Function of SH2 Domains

The potential binding sites for various SH2 domain proteins on cytoplasmic receptor domains as well as on soluble proteins have been precisely mapped for several pTyr-containing target proteins. Apart from the impressive diversity of SH2 domain function, the fundamental property of all SH2 domains refers to the specific recognition of pTyr epitopes. 

---

IV. Structure and Function of SH2 Domains in the Context of Other Protein Modules... 

Initial investigations of SH2 domains focused on understanding how these domains functioned in isolation. As described in Sections II and III, SH2 domains were expressed, purihed, and studied in vitro alone in order to understand their structure and mechanism of binding. WhUe this reductionist approach has allowed extensive characterization of SH2 domains, studies of isolated SH2 domains have not addressed how SH2 domains communicate with other protein domains in order to determine the biological function of SH2 domain-containing proteins. In this section, we describe both structural (Section IV,A) and solution-based biophysical (Section IV,B) investigations which have probed the mechanisms by which SH2 domains function within the context of other domains or of full-length proteins. 

The subcellular localization of protein tyrosine phosphatases is an important aspect of their function. The sequences of cytoplasmic protein tyrosine phosphatases frequently demonstrate sequence signals specifying a particular subcellular localization. This ensures that protein tyrosine phosphatases are only active at defined subcellular sites. The presence of SH2 domains in cytoplasmic protein tyrosine phosphatases also shows that these are coupled, via SH2-phosphotyrosine interactions, to specific substrates, where they then perform their actual function. 

The structure of SH2 domains can be conveniendy divided into two functional regions that involved in coordination of the pTyr of the target and that which contacts the residues G-terminal to the pTyr (Fig. 2A). The pTyr binding cavity is found in the N-terminal half of the protein between the central p-sheet and helix oA, while the specificity determining region is found primarily in the C-terminal half of the protein between the central p-sheet and hehx aB (Fig. 2A). Each of these regions is described in detail below. 

The structures of SH2 domains in the context of other protein modules provided much information on how these modules function within the context of a larger protein. However, the availibility of structures is just the first step of many which are needed to understand how macromolecules function. In this section, we describe solution studies of SH2 domains placed in the context of either larger protein fragments or full-length proteins that have provided information about the various mechanisms by which SH2 domains regidate protein function. 

A similar approach has been taken to prepare aryl phosphoramidates such as 99 [223], as prodmgs of SH2 domain analogs for Src/Lck inhibitors. The compounds exhibit low micromolar growth inhibition in Jurkat T cells, and undergo spontaneous hydrolysis with half-lives of approximately 30 min. The same masking groups have been applied in phosphonate 100 as a suspected SH2 domain blocker through inhibition of mitotic centromere-associated kinesin protein function in a panel of... 

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... 

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... 

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. 

---