Receptor tyrosine kinases ligand-induced dimerization

It is worth mentioning that membrane-bound forms of GC, which can be considered signal transducing enzymes , are structurally homologous to other signal transducing enzymes, such as certain protein tyrosine kinases and phosphatases, which also possess receptor moieties in their extracellular (amino terminus) domain and enzyme catalytic activity in their intracellular domain (see Ch. 24). Activation of many of these receptors occurs upon ligand-induced dimerization of the receptors, and a similar... 

Ligand-induced receptor dimerization is also the first step in cytokine signalling, followed by tyrosine phosphorylation of the receptor by a recruited cytosolic kinase. Principles and structural details of receptor dimerization were discussed in the case of a type I cytokine receptor, the growth hormone (GH) receptor, in Chapter 2. 

The mechanism of activation of the receptor-type guanylate cyclases is not entirely clear. As is the case with other single-transmembrane domain receptor types, such as receptor-type tyrosine kinases, some sort of dimerization may be induced by ligand binding, which serves to activate the intracellular enzyme part of the receptor. As discussed in the next section, dimerization is essential to enzyme activity in soluble guanylate cyclases. 

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