Characteristics of ligands and organization of the receptor apparatus
Every cell contains signal transduction systems whose task is to convert external signals into intracellular changes. Receptors on the cell membrane may be located at a distance from each other or concentrated in specific areas — clustered.
Intracellular receptors are designed to interact with hydrophobic (lipophilic) substances. Classical examples of such ligands are steroid hormones and thyroxine. Due to their chemical nature, these molecules do not need membrane carriers: they can freely diffuse directly through the lipid bilayer of the plasma membrane into the cell.
Structural features of intracellular receptors
Receptors for hydrophobic ligands are localized inside the cell — in the cytosol or directly in the nucleus.
In the absence of a signal, cytosolic receptors are in an inactive state. This inactivity is ensured by special chaperone proteins bound to the receptor. The main function of chaperones is to prevent premature activation of the receptor molecule before the hormone enters the cell.
A key structural feature of both nuclear and cytosolic receptors for steroid and thyroid hormones is the presence of a DNA-binding domain. It is this region of the protein that ensures the ability of the resulting hormone-receptor complex to interact with regulatory DNA sequences in the nucleus.
Signal transduction pathway (genomic mechanism)
The process of signal transmission via intracellular receptors is a nuclear mechanism that leads to a change in the rate of gene transcription. It includes the following stages:
- Penetration: the lipophilic hormone passes freely through the plasma membrane into the target cell.
- Complex formation: in the cytosol (or nucleus), the hormone meets its receptor. A conformational change of the receptor occurs, and a functional unit is formed — the hormone-receptor complex.
- Nuclear translocation: if the complex was formed in the cytosol, it moves into the nucleus.
- DNA binding: the complex attaches to a specific regulatory nucleotide sequence. This can be an enhancer or a silencer.
- RNA polymerase regulation: the interaction of the complex with DNA alters the accessibility of the promoter for the RNA polymerase enzyme. Upon binding to an enhancer, accessibility increases; upon binding to a silencer, it decreases.
- Transcription modification: as a result, the transcription (mRNA synthesis) of specific structural genes is activated or suppressed.
- Processing and export: synthesized mature mRNA molecules exit the nucleus into the cytosol.
- Translation: template biosynthesis takes place on ribosomes in the cytoplasm based on mRNA — the translation rate of specific proteins increases or decreases.
- Biological effect: the change in the number of synthesized proteins directly affects the metabolism and functional state of target cells.
Example of effect execution: the action of calcitriol
The mechanism of action of calcitriol is a classical example of the cytosolic (genomic) signal transduction pathway.
Calcitriol, being a lipophilic hormone, freely penetrates the membrane of the target cell — an enterocyte. In the cytoplasm or nucleus, it binds to a specific intracellular receptor. The resulting hormone-receptor complex dimerizes and is transported into the nucleus.
In the nucleus, the complex binds to regulatory DNA regions called hormone response elements (HREs). This leads to the activation of transcription of a specific gene encoding a calcium ion carrier protein.
Next, translation takes place on ribosomes in the cytoplasm. Specific calcium-binding proteins — calbindins — are synthesized, which ultimately alter cell metabolism, ensuring the execution of its biological function.
Briefly: answers to topical questions
What classes of hormones are ligands for intracellular receptors?
Ligands for intracellular receptors are hydrophobic (lipophilic) substances.
Hormones acting through intracellular receptors include:
- Steroid hormones.
- Thyroid hormones — for example, thyroxine.
These molecules are capable of freely passing through the lipid bilayer of the plasma membrane and interacting with receptors in the cytosol or nucleus.
What functional domains does an intracellular receptor molecule consist of?
The sources mention only the DNA-binding domain.
- DNA-binding domain — ensures the interaction of the hormone-receptor complex with regulatory DNA regions in the nucleus.
To remember the essence of the genomic mechanism, use the PRTI sequence: Penetration (through the membrane) → Reception (in the cytosol) → Translocation (into the nucleus) → Induction/alteration of transcription (binding to DNA).
Class and exam questions
Why do steroid hormones not use membrane receptors?
Steroids are hydrophobic (lipophilic) substances. They easily dissolve in the lipid bilayer of the cell membrane and pass freely through it, which is why their receptors are located inside the cell.
Why are chaperone proteins needed in the cytosol?
Chaperones hold the intracellular receptor in an inactive state. They prevent it from changing conformation and traveling to the nucleus until a hormone molecule binds to it.
What are enhancers and silencers?
These are regulatory DNA regions. Attachment of the hormone-receptor complex to an enhancer increases the accessibility of the gene to RNA polymerase (accelerates transcription), while attachment to a silencer decreases it.
What else to study on this topic
- mRNA processing prior to export from the nucleus into the cytosol
- Mechanism of dimerization of the hormone-receptor complex
- Differences between the nuclear mechanism and the membrane mechanism (via second messengers)
- Functions of calbindins in enterocyte metabolism
- Comparison of paracrine, synaptic, and intracellular signaling types
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Intracellular receptors" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in «Structure and Functions of Biological Membranes»
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.