receptorsproteins

Receptor proteins are proteins that bind signaling molecules, or ligands, and initiate cellular responses. They serve as specific recognition units that translate external or internal cues into biochemical actions, enabling cells to respond to hormones, neurotransmitters, growth factors, nutrients, and other signals. Receptors can reside on the cell surface or inside the cell, and their function depends on their structure and localization.

Most signaling receptors are cell-surface proteins. These include G protein-coupled receptors, receptor tyrosine kinases, ligand-gated ion channels, and enzyme-linked receptors. Binding of a ligand often causes a conformational change that activates intracellular signaling cascades, such as second messenger systems (for example cyclic AMP or calcium), kinase chains, or transcriptional programs. Nuclear or intracellular receptors exist for lipophilic ligands like steroid hormones; upon binding, they typically move to the nucleus to regulate gene expression.

The interaction between a receptor and its ligand is characterized by specificity and affinity. Receptors can amplify signals through cascades, integrate multiple inputs, and produce diverse cellular outcomes, from rapid ion fluxes to long-term changes in gene expression. Receptors are also subject to regulation: they can be desensitized, internalized, recycled to the surface, or degraded in response to ongoing stimulation, allowing cells to tune sensitivity.

Receptor proteins are central to physiology and medicine. They are common drug targets, with therapies aiming to activate, block, or modulate receptor signaling. Examples include insulin receptors, beta-adrenergic receptors, nicotinic acetylcholine receptors, and receptor tyrosine kinases such as EGFR. Understanding receptor function illuminates how cells perceive their environment and coordinate complex biological responses.