Cell surface receptor dimerization

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

Etanercept is a fully human dimeric fusion protein composed of human TNF-a p75 receptor fused to the Fc portion of human IgG 1.41 It acts as a tumor necrosis factor-a (TNF-a) inhibitor by binding to and inactivating TNF-a, thus preventing interactions with its cell surface receptors.41 This agent is useful for chronic moderate to severe plaque psoriasis and for psoriatic... 

Enbrel is a product now approved for medical use that is based upon this strategy. The product is an engineered hybrid protein consisting of the extracellular domain of the TNF p75 receptor fused directly to the Fc (constant) region of human IgG (see Box 13.2 for a discussion of antibody structure) The product is expressed in a CHO cell line from which it is excreted as a dimeric soluble protein of approximately 150 kDa. After purification and excipient addition (mannitol, sucrose and trometamol), the product is freeze-dried. It is indicated for the treatment of rheumatoid arthritis and is usually administered as a twice-weekly s.c. injection of 25 mg product reconstituted in WFI. Enbrel functions as a competitive inhibitor of TNF, a major pro-inflammatory cytokine. Binding of TNF to Enbrel prevents it from binding to its true cell surface receptors. The antibody Fc component of the hybrid protein confers an extended serum half-life on the product, increasing it by fivefold relative to the soluble TNF receptor portion alone. 

Fan, G.-F., Ray, K., Zhao, X., Goldsmith, R K and Spiegel, A. M. (1998) Mutational analysis of the cysteines in the extracellular domian of the human Ca2+ receptor effects on cell surface expression, dimerization and signal transduction. FEBSLett. 436, 353-356. 

The cell surface additionally displays receptors responsible for cell-cell recognition [28]. Members of this class of receptors are selectins [29] that recognize specific carbohydrates from other cells in the presence of calcium. Other cell surface receptors belong to the immunoglobulin superfamily (IgSF) [30] that promote calcium-independent cell-cell adhesion. The third important class are the calcium-dependent cell adhesion molecules, the cadherins [31], which form dimers with cadherin molecules presented on the surfaces of other cells and hence promote aggregation of similar cell types. 

JAKs and signal transducers and activators of transcription (STATs) are functionally analogous with IRS and PI3K. JAKs are physically associated with a cell surface receptor (e.g. for leptin, erythropoietin (EPO), growth factors or cytokines) STATs are free monomeric proteins within the cytosol but following phosphorylation by a JAK, individual proteins dimerize and then move into the nucleus of the cell where they control gene expression. 

Interferons There are two types of interferons Type I, which includes IFN-a and IFN-jS, and Type II consisting of IFN-y. IFN-a and IFN- 8 have about 30% homology in amino acid sequence. There are two more recently discovered Type I interferons they are called IFN-o and IFN-t. IFN-a and IFN- 8 each have 166 amino acids, and IFN-yhas 143. Both IFN-a and IFN-jS are of single chain structure and bind to the same type of cell surface receptors, whereas IFN-y is a dimer of two identical chains and interacts with another type of receptor. All our cells can produce Type I interferons when infected by viruses, bacteria, and fungi. However, only T cells and natural killer cells can produce... 

Fibronectins are typical representatives of adhesive proteins. They are filamentous dimers consisting of two related peptide chains (each with a mass of 250 kDa) linked to each other by disulfide bonds. The fibronectin molecules are divided into different domains, which bind to cell-surface receptors, collagens, fibrin, and various proteoglycans. This is what gives fibronectins their molecular glue" characteristics. 

Figure 8.13. Growth hormone found in the circulation is generally bound to GH-binding proteins. Binding to the cell surface receptor promotes receptor dimerization and phosphorylation and hence activation. This leads to the phosphorylation of various cystolic protein substrates, which mediate intracellular effects of the hormone...

Although both IFN-7 and lL-10 swap the same secondary structural elements (helices E and F) to form the dimers, their quaternary structures are very different. IFN-7 and lL-10 adopt inter-domain angles of approximately 60° and 90°, respectively. More recently, the structure of cmvIL-10 revealed a domain angle of approximately 150° (Jones et al, 2002b). In addition, the two cmvIL-10 peptide chains form an interchain disulfide bond while IFN-7 and cellular IL-10 are non-covalent dimers. The domain orientations of each dimer are essentially fixed at one unique inter domain angle which alters the orientation of the cell surface receptors and may ultimately modulate cellular signal transduction events. 

Here we describe studies of the interaction of interleukin-6 (IL-6) with a soluble form of its cell surface receptor (sIL-6R). A procedure utilising a competition approach is presented which allows the determination of the equilibrium constant in solution thus avoiding any potential problems associated with deviation in kinetic characteristics upon surface immobilisation. In addition, binding characteristics of stable monomeric and dimeric forms of IL-6 are presented to demonstrate both the drastic influence of solute multivalency on kinetic and equilibrium properties and the importance of auxiliary techniques such as analytical ultracentrifugation for the interpretation of SPR data. 

Most transducer proteins that interact with cell-surface receptors are GTP-binding proteins, commonly called G proteins. G proteins typically are composed of three subunits (x, P, and y. When the receptor is in the unbound state, aU three subunits are bound together to form a heterotrimer that is in close association with the membrane-bound receptor. In this state, GDP is bound to the a-subunit (Figure 7-2). Binding of the hormone by the receptor causes a conformational change such that the a-subunit is able to exchange the bound GDP for a molecule of GTP. The GTP-bound a-subunit separates from the Py-dimer and is able to interact with the effector enzyme in either a stimulatory or inhibitory manner, depending on the nature of the a-subunit. Some... 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

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