There is some controversy regarding the location of the steroid hormone receptors prior to their binding to steroid hormones. They have been found in both the cytoplasm and the nucleus, and it appears that a particular steroid hormone receptor is distributed in a characteristic way between both compartments. When the cytoplasmic receptor binds to its specific steroid hormone ligand, however, it translocates (moves) into the nucleus, where its DNA-binding domain binds to the specific hormone-response element of the DNA (see fig. 11.4).
As illustrated in figure 11.5, the hormone-response element of DNA consists of two half-sites, each six nucleotide bases long, separated by a three-nucleotide spacer segment. One steroid receptor, bound to one molecule of the steroid hormone, attaches as a single unit to one of the half-sites. Another steroid receptor, bound to another steroid hormone, attaches to the other half-site of the hormone-response element. The process of two receptor units coming together at the two half-sites is called dimerization (fig 11.5). Since both receptor units of the pair are the same, the steroid receptor is said to form a homodimer. (The situation is different for the nonsteroid family of receptors, as will be described.) Once dimerization has occurred, the activated nuclear hormone receptor stimulates transcription of particular genes, and thus hormonal regulation of the target cell (see fig. 11.4).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.