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

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

known as

halobacteria Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class (biology), class of prokaryotic archaea under the phylum Euryarchaeota, found in water Saturated and unsaturated compounds, saturated or nearly saturated with ...

. 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 Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...

derivative all-trans-

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

. Due to the dedication towards discovering the structure and function of this moleculc, halorhodopsin is one of the few membrane proteins whose

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

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

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...

s and

nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...

s 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 An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

and

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

in opposite directions. Halorhodopsin isoforms can be found in multiple species of halobacteria, including ''

Halobacterium salinarum ''Halobacterium salinarum'', formerly known as ''Halobacterium cutirubrum'' or ''Halobacterium halobium'', is an extremely halophile, halophilic ocean, marine obligate aerobic archaeon. Despite its name, this is not a bacteria, bacterium, but a mem ...

'', 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

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

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

muscle cell A muscle cell, also known as a myocyte, is a mature contractile Cell (biology), cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal muscle, skeletal, smooth muscle, smooth, and Cardiac muscle, cardiac ...

s, 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 potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s 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 The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

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 The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. To achieve higher membrane-localization it was further modified by addition of a golgi export signal and membrane trafficking signal from a

potassium channel Potassium channels are the most widely distributed type of ion channel found in virtually all organisms. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of ...

(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 ''

,'' 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 The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

Applications

Halorhodopsin has been used in

optogenetics Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by Gene expression, expression of Channelrhodopsin, light-sensitive ion channels, Halorhodopsin, pumps or Photoactivated ade ...

to hyperpolarize (inhibit) specific

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

Optogenetics Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by Gene expression, expression of Channelrhodopsin, light-sensitive ion channels, Halorhodopsin, pumps or Photoactivated ade ...

has been proposed as therapeutic approach to neurological conditions for which current treatment methods are not always effective, including

epilepsy Epilepsy is a group of Non-communicable disease, non-communicable Neurological disorder, neurological disorders characterized by a tendency for recurrent, unprovoked Seizure, seizures. A seizure is a sudden burst of abnormal electrical activit ...

and

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a neurodegenerative disease primarily of the central nervous system, affecting both motor system, motor and non-motor systems. Symptoms typically develop gradually and non-motor issues become ...

. NpHR has been used to inhibit excitatory neurons in the subthalamic nucleus of hemiparkinsonian rats, lesioned using the

neurotoxin Neurotoxins are toxins that are destructive to nervous tissue, nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insult (medical), insultsSpencer 2000 that can adversely affect function ...

6-OHDA.

References

External links

Synthetic Neurobiology Group, MIT: Halorhodopsin mediating optical silencing of neurons
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Optogenetics Resource Center

OpenOptogenetics.org, an open wiki about optogenetics
{{Ion pumps 7TM receptors Integral membrane proteins