G Protein-Coupled Receptors (GPCR)
7-Transmembrane receptors (7-TM receptors), also known as G-protein-coupled receptors (GPCRs), are membrane proteins that contain seven membrane-spanning helices. As the name suggests they are coupled to heterotrimeric G proteins on the intracellular side of the membrane. Upon ligand binding, the GPCR undergoes a conformational change which is transmitted to the G protein causing activation. Further signal transduction depends on the type of G-protein.
Ligand-Gated Ion Channels
Ion channels are pore-forming membrane proteins whose functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions, Na+, Ca2+ & Cl-, in and out of a cell through the plasma membrane. Hundreds of different types of ion channel have been identified in the membranes of all biological cells.
Transporters (carrier proteins) are specialized membrane-spanning proteins that assist in the movement of ions, peptides, small molecules, lipids and macromolecules across a biological membrane.
Neurotransmitter Transporters (NT) are a class of transmembrane proteins that carry neurotransmitters across biological membranes to specific cellular and subcellular locations. They are found in the plasma membrane of neurons and glia and many members of this family are sodium-dependent. They often rely on an electrochemical gradient across the membrane to provide energy for neurotransmitter transport, for example Na+ cotransport provides the energy necessary for glutamate transport. NT are found at high concentrations in synaptic clefts where they act to terminate the effects of the neurotransmitter by removing it from the cleft.
Neurotransmitter Transporters are of great clinical interest. They are the pharmacological target of many drugs in the treatment of depression (for example serotonin-noradrenalin reuptake inhibitors (SNRIs)), epilepsy and schizophrenia. In addition, narcotics such as cocaine exert their effects through actions on this class of transporters.
Nuclear Hormone Receptors
Nuclear receptors (also known as nuclear hormone receptors) are a large family of transcription factors that bind directly to DNA to regulate the expression of target genes. They regulate the cellular response to hormones such as sex steroids, vitamin D3, adrenal steroids and other metabolic ligands, and are involved in metabolism, development and reproduction.