holin) Family 1.E.18 - The Lactococcus lactis Phage r1t Holin (r1t Holin) Family 1.E.22 - The Neisserial Phage-associated Holin (NP-Holin) Family 1.E.23 - The Bacillus Spore Morphogenesis and Germination Holin (BSH) Family 1.E.24 - The Bacterophase Dp-1 Holin (Dp-1 Holin) Family 1.E.27 - The BhlA Holin (BhlA Holin) Family 1.E.28 - The Streptomyces aureofaciens Phage Mu1/6 Holin (Mu1/6 Holin) Family 1.E.30 - The Vibrio Holin (Vibrio Holin) Family 1.E.31 - The SPP1 Holin (SPP1 Holin) Family 1.E.32 - The Actinobacterial 1 TMS Holin (A-1 Holin) Family 1.E.33 - The 2 or 3 TMS Putative Holin (2/3 Holin) Family 1.E.35 - The Mycobacterial 1 TMS Phage Holin (M1 Hol) Family 1.E.37 - The Phage T1 Holin (T1 Holin) Family 1.E.38 - The Staphylococcus phage P68 Putative Holin (P68 Hol) Family 1.E.39 - The Mycobacterial Phage PBI1 Gp36 Holin (Gp36 Hol) Family 1.E.42 - The Putative Holin-like Toxin (Hol-Tox) Family 1.E.43 - Putative Transglycosylase-associated Holin (T-A Hol) Family 1.E.44 - The Putative Lactococcus lactis Holin (LLHol) Family 1.E.45 - The Xanthomonas Phage Holin (XanPHol) Family 1.E.46 - The Prophage Hp1 Holin (Hp1Hol) Family 1.E.47 - The Caulobacter Phage Holin (CauHol) Family 1.E.48 - The Enterobacterial Holin (EBHol) Family 1.E.49 - The Putative Treponema 4 TMS Holin (Tre4Hol) Family 1.E.51 - The Putative Listeria Phage Holin (LP-Hol) Family 1.E.52 - The Flp/Fap Pilin Putative Holin (FFPP-Hol) Family 1.E.54 - The Gene Transfer Agent-release Holin (GTA-Hol) Family 1.E.55 - The Brachyspira holin (B-Hol) Family 1.E.56 - The Putative 3 TMS Holin (3-Hol) Family 1.E.57 - The Actinobacterial Phage Holin (APH) Family 1.E.58 - The Erwinia Phage Phi-Ea1h Holin (EPPE-Hol) Family 1.E.59 - The Putative Acholeplasma Phage L2 Holin (L2 Holin) Family 9.B.109 - The Putative Archaeal 2 TMS Holin (A2-Hol) Family 9.B.154 - The Putative Holin-2 (PH-2) Family 9.B.185 - The Putative Bacterial Archaeal Holin (BAH) Family

Worked examples

Example 1 — a first encounter with Holin

Start with the simplest possible case. Write down what Holin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Holin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Holin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Holin

In research: Holin appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Holin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Holin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Holins, Membrane proteins, Protein superfamilies, so understanding it makes those chapters shorter.
In everyday life: Look for Holin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Holin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Holin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Holin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Holin in simple terms?

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 bact…

Why does Holin matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Holin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Holin.