G-proteins: Heterotrimeric G proteins have three subunits: α, β, and γ. The subunits reassociate to form the inactive G-protein, and the cycle starts over. Later, the GTP on the active α subunit of the G-protein is hydrolyzed to GDP and the β subunit is deactivated. One or both of these G-protein fragments may be able to activate other proteins as a result. The subunits of the G-protein then split into the α subunit and the β subunit. Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP. Before the ligand binds, the inactive G-protein can bind to a newly-revealed site on the receptor specific for its binding. All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.Ĭell signaling using G-protein-linked receptors occurs as a cyclic series of events. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane. G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors. The size and extent of each of these domains vary widely, depending on the type of receptor.Ĭell-surface receptors are involved in most of the signaling in multicellular organisms. Cell-surface receptors are also called cell-specific proteins or markers because they are specific to individual cell types.Įach cell-surface receptor has three main components: an external ligand-binding domain (extracellular domain), a hydrophobic membrane-spanning region, and an intracellular domain inside the cell. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. This type of receptor spans the plasma membrane and performs signal transduction, converting an extracellular signal into an intracellular signal. Many intracellular receptors are transcription factors that interact with DNA in the nucleus and regulate gene expression.Ĭell-surface receptors, also known as transmembrane receptors, are cell surface, membrane-anchored, or integral proteins that bind to external ligand molecules. Intracellular Receptors: Hydrophobic signaling molecules typically diffuse across the plasma membrane and interact with intracellular receptors in the cytoplasm. Internal receptors can directly influence gene expression without having to pass the signal on to other receptors or messengers. The ligand-receptor complex moves into the nucleus, binds to specific regulatory regions of the chromosomal DNA, and promotes the initiation of transcription. When the ligand binds to the internal receptor, a conformational change exposes a DNA-binding site on the protein. Gene expression is the cellular process of transforming the information in a cell’s DNA into a sequence of amino acids that ultimately forms a protein. Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis to mediate gene expression. Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane.
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