signaltransduction

Signal transduction refers to the process by which a cell converts an extracellular signal into a functional response. It involves reception by a receptor, relay by intracellular signaling molecules, and execution of a cellular response. Receptors include cell-surface receptors such as G protein-coupled receptors and receptor tyrosine kinases, as well as intracellular receptors for lipophilic signals. Upon ligand binding, receptors undergo conformational changes that initiate signaling cascades.

Transduction pathways employ second messengers such as cyclic AMP, calcium ions, inositol phosphates, and diacylglycerol. Signal propagation is mediated by kinases and phosphatases in cascades, including MAP kinase, PI3K-Akt, JAK-STAT, and Rho-family GTPases. Amplification allows a single extracellular signal to affect many targets. Scaffolding proteins organize modules to enhance specificity and speed, while feedback and cross-talk regulate output.

Signals can be graded or digital, and responses may be rapid or involve changes in gene expression. The spatial organization of signaling components within membranes and organelles, as well as receptor endocytosis and trafficking, influence signal duration and localization.

In biology, signal transduction governs development, metabolism, immune responses, sensory perception, and tissue homeostasis. Dysregulation of signaling pathways is associated with diseases such as cancer, diabetes, and neurodegeneration. Therapeutic approaches target receptors or kinases with antagonists, inhibitors, monoclonal antibodies, or other agents.