Protein tyrosine kinases regulatory domain

Nonreceptor protein tyrosine kinases contain a catalytic domain, as well as various regulatory domains important for proper functioning of the enzyme 416... 

Nonreceptor protein tyrosine kinases contain a catalytic domain, as well as various regulatory domains important for proper functioning of the enzyme. NRPTKs are found in the inner leaflet of the plasma membrane, cytosol, endosomal membranes and nucleus. These include the Src, Jak, Abl, Tec, Ack, Csk, Fak, Fes, Frk and Syk subfamilies (Fig. 24-3). Since a great deal is known about the structure and regulation of the Src family tyrosine kinase, we will use it to illustrate the principles in NRPTK signaling unique features in other subfamilies will be indicated... 

FIGURE 24-10 Schematic structures of nonreceptor protein tyrosine phosphatases (NRPTPs) and receptor protein tyrosine phosphatases (RPTPs). NRPTPs contain a catalytic domain and various regulatory domains. RPTPs are composed of an extracellular domain, a transmembrane domain and an intracellular domain with one or two catalytic domains. Like receptor protein tyrosine kinases, the structural features of the extracellular domains divide the RPTPs into different families. (With permission from reference [12]). 

The domain organization of Src family kinases is found in other protein tyrosine kinases. The N-terminal region of Abl is similar to that of Src kinases, and it has been reported that the Abl-SH3 domain plays a negative regulatory role in the activity of the kinase. The linker between the SH2 and kinase domains in Abl contains the same KP motif found in Src. Mutations in these residues, or in other residues that may couple SH3 domain activity to catalytic repression, cause deregulation of the kinase activity (151). 

Insulin Receptor. Figure 1 Structure and function of the insulin receptor. Binding of insulin to the a-subunits (yellow) leads to activation of the intracellular tyrosine kinase ((3-subunit) by autophosphorylation. The insulin receptor substrates (IRS) bind via a phospho-tyrosine binding domain to phosphorylated tyrosine residues in the juxtamembrane domain of the (3-subunit. The receptor tyrosine kinase then phosphorylates specific tyrosine motifs (YMxM) within the IRS. These tyrosine phosphorylated motifs serve as docking sites for some adaptor proteins with SRC homology 2 (SH2) domains like the regulatory subunit of PI 3-kinase. 

Tyrosine phosphorylated IRS interacts with and activates PI 3-kinase [3]. Binding takes place via the SRC homology 2 (SH2) domain of the PI 3-kinase regulatory subunit. The resulting complex consisting of INSR, IRS, and PI 3-kinase facilitates interaction of the activated PI 3-kinase catalytic subunit with the phospholipid substrates in the plasma membrane. Generation of PI 3-phosphates in the plasma membrane reemits phospholipid dependent kinases (PDKl and PDK2) which subsequently phosphorylate and activate the serine/threonine kinase Akt (synonym protein... 

Receptor tyrosine kinases are integral membrane proteins that have a hgand-binding domain on the extracellular side and a tyrosine kinase domain on the cytosohc side (see Fig. 8.1). The transmembrane portion is made up of just one structural element thus it is assumed that it crosses the membrane in an a-hehcal form. On the cytoplasmic side, in addition to the conserved tyrosine kinase domain, there are also further regulatory sequence portions at which autophosphorylation, and phosphorylation and dephosphorylation by other protein kinases and by protein phosphatases, can take place. 

Fig. 8.7. Structure of the catalytic domain of the insulin receptor. The crystal structure of the tyrosine kinase domain of the insulin receptor (Hubbard et al., 1994) has a two-lobe structure that is very similar to the structure of the Ser/Thr-specific protein kinases. Structural elements of catalytic and regulatory importance are shown. The P loop mediates binding of the phosphate residue of ATP the catalytic loop contains a catalytically essential Asp and Asn residue, found in equivalent positions as conserved residues in many Ser/Thr-specific and Tyr-specific protein kinases. Access to the active center is blocked by a regulatory loop containing three Tyr residues (Tyrll58, Tyrll62 and Tyrll63). Tyrll62 undergoes autophosphorylation in the course of activation of the insulin receptor. MOLSKRIPT representation according to Kraulis, (1991).

Phospholipase C, which initiates the release of phosphatidylinositol derivatives, also requires Ca2+ for activity. It is difficult to determine whether release of Ca2+ is a primary or secondary response. There are three isoenzyme types of phospholipase C-(3, y, and 8- and several sub forms of each with a variety of regulatory mechanisms.298 3"" For example, the y isoenzymes are activated by binding to the tyrosine kinase domain of receptors such as that for epidermal growth factor (see Fig. 11-13). In contrast, the (3 forms are often activated by inhibitory G proteins and also by G, which is specific for inositol phosphate release. 

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