G-protein-linked receptors are protein receptors, located in the plasma membrane of a cell, that work with G-proteins to activate a cell-signaling pathway. These receptors are structured similarly in most organisms, with seven α helices and specific loops for binding sites for signal molecules and G-proteins. When a signal molecule from the extracellular fluid attaches to the signal-binding site it activates the G-protein-linked receptor by changing its shape. When this happens, the G-protein, loosely attached to the cytoplasmic side of the cellular membrane, attaches to its binding side on the receptor protein. The inactive G-protein becomes activated when GDP is displaced by GTP, a molecule similar to ATP. When the signal molecule is released, the G-protein diffuses along the cell membrane and attaches to an inactive enzyme. This newly activated enzyme triggers the cellular response. When the protein detaches itself from the enzyme, it releases a phosphate group turning GTP back into GDP, making the G-protein inactive once again.
Receptor tyrosine kinase is a cell membrane receptor system that can trigger multiple cellular responses simultaneously. It requires two receptor tyrosine kinase proteins, which are initially individual polypeptides that each have a signal-binding site, an α helix spanning the cell membrane, and a tail of multiple tyrosines. When signal molecules bind to both proteins they attach through a process called dimerization, forming a dimer. This process activates, or phosphorylates, the ends of the tyrosines, also known as tyrosine-kinase regions. Once the dimer is activated, multiple inactive relay proteins are able to bind to the tyrosine-kinase regions. Each of these proteins trigger a cellular response.
A ligand-gated ion channel is a cell membrane receptor system that uses a ligand-gated ion channel receptor as a gate to allow ions to pass through the cell membrane. When a signal molecule attaches to the ligand-gated ion channel receptor, it changes the receptor’s shape which “opens the gate” for ions to enter the cell. The ions trigger a cellular response and the signal molecule detaches from the receptor, “closing” the gate.
These tree cell membrane receptor systems are examples of the first steps in signal transduction...