Protein Tyrosine Phosphatase 1B

Definition
Protein Tyrosine Phosphatase 1B (PTP-1B) is a non-receptor PTP. PTP-1B is a member of the protein tyrosine phosphatase family of enzymes.

Discovery
PTP-1B is a widely expressed non-receptor PTP that was originally identified by Fischer and his collaborators from human placenta 1 and determined the amino acid sequence of the major soluble and particulate PTPase, PTP-1B.

Structural Characteristics
PTP-1B possesses a catalytic domain characterized by the 11-amino acid sequence motif, (I/V)HCXAGXXR(S/T)G. This motif contains cysteine (Cys215) and arginine (Arg221) residues critical for the catalytic activity of the enzyme 2. The cDNA sequences for human 3 and rat 4 PTP-1B predict a protein of 50 kDa with 435 and 432 amino acids, respectively. The conserved phosphatase domain of PTP-1B is contained within the domain spanning residues 30 to 278. The COOH-terminal noncatalytic extension of the protein serves a regulatory function. The COOH-terminal 35 residues target the enzyme to the cytoplasmic face of the endoplasmic reticulum 5, whereas the preceding 122 residues are predominantly hydrophilic and contain sites for serine phosphorylation. Recently, the crystal structure of the 321-residue (37-kDa) form of human PTP-1B has been determined, revealing the structural features that provide the protein with its specific enzymatic capacity for phosphotyrosine 6.

Mode of Action
PTP-1B interacts with insulin receptor and negatively regulates insulin signalling by an N-terminal binding7 and it also negatively regulates integrin signalling through a proline-rich region present in the C-terminus 8. A study indicate that PTP1B mutants that are defective in Src homology 3 domain binding fully retain the ability to inhibit insulin signalling, whereas mutants defective in insulin-receptor binding fully retain the ability to inhibit integrin signalling. In contrast, both the C-terminal proline-rich region and the tandem tyrosine residues present in the N-terminal region are required for the activation of Src family kinases. These data show that PTP1B can independently regulate insulin and integrin signals, and that Src might represent a convergence point for regulating signal transduction by this phosphatase 9.

Functions
Increased Insulin Sensitivity and Obesity Resistance in Mice Lacking the PTP-1B gene:
Disruption of the mouse homolog of the gene encoding PTP-1B yielded healthy mice that, in the fed state, had blood glucose concentrations that were slightly lower and concentrations of circulating insulin that were one-half those of their PTP-1B+/+ littermates. The enhanced insulin sensitivity of the PTP-1B-/- mice was also evident in glucose and insulin tolerance tests. The PTP-1B-/- mice showed increased phosphorylation of the insulin receptor in liver and muscle tissue after insulin injection in comparison to PTP-1B+/+ mice. On a high-fat diet, the PTP-1B-/- and PTP-1B+/- mice were resistant to weight gain and remained insulin sensitive, whereas the PTP-1B+/+ mice rapidly gained weight and became insulin resistant. These results demonstrate that PTP-1B has a major role in modulating both insulin sensitivity and fuel metabolism, thereby establishing it as a potential therapeutic target in the treatment of type 2 diabetes and obesity 10.

PTP-1B Complexes with the Insulin Receptor in Vivo and Is Tyrosine-phosphorylated in the Presence of Insulin: In response to insulin, PTP-1B dephosphorylates 95- and 160-180-kDa tyrosine phosphorylated (PY) proteins. Anti-PTPase 1B antibodies coprecipitated a 95-kDa PY protein from insulin-stimulated cells, subsequently identified as the IR ß-subunit. Similarly, anti-IR antibodies coprecipitated the 50-kDa PY- PTP-1B protein from insulin-treated cells. To identify PTP-1B tyrosine (Tyr) residues that are phosphorylated in response to insulin, three candidate sites (Tyr66, Tyr152, and Tyr153) were replaced with phenylalanine. Replacing Tyr66 or Tyr152 and Tyr153 significantly reduced insulin-stimulated PTP-1B phosphotyrosine content, as well as its association with the IR. These results suggest that PTP-1B complexes with the autophosphorylated insulin receptor in intact cells, either directly or within a complex involving additional proteins 11. The interaction requires multiple tyrosine phosphorylation sites within both the receptor and PTP-1B.

References
1. Tonks NK, Diltz CD, Fischer EH (1988). Purification of the major protein-tyrosine-phosphatases of human placenta. J. Biol. Chem., 263:6722-6730.
2. Guan KL, Dixon JE (1991). Evidence for protein-tyrosine-phosphatase catalysis proceeding via a cysteine-phosphate intermediate. J. Biol. Chem., 266:17026-17030.
3. Chernoff J, Schievella AR, Jost CA, Erickson RL, and Neel BG (1990). Cloning of a cDNA for a major human protein-tyrosine-phosphatase. PNAS, 87:2735-2739.
4. Guan KL, Haun RS, Watson SJ, Geahlen RL, and Dixon JE (1991). Cloning and expression of a protein-tyrosine-phosphatase. PNAS, 87:1501-1505.
5. Frangioni JV, Beahm PH, Shifrin V, Jost CA, Neel BG (1992). The nontransmembrane tyrosine phosphatase PTP-1B localizes to the endoplasmic reticulum via its 35 amino acid C-terminal sequence. Cell, 68:545-560
6. Barford D, Flint AJ, Tonks NK. (1994). Crystal structure of human protein tyrosine phosphatase 1. Science, 263:1397-1404.
7. Dadke S, Kusari J, Chernoff J (2000). Down-regulation of insulin signaling by protein-tyrosine phosphatase 1B is mediated by an N-terminal binding region. J. Biol. Chem., 275(31):23642–23647.
8. Liu F, Hill DE, Chernoff J (1996). Direct binding of the proline-rich region of protein tyrosine phosphatase 1B to the Src homology 3 domain of p130(Cas). J. Biol. Chem. 271(49):31290–31295.
9. Dadke S, Chernoff J (2002). Interaction of protein tyrosine phosphatase (PTP) 1B with its substrates is influenced by two distinct binding domains. Biochem. J., 364: 377–383.
10. Elchebly M, Payette P, Michaliszyn E, Cromlish W, Collins S, Loy AL, Normandin D, Cheng A, Himms-Hagen J, Chan CC, Ramachandran C, Gresser MJ, Tremblay ML, Kennedy BP (1999). Increased Insulin Sensitivity and Obesity Resistance in Mice Lacking the Protein Tyrosine Phosphatase-1B Gene . Science 283(5407):1544-1548.
11. Bandyopadhyay D, Kusari A, Kenner KA, Liu F, Chernoff J, Gustafson TA, Kusari J (1997). Protein-Tyrosine Phosphatase 1B Complexes with the Insulin Receptor in Vivo and Is Tyrosine-phosphorylated in the Presence of Insulin. J. Biol. Chem., 272(3):1639-1645.