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Steroid hormone receptor
Steroid hormone receptors are intracellular receptors (typically cytoplasmic) that perform signal transduction for steroid hormones. Steroid hormone receptors are part of the nuclear receptor family that include a group of homologous structured receptors (type II receptors) that bind to non-steroid ligands such as thyroid hormones and vitamin A, as well as to vitamin D, and orphan receptors. All these receptors are transcription factors. Depending upon the steroid hormone that they bind, they are either located in the cytosol and move to the cell nucleus upon activation, or spend their life in the nucleus waiting for the the steroid hormone to enter and activate them. This uptake into the nucleus has to do with Nuclear Localization Signals (NLS) found in a region of the receptor. In most cases this signal is covered up by heat shock proteins which bind the receptor until the hormone is present. Upon binding by the hormone the receptor undergoes a conformational change, the heat shock proteins come off, and the receptor together the with bound hormone enter the nucleus to act upon transcription.
Additional recommended knowledge
Steroid hormone receptors share a common structure of four units that are functionally homologous, so-called "domains":
Only type I receptors have a heat shock protein (hsp) associated with the inactive receptor that will be released when the receptor interacts with the ligand. Type I receptors may be found in homodimer or heterodimer forms. Type II receptors have no hsp, and in contrast to the classical type I receptor are located in the cell nucleus.
There is some evidence that certain steroid hormone receptors can extend through lipid bilayer membranes at the surface of cells and might be able to interact with hormones that remain outside of cells.
Steroid hormone receptors can also function outside of the nucleus and couple to cytoplasmic signal transduction proteins such as PI3k and Akt kinase.
Free (that is, unbound) steroids enter the cell cytoplasm and interact with their receptor. In this process heat shock protein is dissociated, and the activated receptor-ligand complex is translocated into the nucleus.
After binding to the ligand (steroid hormone), steroid receptors often form dimers. In the nucleus the complex acts as transcription factors, augmenting or suppressing transcription of particular genes by its action on DNA. As a result messenger RNA is produced that exits the nucleus and interacts with ribosomes. There, after translation of the genetic message, specific proteins are produced. These specific proteins perform a biological task.
Type II receptors are located in the nucleus. Thus their ligands pass through the cell wall and cytoplasm and enter the nucleus, where they activated the receptor without release of hsp. The activated receptor interacts with the hormone response element, and the transcription process is initiated as with type I receptors.
Action on DNA
The hormone response elements (HRE) for steroid hormone receptors are DNA sequences with the structure of a pair of palindrome or tandem sequences often separated by three nucleotides. These elements resemble each other in their length and arrangement but differ in their sequences.
A given hormone-receptor complex's ability to cause a change in the expression of the gene it regulates depends on the specific HRE sequence, the distance of HRE from the gene and the number of HRE affecting the gene.
The biological response is influenced by the amount of hormones available, the available receptor population, the dissociation rate of the hormone-receptor complex with the specific DNA site, and the replenishment of the receptor population.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Steroid_hormone_receptor". A list of authors is available in Wikipedia.|