Sensory rhodopsin II (SRII), also known as ''pharaonis'' phoborhodopsin (''p''pR), is a

membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

, responsible generating the

phototaxis Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive li ...

signal. Sensory rhodopsin II is found in ''

Halobacterium salinarum ''Halobacterium salinarum'', formerly known as ''Halobacterium cutirubrum'' or ''Halobacterium halobium'', is an extremely halophilic marine obligate aerobic archaeon. Despite its name, this is not a bacterium, but a member of the domain Archaea ...

'' and ''

Natronomonas pharaonis ''Natronomonas'' (common abbreviation ''Nmn.''). is a genus of the Halobacteriaceae.See the NCBIbr>webpage on Natronomonas Data extracted from the Description and significance ''Natronomonas pharaonis'' is an aerobic, extremely haloalkalip ...

''.

Structure

Structure of sensory rhodopsin II is typical for

microbial rhodopsin Microbial rhodopsins, also known as bacterial rhodopsins are retinal-binding proteins that provide light-dependent ion transport and sensory functions in halophilic and other bacteria. They are integral membrane proteins with seven transmembra ...

. It consists of seven

transmembrane A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...

a-helices The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...

with

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 retin ...

molecule connected via

Schiff base In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimine ...

to K205. Notable feature of sensory rhodopsin II is presence of charged

residue Residue may refer to: Chemistry and biology * An amino acid, within a peptide chain * Crop residue, materials left after agricultural processes * Pesticide residue, refers to the pesticides that may remain on or in food after they are applied ...

Y199 on the surface of the

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

region. This residue is responsible for binding of sensory rhodopsin II transducer protein - HtrII. HtrII consists of two transmembrane helices, two HAMP-domains and methyl-accepting signalling domain. In case of ''

'' HtrII also comprises extracellular chemosensor region, which is responsible for

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

sensing.

Function

SRII in the

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

binds its transducer via a number of bonds.

Photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

absorption induces structural changes in SRII, which when conducted to HtrII. HtrII in turn regulates level of unbound

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

s in

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 ...

. Kinases regulate rotation of

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

via phosphorylation. Thus phototaxis signal is converted in bacteria' motion.

Structure

Function

Further reading