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BIS104 Note8 - Lecture 8 Bio Sci 104 Enzyme-linked...

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Lecture 8 1 Bio Sci 104 Winter 2010 Enzyme-linked receptors I. Membrane receptors that possess enzymatic activities A. We have already discussed one major class of cell surface receptors which mediate signaling through trimeric-G-protein complexes. This class of receptors generally has multiple membrane crossing segments, and no "classical" enzymatic activity B. Second major class of ell surface receptors: Enzyme-coupled membrane receptors. ( most of these receptors possess enzymatic activities relating to phosphorylation (kinases or phosphatases). Best characterized are: (1) receptors that are tyrosine protein kinases, (2) a second class of receptors are not themselves tyrosine kinases, but associate with tyrosine kinases, also (3) receptors that are serine/threonine kinases, (4) receptors that are tyrosine phosphatases (5) receptors that are guanylyl cyclases, and (6) receptors that are associated with histidine kinase. II. Receptors that are tyrosine protein kinases (Receptor Tyrosine Kinases, RTK) A. Many are receptors for polypeptide hormones that regulate cell growth and proliferation (growth factors), but these hormones can also play additional roles unrelated to cell proliferation. Examples include receptors for platelet-derived growth factor (PDGF), epidermal growth factor (EGF), nerve growth factor (NGF), macrophage colony stimulating factor (M-CSF), and insulin etc. 1. Growth of most eukaryotic cells in culture requires more than vitamins, sugars, amino acids, minerals. Cell proliferation also requires positive growth- stimulatory signals normally provided within living organism. Search for these factors uncovered a wide assortment of these growth factors . 2. Some growth factors, such as insulin, not only regulate cell proliferation, but also homeostasis (e.g. glucose storage and utilization). 3. Growth factors themselves are small polypeptides, synthesized by a variety of different cell types, and either circulated in blood, or presented on the surface of nearby adjacent cells B. Receptors for growth factors are single transmembrane domain polypeptides, with outside ligand binding domain and inside catalytic domain. Extracellular domains can be quite different among receptors for different growth factors. However, internal catalytic domains generally share strong sequence similarities, act to transfer phosphate from ATP to specific tyrosine residues in target proteins (which tyrosine gets phosphorylated is determined by the context of surrounding sequences). 1. Generally, internal kinase domain is inactive in absence of receptor ligand; activated when extracellular ligand is bound. 2. How is signal transmitted across membrane? Probably by growth factor induced receptor dimerization , either because growth factor itself is a protein dimer, or by inducing the receptor to dimerize through a conformational
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Lecture 8 2 change. (ie. To activate a receptor tyrosine kinase, the ligand usually has to bind simultaneously to two adjacent receptors.) 3. Dimerization of the receptors allows cross-phosphorylation (trans-auto- phosphorylation,
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