RECEPTORS

RECEPTORS

Signals can be perceived by protein receptors or through changes in membrane potential. 

To initiate transduction, a receptor, Most known receptors are present in the plasma membrane, although some are located in the cytosol or other cellular compartments. At least three different classes of cell surface receptors have been detected in animals, but whether all three exist in plants is still uncertain. Most identified receptors have turned out to be proteins. Protein receptors are not easily identified - for example, the breaking of the dormancy of some buds or imbibed seeds by such chemicals as ethanol, ether, azide, or cyanide. Either these chemicals are able to occupy established cellular receptors or, more likely, many of them modify the membrane potential, the voltage across the plasma membrane. The membrane potential can act as a receptor. The plasma membrane uses pups and proteinaceous pores, called channels, to control the flux of ions into and out of the cell. The sequences of many receptors have been determined. Many receptors have seven hydrophobic domains placed strategically throughout the molecule. These hydrophobic domains are thought to represent regions of the receptor that span the plasma membrane. In some transmembrane protein receptors, the C-terminal region is phosphorylated by protein kinases. Two possible families of protein kinases are distinguished on the basis of the amino acids they phosphyorylate on their substrate proteins. Another class of receptors is the so called receptor-like protein kinases. The RLKs of plants typically consist of a large extracytoplasmic domain, a single membrane-spanning segment, and a cytoplasmic domain containing the active site of a protein kinase. Numerous RLKs have been identified in plant cells, including protein kinases with seven membrane-spanning domains. Such RLKs have benn detected in the male reproductive tissues of plants, where they are implicated in incompatibility reactions that prevent fertilization.

§   Intracellular receptors can act as ion channels. Other receptors are located in intracellular membranes and can act as Ca²⁺ channels. The most well-known receptor in this class binds the second messenger inositor 1, 4, 5-triphosphate. Channels for another second messenger, cyclic ADP-ribose (cADPR), have been reported recently. Occupation of the receptor (which may be composed of four subunits) leads to the opening of Ca²⁺ channels and an influx of Ca²⁺ intot he cytoplasm from the vacuole and the ER, each of which contains Ca²⁺ many orders of magnitude greater than the cytosolic concentrations. In contrast to plasma membrane-bound receptors, these protein subunits each have four membrane-spanning domains. Other membrane-spanning proteins also may have important functions in signal transduction.

§   Not all tissues or cell types are able to respond to all signals. For example, fruit tissues become sensitive to ethylene at a certain stage of ripening, whereas guard cells are totally insensitive to high concentrations of the gaseous hormone. Different responses by different tissues to the same signal can in part be explained by families of receptors. Auxin, for example can induce perieyel cells to form adventitious or lateral rots, but in coleoptile cells it promotes elongation. Different receptors are probably involved in each response. However, divergent downstream elements of the signal transduction pathway may also distinguish the developmental responses to auxin exhibited by different cell types. Tissue-specific signals transduction pathways are thus defined not only by the presence or absence of receptors but also by the presence or absence of downstream apparatus required to transduce the responses. Tissues can adapt or desensitize themselves to continuous signals, and receptor concentrations can change during development. For example, when etiolated seedlings are exposed to red or white light, the cellular concentrations of phytochromes decrease rapidly.

§   The gas ethylene regulates ripening, germination, elongation, senescence, and pathogen responses. Several ethylene receptors have been cloned through isolation of ethylene insensitive mutants and subsequent use of molecular technology to identify the mutant gene. ETR1, a 79-kDa protein with a transmembrane domain, was the first receptor cloned from Arabidopsis. The C terminus of ETR1 is homologous to a bacterial two component system hybrid kinase. ETR1 exists as a dimer in the plasma membrane. Ethylene joins the two monomers together and permits intermolecular phosphorylation. Mutations in ETR1 (designated efr1) lead to loss of physiological sensitivity to ethylene. ETRI has also been expressed in yeast to demonstrate that the protein binds ethylene with high affinity. Competitive ethylene antagonists inhibit this binding. Expression of efr1 in yeast leads to loss of ethylene binding, confirming that ETR1 is thus a true ethylene receptor. Genes encoding other ethylene receptors have also been identified, including ERS (ethylene response sensor), Nr (never ripe, .1 developmentally regulated gene from toma to; and LeTAE1 (a tomato ETR1 homolog expressed during flower and fruit senescence.)

Many auxin-binding proteins have been detected, but whether they represent receptors for different auxin-mediated processes is still uncertain. 

Indole 3-acetic acid (IAA, reterred to here as auxin) is a growth regulator with a wide variety of functions in cell division and   expansion. Auxin has been studied intensively for the past 50 years and, not surprisingly, receptors for the auxin signal have been actively sought. Conventioanl pharmacological techniques have uncovered one well-characterized auxin-binding protein (ABP1). The possible receptor function of this protein was controversial for many years but has recently been established.

Phytochorome, a clearly identified receptor for red light, has protein kinase activity incyanobacteria. 

Phytochromes form a family of 120-kDa proteins. The photoreactive moiety (chromophore) of these proteins is an open-chain tetrapyrrole. Two forms of phytochrome, A and B, can each form dimers in solution and physiological evidence suggests that both may dimerize in vivo