Halorhodopsin
Encyclopedia
Halorhodopsin is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea
, known as halobacteria
. It is a seven-transmembrane protein of the retinylidene protein
family, homologous to the light-driven proton pump bacteriorhodopsin
, and similar in tertiary structure (but not primary sequence structure) to vertebrate rhodopsins, the pigments that sense light in the retina
. Halorhodopsin also shares sequence similarity to channelrhodopsin
, a light-driven ion channel. Halorhodopsin contains the essential light-isomerizable vitamin A
derivative all-trans-retinal
. Due to the intense attention on solving the structure and function of this molecule, 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 H. salinarum
, and N. 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 Natromonas (NpHR) has been used 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 (celled 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
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, known as halobacteria
Halobacteria
In taxonomy, the Halobacteria are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms which live in somewhat less concentrated salt water...
. It is a seven-transmembrane protein of the retinylidene protein
Retinylidene protein
Retinylidene proteins are a family of proteins that use retinal as chromophore for light reception. Proteins of this family are also called opsins...
family, homologous to the light-driven proton pump bacteriorhodopsin
Bacteriorhodopsin
Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. 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...
, and similar in tertiary structure (but not primary sequence structure) to vertebrate rhodopsins, the pigments that sense light in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. Halorhodopsin also shares sequence similarity to channelrhodopsin
Channelrhodopsin
Channelrhodopsins are a subfamily of opsin proteins that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis, i.e. movement in response to light. Expressed in cells of other organisms, they enable the use of light to control...
, a light-driven ion channel. Halorhodopsin contains the essential light-isomerizable vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
derivative all-trans-retinal
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
. Due to the intense attention on solving the structure and function of this molecule, halorhodopsin is one of the few membrane proteins whose crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
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
Halobacteria
In taxonomy, the Halobacteria are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms which live in somewhat less concentrated salt water...
, including H. salinarum
Halobacterium salinarum
Halobacterium salinarum is an extremely halophilic marine gram-negative obligate aerobic archaeon. Despite its name, this microorganism is not a bacterium, but rather a member of the Kingdom Archaea. It is found in salted fish, hides, hypersaline lakes, and salterns...
, and N. 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
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
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 Natromonas (NpHR) has been used 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 (celled 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
External links
- Synthetic Neurobiology Group, MIT: Halorhodopsin mediating optical silencing of neurons
- Optogenetics Resource Center
- OpenOptogenetics.org, an open wiki about optogenetics. - Structures and orientations of bacterial rhodopsins in membrane