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4 out of 10

Steroid receptors bind to the following except? (AIIMS MAY 2008)

A Transcription mediators

B Transcription promoters

C Transcription repressors

D Steroid response elements

Ans. C Transcription repressors
  1. Steroid hormone receptors are intracellular receptors (typically cytoplasmic) that perform signal transduction for steroid hormones.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Type I Receptors

Sex hormone receptors (sex hormones)

a. Androgen receptor

b. Estrogen receptor

c. Progesterone receptor

d. Glucocorticoid receptor (glucocorticoids)

e. Mineralocorticoid receptor (mineralocorticoids)

Type II Receptors

a. Vitamin A receptor (Vitamin A)

b. Vitamin D receptor (Vitamin D)

c. Retinoid receptor

d. Thyroid hormone receptor

e. Orphan receptors

  1. Steroid hormone receptors are proteins that have a binding site for a particular steroid molecule.
  2. Their response elements are DNA sequences that are bound by the complex of the steroid bound to its receptor.
  3. The response element is part of the promoter of a gene.
  4. Binding by the receptor activates or represses, as the case may be, the gene controlled by that promoter.
  5. It is through this mechanism that steroid hormones turn genes on (or off).
  6. For a steroid hormone to turn gene transcription on, its receptor must: bind to the hormone bind to a second copy of itself to form a homodimer be in the nucleus, moving from the cytosol if necessary bind to its response element activate other transcription factors to start transcription.