<Li> Type 2: G protein - coupled receptors (metabotropic)--This is the largest family of receptors and includes the receptors for several hormones and slow transmitters e.g. dopamine, metabotropic glutamate . They are composed of seven transmembrane alpha helices . The loops connecting the alpha helices form extracellular and intracellular domains . The binding - site for larger peptide ligands is usually located in the extracellular domain whereas the binding site for smaller non-peptide ligands is often located between the seven alpha helices and one extracellular loop . The aforementioned receptors are coupled to different intracellular effector systems via G proteins . </Li> <Li> Type 3: Kinase - linked and related receptors (see "Receptor tyrosine kinase", and "Enzyme - linked receptor") - They are composed of an extracellular domain containing the ligand binding site and an intracellular domain, often with enzymatic - function, linked by a single transmembrane alpha helix . The insulin receptor is an example . </Li> <Li> Type 4: Nuclear receptors--While they are called nuclear receptors, they are actually located in the cytoplasm and migrate to the nucleus after binding with their ligands . They are composed of a C - terminal ligand - binding region, a core DNA - binding domain (DBD) and an N - terminal domain that contains the AF1 (activation function 1) region . The core region has two zinc fingers that are responsible for recognizing the DNA sequences specific to this receptor . The N terminus interacts with other cellular transcription factors in a ligand - independent manner; and, depending on these interactions, it can modify the binding / activity of the receptor . Steroid and thyroid - hormone receptors are examples of such receptors . </Li> <P> Membrane receptors may be isolated from cell membranes by complex extraction procedures using solvents, detergents, and / or affinity purification . </P>

Which type of receptor produces a change in gene expression when activated