Halorhodopsin

Key takeaways

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

Reference excerpt

Halorhodopsin is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is a chloride-specific light-activated ion pump found in archaea known as halobacteria. It is activated by green light wavelengths of approximately 578 nm. Halorhodopsin also shares sequence similarity to channelrhodopsin, a light-gated ion channel. Halorhodopsin contains the essential light-isomerizable vitamin A derivative all-trans-retinal. Due to the dedication towards discovering the structure and function of this moleculc, halorhodopsin is one of the few membrane proteins whose crystal structure is known. Halorhodopsin uses the energy of green/yellow light to move chloride ions into the cell, overcoming the membrane potential. Beside chlorides it transports other halides and nitrates into the cell. Potassium chloride uptake by cells helps to maintain osmotic balance during cell growth. By performing the same task, light-driven anion pumps can considerably reduce the use of metabolic energy. Halorhodopsin has been the subject of much study and its structure is accurately known. Its properties are similar to those of bacteriorhodopsin, and these two light-driven ion pumps transport cations and anions in opposite directions. Halorhodopsin isoforms can be found in multiple species of halobacteria, including Halobacterium salinarum, and Natronobacterium pharaonis. Much ongoing research is exploring these differences, and using them to parse apart the photocycle and pump properties. After bacteriorhodopsin, halorhodopsin may be the best type I (microbial) opsin studied. Peak absorbance of the halorhodopsin retinal complex is about 570 nm. Just as the blue-light activated ion channel channelrhodopsin-2 opens up the ability to activate excitable cells (such as neurons, muscle cells, pancreatic cells, and immune cells) with brief pulses of blue light, halorhodopsin opens up the ability to silence excitable cells with brief pulses of yellow light. Thus halorhodopsin and channelrhodopsin together enable multiple-color optical activation, silencing, and desynchronization of neural activity, creating a powerful neuroengineering toolbox. Halorhodopsin from Natronomonas (NpHR) has been used to achieve inhibition of action potentials in neurons in mammalian systems. Since light activation of NpHR leads to an influx of chloride ions which is a part of the natural process for generating hyperpolarization, NpHR induced inhibition works very well in neurons. Original NpHR channels when expressed in mammalian cells, showed a tendency to get accumulated in the endoplasmic reticulum of the cells. To overcome the sub-cellular localization issues, an ER export motif was added to the NpHR sequence. This modified NpHR (called eNpHR2.0) was utilized successfully to drive aggregate-free, high level expression of NpHR in vivo. However, even the modified form of NpHR showed poor localization at the cell membrane. To achieve higher membrane-localization it was further modified by addition of a golgi export signal and membrane trafficking signal from a potassium channel (Kir2.1). The addition of Kir2.1 signal significantly improved the membrane localization of NpHR and this engineered form of NpHR was labeled eNpHR3.0.

History Halorhodopsin was discovered in 1980 in Halobacterium salinarum, a salt-loving (halophilic) type of archaeon.

Etymology The name Halorhodopsin is of Greek origin, the halo- prefix emerging from ἅλς (háls) meaning "salt" or "sea". The suffix -rhodopsin originates from ῥόδον (rhódon, “rose”), due to its pinkish color, and ὄψις (ópsis, “sight”).

Structure Halorhodopsin folds into a seven-transmembrane helix topology and has a similar tertiary structure (but not primary sequence structure) to vertebrate rhodopsins, the pigments that sense light in the retina.

Applications Halorhodopsin has been used in optogenetics to hyperpolarize (inhibit) specific neurons. Optogenetics has been proposed as therapeutic approach to neurological conditions for which current treatment methods are not always effective, including epilepsy and Parkinson's disease. NpHR has been used to inhibit excitatory neurons in the subthalamic nucleus of hemiparkinsonian rats, lesioned using the neurotoxin 6-OHDA.

References

External links Synthetic Neurobiology Group, MIT: Halorhodopsin mediating optical silencing of neurons Halorhodopsin at the U.S. National Library of Medicine Medical Subject Headings (MeSH) Optogenetics Resource Center OpenOptogenetics.org, an open wiki about optogenetics.

Illustrations

Worked examples

Example 1 — a first encounter with Halorhodopsin

Start with the simplest possible case. Write down what Halorhodopsin 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 Halorhodopsin 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 Halorhodopsin 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 Halorhodopsin

In research: Halorhodopsin 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 Halorhodopsin 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: Halorhodopsin is common in secondary-school and first-year university syllabi. It links to neighbouring topics 7TM receptors, Integral membrane proteins, so understanding it makes those chapters shorter.
In everyday life: Look for Halorhodopsin 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 Halorhodopsin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Halorhodopsin 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 Halorhodopsin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Halorhodopsin in simple terms?

Halorhodopsin is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is a chloride-specific light-activated ion pump found in archaea known as halobacteria.

Why does Halorhodopsin 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 Halorhodopsin?

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 Halorhodopsin.