Phosphotyrosine recognition

The phosphotyrosine-recognition subdomain confers substrate specificity of PTPs by creating a deep pocket (9 A) so that only the phosphotyrosine moiety is long enough to reach the cysteine nucleophile located at the base of this pocket phosphoserine and phosphothreonine... 

SH2 Src homology 2, a phosphotyrosine-recognition protein domain of about 100 amino acid residues hrst... 

Waksman, G. Kominos, D. Robertson, S.C. Pant, N. Baltimore, D. Birge, R.B. Cowburn, D. Hanafusa, H. Mayer, B.J. Overduin, M. et al. Crystal structure of the phosphotyrosine recognition domain SH2 of v-src com-plexed with tyrosine-phosphorylated peptides. Nature, 358, 646-653... 

Fig. 8.9. Crosslinking of signal proteins with the help of protein modnles. A hypothetical protein is shown which contains SH2, SH3, PTB and PH domains. Recognition of phosphotyrosine residues occurs with the help of SH2 or PTB domains SH3 domains bind to proline-rich sequences (Pro in Protein 3) whilst the pleckstrin homology domains (PH domains) mediate binding to phosphatidyl-inositol-phosphates (PtdInsP) in the membrane. In an idealized scheme, the modular protein can associate several proteins (Protein 1 - Protein 3) and mediate interactions between these proteins (shown as broken arrows). The PH domain helps to recruit the complex to the cell membrane favoring interactions with other membrane-associated proteins (Protein X).

The crystal structures of the SH2 domains of Src tyrosine kinase and Lck tyrosine kinase in complex with Tyr phosphorylated peptides have enabled important insight to be obtained into recognition of the phosphotyrosine residue and the neighboring amino acids in class lA of SH2 domains. The phosphate residue is boimd in a deep pocket of the SH2 domain, at the end of which an invariant Arg residue (Arg PB5) is located which contacts the negatively charged phosphate by a two-pronged interaction. It can be estimated that a phosphoserine or phosphothreonine residue would be too short to enter into a similar interaction with the Arg residue. 

Fig. 8.11. Recognition of phosphotyrosine-containing substrate peptides by SH2 domains of Src kinase and phospholipase Cyl. Binding of phosphotyrosine-containing peptides to SH2 is shown schematically, based on crystal structures of the complexes. The SH2 domain of Src kinase has a basic binding pocket for the phosphotyrosine residue and a hydrophobic pocket for the isoleucine residues at position -1-3 of the peptide substrate. The SH2 domain of PL-Cyl has a hydrophobic binding surface to which the C-terminal part of the peptide P-Tyr-Ile-Ile-Pro-Leu-Pro-Asp binds. According to Cohen, (1995).

To sum up while the SH2 domains recognize C-terminal residues adjacent to the phosphotyrosine, the PTB domains recognize the N-terminal residues adjacent to the phosphotyrosyl residue. Moreover, in PTB domains a P-turn with the motif NPXY (N is asparagine, P is proline, X is any amino acid, and Y is a phosphotyrosine) is critical for recognition. However, the main difference between PTB domains and SH2 domains is that the former also bind to unphosphorylated peptides and proteins, whereas the latter only recognize phosphorylated proteins. 

Phosphotyrosine phosphatases are integrated just like tyrosine Idnases into signalling pathways. They interact with receptors and have recognition motifs that direct diem to their targets. 5 Protein phosphotyrosine phosphatases downregulate tyrosine phosphorylation and play a role in cellular regulation as important as that of protein tyrosine kinases. 

The significance of tliese observations was not immediately appreciated, but a few further important examples, including the observation of several amino hydrogen bonds to the ring of the phosphotyrosine in its recognition by SH2 domains, as shown in Fig. 6.2 [20], and a later review by Perutz [21] stimulated both structural analyses and dieoretical studies of D-H - -Tt-acceptor systems in proteins [22, 23]. It has been shown that such bonds are relatively rare compared to the usual... 

Jia Z, Barford D, Flint AJ et al (1995) Structural basis for phosphotyrosine peptide recognition by protein tyrosine phosphatase IB. Science 268 1754-1758... 

Fig. 8.13 Recognition of phosphotyrosine-contai-ning substrate peptides by SH2 domains of Src kinase and phospholipase Cyl. Binding of phos-photyrosine-containing peptides to SH2 is shown schematically, based on crystal structures of the complexes. The SH2 domain of Src kinase has a...

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