Hormonal Control of Gene Expression
Hormones are molecules that are produced in one cellular location in an organism, and whose effects are seen in another tissue or cell type. In mammals hormones can be proteins or steroids. The protein hormones do not enter the cell, but bind to receptors in the cell membrane and mediate gene expression through intermediate molecules. Steroids, though actually enter the cell and interact with steroid receptor proteins to control gene expression.
Glucocorticoid is one type of steroid whose method of controlling gene expression has been determined. The steroid interacts with a receptor protein, and this interaction serves two function. First, binding stimulates the release of the protein Hsp90 that is bound to the receptor protein. When Hsp90 is bound to the receptor protein, gene expression is not activated. This would be expected, if the steroid is the signal required for the expression of specific genes in the tissue. When the steroid is bound and Hsp90 is released, the receptor protein forms a dimer (two proteins together) with another copy of the receptor protein. This complex then binds to specific enhancer sequences and gene expression is activated.
A number of steroid receptor proteins have been characterized, and a number of features are in common among them. First, the N-teriminal region of the protein is required to activate transcription in some manner, but the mechanism is not known. This is the least conserved region among the eight proteins. The central portion of the protein is required for DNA binding, and this region is highly conserved (42-94% amino acid identity). The C-terminal region is required for steroid binding and is moderately conserved (15-52% amino acid identity). This overall conservation suggests that an ancestral gene may have been the model for each of these genes.
Copyright © 1997. Phillip McClean