Most signal receptors are plasma membrane proteins. Most signal molecules are water-soluble and too large to pass through the plasma membrane. They influence cell activities by binding to receptor proteins on the plasma membrane. Binding leads to changes in the shape of the receptor or to the aggregation of receptors. These cause changes in the intracellular environment. There are three major types of membrane receptors: G-protein-linked receptors, receptor tyrosine kinases, and ion-channel receptors. A G-protein-linked receptor consists of a receptor protein associated with a G protein on the cytoplasmic side. Seven alpha helices span the membrane. G-protein-linked receptors bind many different signal molecules, including yeast mating factors, epinephrine and many other hormones, and neurotransmitters.
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