Signals transduction

1 BASIC CHARACTERISTICS OF TRANSMEMBRANE SIGNALLING THROUGH THE INSULIN RECEPTOR KINASE 

2 AUTOACTIVATION OF THE RECEPTOR KINASE BY TYROSINE PHOSPHORYLATION THE AUTOPHOSPHORYLATION CASCADE 

It appears that after kinase activation a signal amplification occurs through autophosphorylation. Several tyrosine residues in the /3-subunit are involved in the autophosphorylation reaction including Tyr-1146, -1150 and -1151 in the regulatory region and Tyr-1316 and -1322 at the C-terminus (Tornqvist etal., 1987, 1988 Tavare et al., 1988 White etal., 1988b). Autophosphorylation proceeds through a sequential mechanism in which Tyr-1146 and either 

Interactions between insulin and its receptor probably occur, as outlined above, at amino acids 83-103 and 205-316. It is speculated that binding of 

There is increasing evidence that the conformational change in the a-subunit is transduced to the /3-subunit and modulates the tyrosine kinase activity of the /3-subunit. The coupling of the a-subunit to the extracellular part of the /3-subunit occurs through disulphide bonds. Following the idea that the unoccupied a-subunit functions as an inhibitor of the /3-subunit (Herrera et al., 1988 Shoelson et al., 1988), it seems possible that the insulin-binding-induced conformational change in the a-subunit is transduced to the /3-subunit and releases the catalytic domain from inhibition. 

Within cells, including nerve cells, fluxes of Ca ions play an important role in signal transduction (Chapter 11). Most eukaryotic cells export calcium across the plasma membrane or deposit it in membrane-enclosed storage sites in order to maintain free cytosolic Ca levels at 100—200 nM, roughly 10,000 times less than in the extracellular space. This allows calcium to function as a second messenger and also as a carrier of biological 

Synapses are the local sites of communication between neurons. 

Synaptotagmins are yet another family of Ca -binding proteins, localised on the membranes of synaptic vesicles, where they seem to be involved in the release of neurotransmitters. While the mechanism by which they are involved in Ca -mediated synaptic transmission is unclear, it seems likely that the neurotoxicity of heavy metals, such as Pb, is due to a higher affinity of synaptotagmins for Pb than for Ca.  

Astrocytes can exocytotically release the ghotransmitter glutamate from vesicular compartments. Increased 

Inhibition of DNA repair may result in an adverse event within the cell. As(III) has been shown to inhibit DNA ligase in vitro, but it does not appear to occur by the direct interaction of As(III) with this repair enzyme (Li and Rossman, 1989). It has been suggested that As(III) alters redox levels or affects signal transduction pathways involved with DNA ligase (Hu, Su and Snow, 1998). The nucleotide excision repair system is also impaired by arsenic (Okui and Fujiwara, 1986 Hartwig et al., 1997). 

Transferrin, which is the major iron-transport protein, holds two Fe(III) atoms per molecule, and it accounts for nearly all the iron in plasma, where its concentration is usually 2-5x 10-5 M [149]. In cells and tissues, the iron release from transferrin would be controlled by local pH variations in the presence of Fe(III) chelators [149]. Conflicting reports have been published on the ability of superoxide to initiate transferrin-promoted Fenton reactions [154]. 

The existence of a transit pool of free iron , which would be in equilibrium with iron-containing proteins, is a thermodynamic necessity, but its size is probably extremely small in normal conditions. Microsomal membranes contain nonheme-iron which is released upon incubation with some intermediates of heme synthesis (reviewed in ref. [155]). The resulting free-iron pool does initiate lipid peroxidation in vitro. In the cytosol, the iron-transit pool would be composed of Fe(III) and/or Fe(II), complexed with low-molecular-weight chelators, possibly in the form of polynuclear clusters [149], 

It is noteworthy that Aust and coworkers have shown that in vitro lipid peroxidation initiated by ferrous-ADP or ferrous-AMP complexes was strongly stimulated by the presence of the analogous ferric complexes, with no effect of either SOD, catalase or OH scavengers [156,157]. This suggests that a ferrous-dioxygen-ferric chelate may serve as a potent free radical initiator of its own. 

In the absence of strong chelating agents, lactoferrin, transferrin and ceruloplasmin do not promote hydroxyl-radical production at pH 7.4 [158,159], which is consistent with their protecting role at sites of inflammation. At present, the possible involvement of ferritin and that of the iron-transit pool in Fenton-type reactions cannot be excluded, and this may be of great importance in some pathophysiological situations such as post-ischemic reperfusion of tissues. 

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Calculation of Conformational Free Energies for a Model of a Bilobal Enzyme Protein kinases catalyze the transfer of phosphate from adenosine triphosphate (ATP) to protein substrates and are regulatory elements of most known pathways of signal transduction. 

The spatial and steric requirements for high affinity binding to protein kinase C (PKC), a macromolecule that has not yet been crystallized, were determined. Protein kinase C plays a critical role in cellular signal transduction and is in part responsible for cell differentiation. PKC was identified as the macromolecular target for the potent tumor-promoting phorbol esters (25). The natural agonists for PKC are diacylglycerols (DAG) (26). The arrows denote possible sites of interaction. 

Excitation of smooth muscle via alpha-1 receptors (eg, in the utems, vascular smooth muscle) is accompanied by an increase in intraceUular-free calcium, possibly by stimulation of phosphoUpase C which accelerates the breakdown of polyphosphoinositides to form the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 releases intracellular calcium, and DAG, by activation of protein kinase C, may also contribute to signal transduction. In addition, it is also thought that alpha-1 adrenergic receptors may be coupled to another second messenger, a pertussis toxin-sensitive G-protein that mediates the translocation of extracellular calcium. 

In this chapter we describe some examples of structures of membrane-bound proteins known to high resolution, and outline how the elucidation of these structures has contributed to understanding the specific function of these proteins, as well as some general principles for the construction of membrane-bound proteins. In Chapter 13 we describe some examples of the domain organization of receptor families and their associated proteins involved in signal transduction through the membrane. 

The augmentation of a p sheet in one protein by a strand emanating from another is a mode of protein association not restricted to viral shells. Small domains involved in intracellular signal transduction bind to "arms" of other proteins by presenting the edge of a sheet on which those arms can form an additional strand. 

Depletion of ATP in the cells prevents maintenance of the membrane potential, inhibits the functioning of ion pumps, and attenuates cellular signal transduction (e.g., formation of second messengers such as inositol phos phates or cyclic AMP). A marked ATP depletion ultimately impairs the activ-itv of the cell and leads to ceil death. 

Felder, C. C. (1995). Muscarinic acetylcholine receptors . Signal transduction through multiple effectors. FASEB j. 9, 619-625. 

Cells make use of many different types of membranes. All cells have a cytoplasmic membrane, or plasma membrane, that functions (in part) to separate the cytoplasm from the surroundings. In the early days of biochemistry, the plasma membrane was not accorded many functions other than this one of partition. We now know that the plasma membrane is also responsible for (1) the exclusion of certain toxic ions and molecules from the cell, (2) the accumulation of cell nutrients, and (3) energy transduction. It functions in (4) cell locomotion, (5) reproduction, (6) signal transduction processes, and (7) interactions with molecules or other cells in the vicinity. 

Airother interesting facet of lipid anchors is that they are transient. Lipid anchors can be reversibly attached to and detached from proteins. This provides a switching device for altering the affinity of a protein for the membrane. Reversible lipid anchoring is one factor in the control of signal transduction pathways in eukaryotic cells (Chapter 34). 

Thiophene, 2,2-bithiophene, and 2,2, 5, 2"-terthiophene derivatives from Chinese medicinal plants as oncogene signal transduction inhibitors (proteinki-nase C inhibitors) 99PAC1101. 

Heldin, C. H. (1995). Dimerization of cell surface receptors in signal transduction. Cell 80 213-223. 

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