SH2, Domains

Docking also provided insight into the 3D hydrophobic requirements for binding of nonpeptide molecules in the SH2 domain of ppGOsrc at ARIAD Pharmaceuticals [92]. 

SH2 domains bind to phosphotyrosine-containing regions of target molecules... 

Figure 13.26 Schematic diagram of the SH2 domain from the Src tyrosine kinase with bound peptide. The SH2 domain (blue) comprises a central p sheet surrounded by two a helices. Three positively charged residues (green) are involved in binding the phosphotyrosine moiety of the bound peptide (red). (Adapted from G. Waksman et al.. Cell 72 779-790, 1993.)...

The Src SH2 domain typifies a large number of those characterized to date. The pTyr fits into a pocket on the opposite side of the central sheet to the pY-r3 pocket (Figure 13.27a). All known SH2 domains bind pTyr in essentially the same way, but some have a different pattern of contacts for the residues that follow. For example, in the Grb2 SH2 domain, a tryptophan side chain from the small sheet fills the pY-r3 pocket, and the bound peptide takes a different course, with important interactions to an asparagine at pY-r2. Screens of peptide libraries have detected the importance of this asparagine. The SH2 domain from PFC-yl contacts five mainly hydrophobic residues that follow pTyr. 

The polypeptide chain of Src tyrosine kinase, and related family members, comprises an N-terminal "unique" region, which directs membrane association and other as yet unknown functions, followed by a SH3 domain, a SH2 domain, and the two lobes of the protein kinase. Members of this family can be phosphorylated at two important tyrosine residues—one in the "activation loop" of the kinase domain (Tyr 419 in c-Src), the other in a short... 

Figure 13.30 Ribbon diagram of the structure of Src tyrosine kinase. The structure is divided in three units starting from the N-terminus an SH3 domain (green), an SH2 domain (blue), and a tyrosine kinase (orange) that is divided into two domains and has the same fold as the cyclin dependent kinase described in Chapter 6 (see Figure 6.16a). The linker region (red) between SH2 and the kinase is bound to SH3 in a polyproline helical conformation. A tyrosine residue in the carboxy tail of the kinase is phosphorylated and bound to SH2 in its phosphotyrosine-binding site. A disordered part of the activation segment in the kinase is dashed. (Adapted from W. Xu et al.. Nature 385 595-602, 1997.)...

Figure 13.31 Space-filling diagram of Src tyrosine kinase in the same view as Figure 13.30. The SH2 domain makes only a few contacts with the rest of the molecule except for the tail region of the kinase. The SH3 domain contacts the N-domain of the kinase in addition to the linker region. There are extensive contacts between the N- and C-domains of the kinase. (Adapted from W. Xu et al., Nature 385 596-602, 1997.)...

Src tyrosine kinase contains both an SH2 and an SH3 domain linked to a tyrosine kinase unit with a structure similar to other protein kinases. The phosphorylated form of the kinase is inactivated by binding of a phosphoty-rosine in the C-terminal tail to its own SH2 domain. In addition the linker region between the SH2 domain and the kinase is bound in a polyproline II conformation to the SH3 domain. These interactions lock regions of the active site into a nonproductive conformation. Dephosphorylation or mutation of the C-terminal tyrosine abolishes this autoinactivation. 

Waksman, G., et al. Binding of a high affinity phosphoty-rosyl peptide to the Src SH2 domain crystal structures of the complexed and peptide-free forms. Cell 72 779-790, 1993. 

PTB domains recognize small peptides containing a phosphotyrosine, usually with the consensus sequence, NPXpY. Some PTB-containing proteins, such as Numb, are able to bind to the consensus peptide in the absence of phosphorylated tyrosine, suggesting phosphotyrosine is dispensable for the function of certain PTB domains. Hydrophobic residues N-termi-nal to the phosphotyrosine provide some specificity of target and distinction from SH2 domains. PTB domains appear to be particularly important in docking... 

Phosphorylation is a common method of regulation. As described above, SH2 domains bind to phosphorylated tyrosine residues. Conversely, phosphorylation of serines and threonines proximal to SH3 and PDZ domains uncouples them from their target motifs. Therefore modulation of protein kinase activity in cells regulates interactions between adaptor proteins and their target proteins. 

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... 

Immunreceptor based activation motif. The classical ITAM motif comprises the consensus sequence Yxxl/ Lx(6-12)YxxI/L (where Y stands for tyrosine, I stands for isoleucine, L stands for leucine, and x can be any amino acid). Kinases containing tandem SH2 domains, as for example ZAJP-70 or SYK, recognize the phosphorylated ITAMs, thereby initiating downstream signaling events. 

All phosphoinositides are found in the cytosolic half of the lipid bilayer of the plasma or intracellular compartment membranes (left part). The different kinases acting on phosphoinositides in mammalian cells are shown in solid lines and the phosphoinositide 3-kinases, in bold. The phosphoinositides counterpart pathways catalysed by known phosphatases are represented by dashed lines. The best known phosphatases are PTEN (Phosphatase and tensin homolog deleted on chromosome 10) and SHIP (SH2 domain-containing inositol 5-phosphatase). 

Phospholipid phosphatases are enzymes such as SHIP (SH2-domain containing inositide-5-phosphatase) or PTEN (phosphatase and tensin homolog deleted on chromosome 10) which dephosphorylate phosphoino-sitides. Whereas SHIP removes phosphate from the 5 ... 

Protein-protein interaction domain that recognizes short sequences containing a phosphotyrosine. Hydrophobic residues N-terminal to the phosphotyrosine residue provide distinction from SH2 domains. Particularly important in assembling protein complexes at activated receptors. 

The Src homology 2 domain (or SH2-domain) is a protein domain of about 100 amino acid residues first identified in the tyrosine kinase Src. SH2-domain... 

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