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In response to insulin, protein-tyrosine phosphatase 1B (PTPase 1B) dephosphorylates 95-and 160 -180-kDa tyrosine phosphorylated (PY) proteins (Kenner, K. A., Anyanwu, E., Olefsky, J. M., and Kusari, J. (1996) J. Biol. Chem. 271, 19810 -19816 Studies using mutant IRs demonstrated that IR autophosphorylation is necessary for the PTPase 1B-IR interaction. These results suggest that PTPase 1B complexes with the autophosphorylated insulin receptor in intact cells, either directly or within a complex involving additional proteins. The interaction requires multiple tyrosine phosphorylation sites within both the receptor and PTPase 1B.Insulin is a potent metabolic and growth-promoting hormone that has pleiotropic effects at the level of the cell and within the intact organism. Insulin acts on cells to stimulate glucose, protein, and lipid metabolism, as well as RNA and DNA synthesis, by modifying the activity of a variety of enzymes and transport processes (1). As a first step in initiating these responses, insulin binds to its plasma membrane receptor. The insulin receptor is a heterotetrameric protein consisting of two ␣ and two ␤ subunits linked by disulfide bonds to form a ␤-␣-␣-␤ structure. After insulin binding, the insulin receptor undergoes autophosphorylation on tyrosine residues. Autophosphorylation increases the tyrosine kinase activity of the insulin receptor, which in turn phosphorylates one or more cellular substrates, leading to a cascade of secondary phosphorylation and dephosphorylation reactions (2).As the molecular mechanism of insulin action is defined with increasing clarity, so too is our appreciation of the central role played by protein tyrosine phosphorylation. Regulated tyrosine phosphorylation represents a balance of protein-tyrosine kinase (PTKase) 1 and protein-tyrosine phosphatase (PTPase) activities. To date, most attempts to assess the role of proteintyrosine phosphorylation in insulin signal transduction have focused on the action of kinases and thus furnish an incomplete picture of this dynamic process. PTPases can be used as probes to test the role of protein tyrosine phosphorylation, complementing studies performed on the PTKases.Extensive progress in the identification and characterization of PTPases has been made in recent years (3), partially as a result of our appreciation of the PTKases (4). PTPase 1B was the first PTPase to be isolated in homogeneous form and sequenced (5, 6). PTPase 1B possesses a catalytic domain characterized by the 11-amino acid sequence motif, (I/V)HCX-AGXXR(S/T)G. This motif contains cysteine (Cys 215 ) and arginine (Arg 221 ) residues critical for the catalytic activity of the enzyme (7-9). The cDNA sequences for human (10, 11) and rat (8) PTPase 1B predict a protein of 50 kDa with 435 and 432 amino acids, respectively. The conserved phosphatase domain of PTPase 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 ...

Particulate and cytosolic protein tyrosine phosphatase (PTPase) activity was measured in skeletal muscle from 15 insulin-sensitive subjects and 5 insulin-resistant nondiabetic subjects, as well as 18 subjects with non-insulin-dependent diabetes mellitus (NIDDM). Approximately 90% of total PTPase activity resided in the particulate fraction. In comparison with lean nondiabetic subjects, particulate PTPase activity was reduced 21% (P < 0.05) and 22% (P < 0.005) in obese nondiabetic and NIDDM subjects, respectively. PTPase1B protein levels were likewise decreased by 38% in NIDDM subjects (P < 0.05). During hyperinsulinemic glucose clamps, glucose disposal rates (GDR) increased approximately sixfold in lean control and twofold in NIDDM subjects, while particulate PTPase activity did not change. However, a strong positive correlation (r = 0.64, P < 0.001) existed between particulate PTPase activity and insulin-stimulated GDR. In five obese NIDDM subjects, weight loss of -10% body wt resulted in a significant and corresponding increase in both particulate PTPase activity and insulin-stimulated GDR. These findings indicate that skeletal muscle particulate PTPase activity and PTPasel B protein content reflect in vivo insulin sensitivity and are reduced in insulin resistant states. We conclude that skeletal muscle PTFPase activity is involved in the chronic, but not acute regulation of insulin action, and that the decreased enzyme activity may have a role in the insulin resistance of obesity and NIDDM. (J.

We have isolated and sequenced cDNA clones encoding the poly(A)-binding protein of Xenopus laevis oocytes. Polyclonal antiserum was raised against a fusion protein encoding 185 amino acids of the Xenopus poly(A)-binding protein. This antiserum localizes the poly(A)-binding protein to subcellular sites associated with protein synthesis; in the retina, immunoreactive protein is detected in the synthetically active inner segment of the photoreceptor but not in the transductive outer segment. Transcripts encoding the poly(A)-binding protein are present in oocytes, although no protein is detected on protein blots. In contrast, the levels of both transcripts and protein increase in development, which correlates with the observed increase in total poly(A) during Xenopus embryogenesis (N. Sagata, K. Shiokawa, and K. Yamana, Dev. Biol. 77:431-448, 1980).