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RECEPTORS


TABLE OF CONTENTS

Receptor Basics
Membrane-Bound Receptors/Cell Surface Receptors
Mechanism of G-Protein Coupled Hormone Action
Enzyme-Linked Receptors
Cytokine Receptors
Receptor Regulation
Intracellular Receptors/Nuclear Receptors
Related Topics

RECEPTOR BASICS

Hormones are chemicals produced by the glands of the endocrine system. These chemicals are secreted into the bloodstream, which carries them to almost all the cells in the body. However, not all cells respond to these hormones. In fact, these hormones are produced only for specific organs or a group of cells in an organ. So, why do only these organs respond to the hormones? Each type of hormone has its own receptor that is located on or inside the cell. Only the cells that have these receptors will be able to respond to the hormone. In many cases, however, hormones of similar origin have the same or very similar receptors. These will be discussed later in more detail.


Receptor properties:

1. They are proteins located on either the surface or inside
 the cell.
2. They are highly specific for ligand.
3. They form a complex with the ligand, (R:L).
4. They have an equilibrium constant for the forward and
reverse reactions, (K
1 & K2, respectively).
5. The concentration of the ligand and the receptor set the
amplitude of the response. More hormones/receptors will
yield a stronger response.
6. The receptors can dimerize to increase their activity.

In most cases, binding of a hormone/ligand to the receptor promotes a conformational change in the receptor which transmits the information to the other side of the membrane. The conformational changes trigger a set of reactions which are termed "Signal Transduction." There are some hormone receptors that are actually ion channels which open or close due to hormone binding. These are usually associated with, but not limited to the neuroendocrine system.

There are three major types of hormones (see HORMONES for more details): peptide hormones, steroid hormones, and eicosanoids. Hormones of each catagory have different types of receptors. Peptide hormones bind to membrane-bound receptors or cell surface receptors, steroid-derived hormones bind to receptors found on intracellular receptors, and eicosanoids, which are derivatives of arachidonic acid, also bind to membrane-bound receptors.

MEMBRANE-BOUND RECEPTORS/CELL SURFACE RECEPTORS

These are peptide hormone receptors. Membrane-bound receptors are proteins located on the plasma membrane. They can be single polypeptide chains or have up to four subunits. Some of them have up to seven transmembrane domains (see figure 1.1 below). Some of the hormones that these receptors bind to are prostaglandins, ACTH, glucogon, catecholamines, parathyroid hormone and others. Many other hormones bind to receptors with less domains (see ENZYME- LINKED RECEPTORS).

Seven-Transmembrane Receptor

Binding of a hormone to the receptor initiate signal transductions. Many of these signal transductions involve second messengers, which are compounds that are activated or produced when the receptors bind the ligands. These second messengers are usually cyclic adenosine monophosphate (cAMP) molecules or cyclic guanine monophosphate(cGMP) molecules, Ca
2+, or protein kinase C (PKC). During signal transduction, many of these compounds form complexes with each other and activate or inactivate other molecules inside the cell. These reactions are also coupled with G proteins, discussed in detail below. Other receptors use tyrosine kinase, serine kinase and guanyl cyclase domains. These are known as Enzyme-Linked receptors. They will be discussed in more detail later.

MECHANISM OF G-PROTEIN COUPLED HORMONE ACTION

There are different types of G proteins. Activated Gs (G stimulatory) proteins activate other enzymes and proteins, mainly through the adenylyl cyclase pathway. Acitivated Gp proteins activate enzymes and proteins throught the phosphotidylinositol pathway. And finally, Activated Gi (G inhibitory) proteins inhibit the activities of Gs and Gp proteins.

In figure 1.1, there are three cytoplasmic loops: loops I, II & III. Loop III and the carboxyl tail have a kinase activity, which enables them to autophosphorylate. The seven-transmembrane receptor is coupled with a Guanine Nucleotide Binding Protein (G-protein). G-proteins are heterotrimeric (three non-identical subunits): a, b & g subunits. The a subunits has a GDP bound to it when it is inactive (see figures 1.2 & 1.3). It also has an intrinsic GTPase activity. When the hormone binds to the binding domain of the receptor, loop III and the carboxyl tail autophosphorylate which leads to negative charge repulsion between loop III and the tail. This forces the carboxyl tail to move away from loop III and allow a G-protein to bind. Once the G-protein binds to loop III, the a subunit looses the GDP and obtains a GTP. The three subunits separate and the G-alpha becomes active. This receptor will activate G-proteins until the ligand leaves or it is degraded. Usually, one receptor activates from 100 to 5000 G-proteins. This is crucial in signal transduction and is termed "signal amplification." Activated G-alpha binds to Adenylate cyclase, which is another membrane bound protein, and activates it. Adenylate cyclase takes an ATP molecule and converts it to cAMP. cAMP is the second messenger, which binds to the regulatory subunit of protein kinase A (PKA). The catalytic subunit of PKA separates from the regulatory subunit and can either phosphorylate cytosolic enzymes (eg. phosphorylase kinase), or it can go into the nucleus and phosphorylate the cyclic AMP response element binding protein (CREB). This protein is already bound to the DNA. Phosphorylation of CREB leads to its activation and promotes transcription.

Another G-protein effector system is the phospholipase C beta (PLC
b). Once PLC is activated, it leads to the cleavage of phosphoinositol 4,5-bisphospate (PIP2). The two products of this reaction are, phosphoinositol 1,4,5- trisphosphate (IP3) and diacylglycerol (DAG). DAG stays in the membrane and activates membrane bound protein kinase C. PKC, when activated, can either phosphorylate cytoplasmic membrane proteins and other free floating enzymes, or it can enter the nucleus and activate or deactivate transcription enzymes. IP3, on the other hand, floats off into the cytoplasm and binds to receptors on endoplasmic reticulum. These receptors are actually Ca2+ channels, which become activated when bound to IP3. Increased concentration of Ca2+ in the cytoplasm can have a variety of effects on cells. Ca2+ in the cytoplasm almost always binds to a "carrier" protein. In many cases, this carrier protein is calmodulin, which binds to four Ca2+ ions. They form a Ca2+:Calmodulin complex, which has the potential to activate PKC, affect ion channels, and induce smooth muscle contraction.

Inactive GTP Active GTP

As discussed above, a small amount of hormone can induce the activation of thousands of proteins. As the process continues, most of the other components are further amplified. How does this system shut itself off? Cells use a few different methods that make sure that the effects of the hormone cease once the hormone leaves the receptor binding site. Previously, it was mentioned that the Ga subunit has an intrinsic GTPase activity. This region of the Ga subunit dephosphorylates the GTP that is bound to it, thus deactivating the G-protein. After the GTP is converted to GDP, the other b and g subunits of the protein bind to the a subunit. The cyclic AMP in the cytosol is also converted to AMP by the enzyme phosphodiesterase. PKA requires the presence of cAMP molecules to become activated, thus in the absence of cAMP, PKA is deactivated. There are many other types of protein phosphatases present in the cytosol, which makes sure that the enzymes that have been phosphorylated due to the effects of the hormone are dephosphorylated.

ENZYME-LINKED RECEPTORS

Enzyme-linked receptors usually span the plasma membrane only once. Among the many types of hormones which use this type of receptors are, insulin, insulin-like growth factor (IGF), and platelet-derived growth factor (PDGF). These hormones typically pormote cell proliferation and cell growth. The carboxyl tail of these receptors have tyrosine residues which are phosphorylated by the receptor itself through the action of intrinsic kinase. This phosphorylation proceeds a ligand binding to the amino terminus of the receptor. The phosphorylated tyrosine residues act as the catalytic sitea for accessory proteins such as PLC
g, phosphoinositol 3' kinase, GTPase-activating protein (GAP), and growth factor receptor-bound protein-2 (GRB2). These proteins interact with the tyrosine residues through highly conserved 2 src homology domains (SH2). However, each protein interacts with different sets of tyrosine residues on the catalytic site of the receptor, thus showing selectivity for the accessory proteins. More proteins with other homology domains such as SH3 may bind to the R:SH2 complex which leads to a much more complex mechanisms. In many instances, enzyme-linked receptors can be found in dimerized form. For example, insulin receptor is a dimer. This feature enables the cell to have a heightened response to the hormone.

CYTOKINE RECEPTORS


Cytokine receptors include the receptors for erythropoietin,colony-stimulating factor, prolactin, growth hormone(Note that this is not the same as growth factor. Growth factors have receptors with tyrosine kinase domain). These receptors don't have the tyrosine kinase domain. These receptors use accessory proteins which have intrinsic tyrosine kinase activity. One such protein is JAK2. The binding of a ligand induces these accessory proteins to bind the cytoplasmic side of the receptor and autophosphorylate the tyrosine residues. Phosphorylated JAK2 proteins trigger more reactions in the cell.

RECEPTOR REGULATION

Regulation of hormone receptors is very important for a normal functioning cell. There are several ways a cell regulates its hormone receptors. Below is an outline of such regulatory functions:

A) Regulating the expression of receptors - changing the number of receptors on the plasma membrane.
1. Up regulation - increasing the number of receptors
2. Down regulation - decreasing the number of receptors

Mechanism:
-internalization - endocytosis of receptors
-modify transcription - inhibiting or stimulating transcription factors
-modify receptor half-life - adding groups to the receptors which will degrade them faster

B) Receptor modification - addition of inhibitors or stimulating factors to the receptor.

INTRACELLULAR RECEPTORS/NUCLEAR RECEPTORS

Intracellular receptors, as mentioned above, bind steroid hormones. All steroid hormones are derived from cholesterol, which is a hydrophobic molecule. Thus, steroid hormones can diffuse through the plasma membrane and interact with receptors found inside the cell. In contrast to peptide hormones, steroid hormones do not bind to membrane-bound receptors. Steroid receptors are also not membrane spanning proteins. They are either free floating in the cytoplasm, or they are in the nucleus. Most steroid receptors have specific sites which interact with DNA and affect transcription (this is discussed in more detail below). These receptors are mostly found in multimeric complexes when they are inactive. Some of these structures include complexes with heat shock proteins. Complex formation of steroid hormones with these receptors release the heat shock protein and become activated. These active complexes enter the nucleus and interact with Hormone Response Elements (HRE) which enhances or sometimes represses transcription.

Intracellular receptors are made up of single protein chains. Three domains common to all steroid receptors have been identified (see figure 1.4). The first region is the N-terminal domain or the Hypervariable Domain (HVD). This domain, as its name suggest, is the most variable domain among the others in the protein. It varies greatlyin length, ranging from 603 amino acids to 24 amino acids. HVD is grouped according to its length: 420-603 a.a. long chain HVD mainly binds to androgens, progestrone, glucocorticoids & mineralocorticoids (see section for Hormones), 24-185 amino acid short chain HVD mainly bind to estrogens, thyroid hormones & vitamin D. Regardless of this information, the exact function of HVD's is not known. The next domain is the DNA binding domain (DBD), which is the most highly conserved domain in this family. It is between 66 to 68 amino acids long and contains a structure called "Zinc Fingers." These Zinc fingers contain eight cysteine residues which interact with zinc ions (see figure 1.5). Zinc fingers bind to hormone response sequences on the DNA. As shown in figure 1.4, there are two zinc fingers on the DBD. The one closest to the amino terminus is the P-box which sets gene specificity. The D-box, closest to the carboxyl terminus controls receptor dimerization. This is a very important region because it is responsible for the amplitude of the response (eg. heightened response when the receptor dimerizes). The last domain is the Ligand Binding Domain or LBD. It is about 200 amino acids long. This domain sets the specificity for the ligand/hormone and enhances transcription and dimerization. In the absence of a hormone, this domain inhibits the DNA-Binding Domain. This is easily demonstrated by the removal of the Ligand Binding Domain. Once it is removed, the receptor acts as if it has a hormone bound to it. This shows that LBD also controls the receptor activation system. When the receptor binds to the hormone, there is a conformational change in the LBD which causes changes in the rest of the receptor and activates it.

Intracellular Receptor

Zinc Fingers

Heat shock proteins are named based on their size: hsp 90, hsp 70 and hsp 56. They help block the DNA-Binding Domain of receptor molecules. These proteins also reverse the degradation of newly synthesized peptide chains. Their synthesis increases with the temperature and they bind to newly synthesized proteins to protect them from degradation or aggregation, hence the name, "heat shock" proteins. These proteins dissociate as the ligand forms a complex with the receptor. This uncovers the site where the receptor interacts with DNA or another protein in the signalling system.


Related Topics:
	Protein Kinase A
	Hormones
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