Phosphotyrosine proteins

Whereas the majority of receptor phospho-acceptor sites are in serine-and-threonine-rich regions of the intracellular domains, there is also evidence that GPCRs are phosphorylated by different kinases (CK2, CK(, Akt, PKB) on tyrosine residues (Paxton et al. 1994 Fan et al. 2001) in a manner that can, in some instances, generate classical phosphotyrosine protein-interaction motifs (Karoor et al. 1998). 

D. Degl lnnocenfi, A. Caselfi, F. Rosati, R. zocchini G. Manao, G. Caniici, and G. Rainponi, Thiolation of low-M (r) phosphotyrosine protein phosphatase by thiol-disulfides, lUBMB Life 48 (1999)505-511. 

Acid phosphatase 1, soluble ACPI 52 Acid phosphatase of erythrocyte Low molecular weight phosphotyrosine protein phosphatase HAAP... 

Vincent, C. Doublet, P. Grangeasse, C. Vaganay, E. Cozzone, A.J. Duclos, B. Cells of Escherichia coli contain a protein-tyrosine kinase, Wzc, and a phosphotyrosine-protein phosphatase, Wzb. J. BacterioL, 181, 3472-3477 (1999)... 

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... 

Several different types of protein domains are known to function in binding to phosphotyrosine, including... 

Noncatalytic phosphotyrosine binding (PTB) domains are 100-150 residue modules, which bind Asn-Pro-X-Tyr motifs. PTB-domain binding specificity is determined by residues at the amino-terminal side of the phosphotyrosine. In most cases, the tyrosine residue must be phosphorylated in order to mediate binding. PTB domain containing proteins are often found in signal transduction pathways. 

Besides cytoplasmic protein kinases, membrane receptors can exert protein kinase activity. These so-called receptor tyrosine kinases (RTK) contain a ligandbinding extracellular domain, a transmembrane motif, and an intracellular catalytic domain with specificity for tyrosine residues. Upon ligand binding and subsequent receptor oligomerization, the tyrosine residues of the intracellular domain become phosphory-lated by the intrinsic tyrosine kinase activity of the receptor [3, 4]. The phosphotyrosine residues ftmction as docking sites for other proteins that will transmit the signal received by the RTK. 

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. 

Benes CH, Wu N, Elia AE, Dharia T, Cantley LC, Soltoff SP. The C2 domain of protein kinase C-delta is a phosphotyrosine binding domain. Cell 2005 121 (2) 271-280. 

The endothelin B receptor is an example of characterization of a homogeneous, affinity purified protein (Roos et al., 1998). Significant progress has been made in the development of techniques for more high-throughput identification of phosphorlyation events. Analysis of large sets of phosphorylated proteins is facilitated by the availability of affinity purification methods such as anti-phosphotyrosine or anti-phosphoserine antibodies or metal affinity chromatography (Neubauer and Mann, 1999 Soskic et al., 1999). These methods are not specific to a particular protein but rather are used to fractionate all proteins that are phosphorylated. 

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