Mitogen-activated protein (MAP) kinases are important players in signal transduction pathways activated by a range of stimuli and mediate a number of physiological and pathological changes in cell function. MAP kinase activation requires phosphorylation on a threonine and tyrosine residue located within the activation loop of kinase subdomain VIII. This process is reversible even in the continued presence of activating stimuli, indicating that protein phosphatases provide an important mechanism for MAP kinase control. Dual specificity phosphatases (DSPs) are an emerging subclass of the protein tyrosine phosphatase (PTP) gene superfamily, which appears to be selective for dephosphorylating the critical phosphothreonine and phosphotyrosine residues within MAP kinases. Some DSPs are localized to different subcellular compartments and moreover, certain family members appear highly selective for inactivating distinct MAP kinase isoforms. This enzymatic specificity is due in part to powerful catalytic activation of the DSP phosphatase after tight binding of its amino-terminal to the target MAP kinase. DSP gene expression is induced strongly by various growth factors and/or cellular stresses, providing a sophisticated transcriptional mechanism for targeted inactivation of selected MAP kinase activities.

MAP kinase phosphatase-3 (MKP-3) dephosphorylates phosphotyrosine and phosphothreonine and inactivates selectively ERK family mitogen-activated protein (MAP) kinases. MKP-3 was activated by direct binding to purified ERK2. Activation was independent of protein kinase activity and required binding of ERK2 to the noncatalytic amino-terminus of MKP-3. Neither the gain-of-function Sevenmaker ERK2 mutant D319N nor c-Jun amino-terminal kinase-stress-activated protein kinase (JNK/SAPK) or p38 MAP kinases bound MKP-3 or caused its catalytic activation. These kinases were also resistant to enzymatic inactivation by MKP-3. Another homologous but nonselective phosphatase, MKP-4, bound and was activated by ERK2, JNK/SAPK, and p38 MAP kinases. Catalytic activation of MAP kinase phosphatases through substrate binding may regulate MAP kinase activation by a large number of receptor systems.Signal transduction pathways that lead to activation of MAP kinases control many diverse and essential functions in yeast, worms, flies, and mammals. Extracellular signal-regulated kinase-1 (ERK1) and ERK2 exemplify one class of MAP kinase that undergoes activation by a range of stimuli including growth factors, cytokines, cell adhesion, tumor-promoting phorbol esters, and oncogenes (1). Specific functions assigned to ERK activity include chemotaxis, neuronal differentiation, and synaptic changes underlying memory and learning, as well as cellular mitogenesis and oncogenic transformation (1, 2).Full activation of ERK requires phosphorylation of threonine and tyrosine residues by a class of MAP kinase/ERK kinase (MEK) exemplified by MEK-1 (1, 3). Conversely, an emerging family of dual-specificity phosphatases that act on both phosphotyrosine and phosphothreonine reverse this process and also appear to be critical regulators of MAP kinase activity. CL100/3CH134 or MAP kinase phosphatase-1 (MKP-1) is the archetypal member of this gene family and has high substrate specificity for MAP kinases (4). Up to nine other mammalian dual-specificity phosphatases have been identified, and several of these are under tight transcriptional control and display distinct tissue, cell, and subcellular expression patterns (5, 6). MKP-3 appears exceptional in that it specifically inactivates ERK as compared with c-Jun NH 2 -terminal kinases/stress-activated protein kinases (JNK/SAPK) or p38 MAP kinases (7). We now show that ERK, but not other MAP kinases, cause substratetriggered activation of MKP-3.We purified various MKP-3 deletion mutants expressed in Escherichia coli and found that the NH 2 -terminal noncatalytic domain (amino acids 1 to 221) binds tightly to its target MAP kinases p44 ERK1 and p42 ERK2 (8). Binding to purified ERK2 (9) stimulates p-nitrophenyl phosphate (p-NPP) phosphatase activity of full-length MKP-3 by up to 30-fold (Fig. 1A) (10). Both glutathione-S-transferase (GST)-ERK2 and ERK2 caused similar activation of either the fusion protein GST-MKP-3, His-tagged MKP-3, or free MKP-3 (11). Activation of MKP-3 was dose-dependent and saturable wit...

FSH, a glycoprotein hormone, and the FSH receptor (FSHR), a G protein-coupled receptor, play central roles in human reproduction. We report the crystal structure of FSH in complex with the entire extracellular domain of FSHR (FSHR ED ), including the enigmatic hinge region that is responsible for signal specificity. Surprisingly, the hinge region does not form a separate structural unit as widely anticipated but is part of the integral structure of FSHR ED . In addition to the known hormone-binding site, FSHR ED provides interaction sites with the hormone: a sulfotyrosine (sTyr) site in the hinge region consistent with previous studies and a potential exosite resulting from putative receptor trimerization. Our structure, in comparison to others, suggests FSHR interacts with its ligand in two steps: ligand recruitment followed by sTyr recognition. FSH first binds to the high-affinity hormone-binding subdomain of FSHR and reshapes the ligand conformation to form a sTyr-binding pocket. FSHR then inserts its sTyr (i.e., sulfated Tyr335) into the FSH nascent pocket, eventually leading to receptor activation.F SH is a gonadotropin that stimulates steroidogenesis and gametogenesis in the gonads. Secreted by the anterior pituitary gland, FSH regulates the menstrual cycle and ovarian follicular maturation in women and supports sperm production in men. FSH acts by binding to the FSH receptor (FSHR) on the granulosa cell surface in ovaries and the Sertoli cell surface in testes. The stimulated receptor leads to the dissociation of α-and βγ-subunits of G protein heterotrimer inside the cell. The α-subunit activates adenylyl cyclase, resulting in an increase of cAMP levels, and ultimately leads to the increased steroid production that is necessary for follicular growth and ovulation in women. The free βγ dimers recruit G protein-coupled receptor (GPCR) kinases to the receptor, which, in turn, lead to the recruitment of β-arrestin to the receptor (1). FSH is used clinically for controlled ovarian stimulation in women treated with assisted reproductive technologies and also for the treatment of anovulatory infertility in women and hypogonadotropic hypogonadism in men. The central role of FSH in human reproduction makes its receptor a unique pharmaceutical target in the field of fertility regulation (2-4).The glycoprotein hormone (GPH) family has four members: FSH; two other pituitary hormones, luteinizing hormone and thyroid-stimulating hormone (TSH); and one placental hormone, chorionic gonadotropin. The four members are homologous in sequence, structure, and function. Each member is a heterodimer composed of a common α-subunit and a hormone-specific β-subunit. The crystal structures of FSH and human CG (hCG) revealed that both α-and β-subunits adopt similar folds of cystineknot architecture (5-7). The assembled α-and β-heterodimers bind to their respective receptors with high affinity and hormone specificity, resulting in similar signaling pathways but distinct biological responses.

MKP-1 (also known as CL100, 3CH134, Erp, and hVH-1) exemplifies a class of dual-specificity phosphatase able to reverse the activation of mitogen-activated protein (MAP) kinase family members by dephosphorylating critical tyrosine and threonine residues. We now report the cloning of MKP-3, a novel protein phosphatase that also suppresses MAP kinase activation state. The deduced amino acid sequence of MKP-3 is 36% identical to MKP-1 and contains the characteristic extended activesite sequence motif VXVHCXXGXSRSXTXXXAYLM (where X is any amino acid) as well as two N-terminal CH2 domains displaying homology to the cell cycle regulator Cdc25 phosphatase. When expressed in COS-7 cells, MKP-3 blocks both the phosphorylation and enzymatic activation of ERK2 by mitogens. Northern analysis reveals a single mRNA species of 2.7 kilobases with an expression pattern distinct from other dual-specificity phosphatases. MKP-3 is expressed in lung, heart, brain, and kidney, but not significantly in skeletal muscle or testis. In situ hybridization studies of MKP-3 in brain reveal enrichment within the CA1, CA3, and CA4 layers of the hippocampus. Metrazole-stimulated seizure activity triggers rapid (<1 h) but transient up-regulation of MKP-3 mRNA in the cortex, piriform cortex, and some amygdala nuclei. Metrazole stimulated similar regional up-regulation of MKP-1, although this was additionally induced within the thalamus. MKP-3 mRNA also undergoes powerful induction in PC12 cells after 3 h of nerve growth factor treatment. This response appears specific insofar as epidermal growth factor and dibutyryl cyclic AMP fail to induce significant MKP-3 expression. Subcellular localization of epitope-tagged MKP-3 in sympathetic neurons reveals expression in the cytosol with exclusion from the nucleus. Together, these observations indicate that MKP-3 is a novel dual-specificity phosphatase that displays a distinct tissue distribution, subcellular localization, and regulated expression, suggesting a unique function in controlling MAP kinase family members. Identification of a second partial cDNA clone (MKP-X) encoding the C-terminal 280 amino acids of an additional phosphatase that is 76% identical to MKP-3 suggests the existence of a distinct structurally homologous subfamily of MAP kinase phosphatases.A wide range of cell-surface stimuli, including growth and differentiation factors and cytokines as well as ultraviolet radiation and osmotic shock, trigger rapid and powerful activation of mitogen-activated protein (MAP) 1 kinase family members (1-5). Currently, three major subclasses of MAP kinase can be identified, and these comprise the ERK, SAPK/JNK, and p38/HOG1 families (2, 3, 6). Full activation of MAP kinase requires phosphorylation on critical tyrosine and threonine residues, and several upstream dual-specificity kinases catalyzing this modification have now been identified (1-3, 6). Once activated, MAP kinases phosphorylate and regulate several cellular proteins, including additional protein kinases, cytoskeletal elements, stathmin,...

We have studied the phosphorylation of the Bcl-2 family of proteins by different mitogen-activated protein (MAP) kinases. Purified Bcl-2 was found to be phosphorylated by the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) p54-SAPK␤, and this is specific insofar as the extracellular signal-regulated kinase 1 (ERK1) and p38/RK/CSBP (p38) catalyzed only weak modification. Bcl-2 undergoes similar phosphorylation in COS-7 when coexpressed together with p54-SAPK␤ and the constitutive Rac1 mutant G12V. This is seen by both 32 PO 4 labeling and the appearance of five discrete Bcl-2 bands with reduced gel mobility. As anticipated, both intracellular p54-SAPK␤ activation and Bcl-2 phosphorylation are blocked by co-transfection with the MAP kinase specific phosphatase MKP3/PYST1. MAP kinase specificity is also seen in COS-7 cells as Bcl-2 undergoes only weak phosphorylation when co-expressed with enzymatically activated ERK1 or p38. Four critical residues undergoing phosphorylation in COS-7 cells were identified by expression of the quadruple Bcl-2 point mutant T56A,S70A,T74A,S87A. Sequencing phosphopeptides derived from tryptic digests of Bcl-2 indicates that purified GST-p54-SAPK␤ phosphorylates identical sites in vitro. This is the first report of Bcl-2 phosphorylation by the JNK/SAPK class of MAP kinases and could indicate a key modification allowing control of Bcl-2 function by cell surface receptors, Rho family GTPases, and/or cellular stresses.