Membrane surface receptors pass their extracellular signal across the plasma membrane to cytoplasmic secondary signalling molecules. Examples of these receptors include the non-lipid-soluble ligands such as growth hormone (GH), insulin, insulin-like growth factor (IGF) and luteinizing hormone (LH). These membrane-bound receptors can be subclassified according to the mechanism by which they activate signalling molecules:

• ion channel linked (see above)
• G-protein linked
• enzyme linked.

Structurally these plasma membrane receptors can be:

• serpentine (seven transmembrane domains, e.g. the LH receptor)
• transmembrane with large extra- and intracellular domains (e.g. the epidermal growth factor (EGF) receptor)
• transmembrane with a large extracellular domain only
• entirely linked onto the outer membrane leaflet by a lipid moiety known as a GPI (glycan phosphatidylinositol) anchor (e.g. T-cell receptor).

The function of these membrane receptors is to initiate a secondary message that ultimately results in activation of a specific enzyme or a DNA-binding protein. This may involve translocation to the nucleus and initiation of transcription of a specific set of genes.

G-protein-linked receptors

The G-protein-linked receptor, once activated by a ligand, binds a trimeric complex (α,β,γ) which is anchored to the inner surface of the plasma membrane. This complex is a GTP-binding protein, or G-protein. The G-protein binds GTP rather than GDP, and then interacts with enzyme complexes anchored into the inner leaflet of the membrane. These complexes in turn activate one or all three of the secondary messengers:

• cyclic AMP (cAMP)
• Ca²⁺ ions
• inositol 1,4,5-trisphosphate/diacylglycerol (IP₃/DAG).

Enzyme-linked surface receptors

These receptors usually have a single transmembrane-spanning region, and a cytoplasmic domain that has intrinsic enzyme activity or will bind and activate other membrane-bound or cytoplasmic enzyme complexes. Four classes of enzymes have been designated:

• Guanylyl cyclase-linked receptors (e.g. the atrial natriuretic peptide receptor), which produce cyclic GMP. This in turn activates a cGMP-dependent kinase (G-kinase), which binds to and phosphorylates serine and threonine residues of specific secondary messengers.
• Tyrosine kinase receptors (e.g. the platelet-derived growth factor (PDGF) receptor), which either specifically phosphorylate kinases on a small set of intracellular signalling proteins, or associate with proteins that have tyrosine kinase activity.
• Tyrosine phosphatase receptors (e.g. CD45), which remove phosphates from tyrosine residues of specific intracellular signalling proteins.
• Serine/threonine kinase receptors (e.g. the transforming growth factor-beta (TGF-β) receptor), which phosphorylate specific serine and threonine residues of intracellular signalling proteins.

There are many intracellular receptors that bind lipid-soluble ligands such as steroid hormones (e.g. progesterone, cortisol, T3 and T4). These cytoplasmic receptors often change shape in response to binding their ligands, form dimers, enter the nucleus and interact directly with specific DNA sequences.

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