SH2 domain Function

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. 

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 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. 

The SH2 DOMAIN links an activated (autophosphorylated) receptor to various downstream pathways. Each of the proteins has a common SH2 adaptor domain fused to another domain that performs the downstream function. The same signal can then activate multiple pathways. GNRP and GAP represent adaptors that can affect G-protein signaling (see the following section G-Protein Coupled Receptors ). 

The Novartis group used the X-ray structure of a Grb2-peptide complex [68] as the structural basis for a design attempt that yielded entirely new non-peptide SH2 domain ligands [164]. As mentioned several times throughout this contribution, the interaction of the pTyr sidechain and the Asn sidechain in pTyr+2 position of the peptide ligand have been identified as key elements for molecular recognition (see Fig. 10). The obvious relevance of these two sidechain functionalities allowed the definition of a minimal pharmacophore pattern that... 

Although domains are often mobile and occur in many different modular architectures, it is notable that the co-occurrence of domains within single polypeptides is far from random, since a domain is usually found to co-occur only with a small subset of all domain types. When two domain types are not observed within the same molecule, it is likely that their activities are antagonistic, thereby effectively neutralizing the overall function of the molecule. Such an example is provided by protein kinase and phosphatase domains that are not currently known to cooccur within the same molecule. However, the reasons that functionally distinct and otherwise widespread domains have never yet been found together, such as signaling PDZ and SH2 domains, remains elusive. 

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