Purpose of cellular signaling
Cells of a multicellular organism constantly exchange information to coordinate growth, division, and functional activity. Signal transmission is carried out using signaling molecules: hormones, neurotransmitters, immunomodulators, and growth factors. In response to changes in the external or internal environment, the source cell secretes such molecules, and the target cell alters its activity—activating or suppressing intracellular processes, dividing, or secreting proteins.
The interaction between a signaling molecule and a cell is described by the "lock and key" principle: the ligand has a unique conformation and active sites that are complementary to a strictly specific receptor. This ensures the specificity of signaling—every hormone corresponds to its own receptor.
Receptor types and their domain organization
Receptors are divided into two groups:
- Membrane receptors—embedded in the plasma membrane;
- Cytoplasmic receptors—located in the cytoplasm or the cell nucleus.
A membrane receptor has three distinct domains: the extramembrane (receptor) domain, which binds the ligand; the transmembrane domain, which spans the bilayer; and the submembrane (effector) domain, which is responsible for the intracellular response.
A cytoplasmic receptor contains a signal-binding domain, a DNA-binding domain, and an effector domain. The presence of a DNA-binding site means that such a receptor acts on template processes—transcription and translation.
Fast and slow responses
The type of cellular response is determined by the receptor type:
- Membrane receptor → fast response. Upon ligand binding, the effector domain alters the activity of preexisting intracellular proteins (phosphorylation, dephosphorylation, conformational changes). This takes seconds.
- Cytoplasmic receptor → slow response. A lipophilic signaling molecule diffuses through the membrane, binds to a receptor in the cytoplasm or nucleus, the complex interacts with DNA, transcription and translation are initiated, and a new protein is synthesized. This process requires significantly more time.
Chemical synapse as an example of a fast response
Ca²⁺ enters the axon terminal through voltage-gated channels. Calcium stimulates the fusion of vesicles with the presynaptic membrane and the release of acetylcholine into the synaptic cleft. The neurotransmitter binds to a receptor on the postsynaptic membrane (e.g., of a muscle cell), leading to the opening of the sodium channel. The change in membrane potential opens voltage-gated calcium channels, and the incoming calcium triggers muscle contraction.
Adenylate cyclase and guanylate cyclase systems
A hormone binds to a membrane receptor, which activates a G protein consisting of α, β, and γ subunits. The α subunit exchanges GDP for GTP, dissociates, and diffuses along the membrane to the enzyme adenylate cyclase. The enzyme converts ATP into cAMP, a secondary messenger. cAMP activates protein kinase A, which phosphorylates target proteins, turning them on or off.
The guanylate cyclase system is arranged similarly: the enzyme guanylate cyclase synthesizes cGMP from GTP, activating protein kinase G. The result is protein phosphorylation, for example, the relaxation of vascular smooth myocytes.
Phosphodiesterase degrades cAMP into AMP, inactivating the signal, and acts as an antagonist to the adenylate cyclase system.
Phospholipase system
Following G-protein activation, the GTP-bound α subunit activates phospholipase C. The enzyme cleaves a membrane phospholipid into two secondary messengers:
- Diacylglycerol (DAG)—remains in the membrane, diffuses to protein kinase C, which phosphorylates proteins;
- Inositol 3-phosphate (IP₃)—binds to the endoplasmic reticulum receptor, opening calcium channels. Ca²⁺ enters the cytoplasm as a tertiary messenger and triggers muscle contraction, exocytosis, and other processes.
Clinical and pharmacological examples
Caffeine enhances the adenylate cyclase system and inhibits phosphodiesterase, causing cAMP to accumulate and protein kinase A to work more intensively. The alerting effect is not associated with the generation of new energy, but with the maximal utilization of existing metabolic reserves.
Cholera toxin from Vibrio cholerae irreversibly activates the G protein of enterocytes, leading to sustained adenylate cyclase activity and cAMP accumulation. Protein kinase A phosphorylates the chloride channel—Cl⁻ exits into the intestinal lumen, followed by Na⁺ and water down a concentration gradient. Profuse diarrhea and severe dehydration develop.
Key points
- Signaling molecules interact with receptors according to the "lock and key" principle.
- Membrane receptors provide a fast response by altering the activity of preexisting proteins.
- Cytoplasmic receptors trigger a slow response via the synthesis of new proteins.
- The G protein is a universal switch: the GDP/GTP exchange of the α subunit triggers a cascade.
- Adenylate cyclase system: cAMP → protein kinase A → protein phosphorylation.
- The phospholipase system yields two messengers—DAG (PKC) and IP₃ (Ca²⁺ release from the ER).
- Phosphodiesterase degrades cAMP and suppresses the adenylate cyclase pathway.
- Cholera toxin and caffeine serve as clinical illustrations of pathological and pharmacological stimulation of cAMP signaling.