Differential accumulation of DPTP61F alternative transcripts: regulation of a protein tyrosine phosphatase by segmentation genes

DPTP61F is a non-receptor protein tyrosine phosphatase that is expressed during Drosophila oogenesis and embryogenesis. DPTP61F transcripts are alternatively spliced to produce two isoforms of the protein which are targeted to different subcellular locations. DPTP61Fn accumulates in the nucleus, and DPTP61Fm associates with the membranes of the reticular network and the mitochondria. We have examined the spatial and temporal expression of the two alternative transcripts of dptp61F during Drosophila embryogenesis. Our observations indicate that the two isoforms are expressed in distinct patterns. The DPTP61Fn transcript is expressed in the mesoderm and neuroblast layer during germband extension and later in the gut epithelia. In comparison, the transcript encoding DPTP61Fm accumulates in 16 segmentally repeated stripes in the ectoderm during germband extension. These stripes are flanked by, and adjacent to, the domains of engrailed and wingless gene expression in the anterior/posterior axis. In stage 10 embryos, the domains of DPTP61Fm transcript accumulation are wedge shaped and roughly coincide with the area lateral to the denticle belts that will give rise to naked cuticle. The DPTP61Fm transcript is also expressed later in embryogenesis in the central nervous system. The segmental modulation of DPTP61Fm transcript accumulation in the A/P axis of the germband is regulated by the pair-rule genes, and the intrasegmental pattern of transcript accumulation is regulated by the segment polarity genes.

PTP NE-6: a brain-enriched receptor-type protein tyrosine phosphatase with a divergent catalytic domain

A receptor-type protein tyrosine phosphatase, PTP NE-6, was identified from rat olfactory epithelial cDNA and cloned from a rat brain cDNA library. PTP NE-6 mRNA is abundant in brain and expressed at lower levels in olfactory tissue and adrenal gland. In situ hybridization demonstrates that PTP NE-6 mRNA is expressed throughout the brain, with highest levels in the medial habenula and at intermediate levels in layer IV of cortex, medial geniculate nucleus, inferior colliculus, hypothalamus, and thalamus. The predicted amino acid sequence demonstrates that PTP NE-6 contains a single catalytic domain that diverges from the consensus protein tyrosine phosphatase catalytic domain by expressing an aspartate instead of the conserved alanine residue in the catalytic site. Recombinantly expressed PTP NE-6 does not exhibit detectable phosphatase activity. Upon mutation of the aspartate to the consensus alanine, phosphatase activity toward p-nitrophenyl phosphate is observable with a k(cat) value of 3.7 s(-1) and a Km of 980 microM. These data demonstrate that the inactivity of native PTP NE-6 toward p-nitrophenyl phosphate is due to the divergent aspartate in the catalytic site and not to variant amino acids within the phosphatase domain.

Roles of aspartic acid-181 and serine-222 in intermediate formation and hydrolysis of the mammalian protein-tyrosine-phosphatase PTP1

Protein tyrosine phosphatases (PTPases) share a number of conserved amino acid residues, including the active site sequence HCXXGXXRS(T), which are strongly implicated in catalysis. The roles of two conserved active site residues, Asp-181 and Ser-222, were investigated using a combination of site-directed mutagenesis and kinetic analysis in the mammalian PTPase PTP1. The pH profiles for k(cat)/K(m) and k(cat) of the wild-type enzyme indicate that two ionizable groups, of pK(a) values 5.1 and 5.44, must be deprotonated and one group with a pK(a) value of 4.93 must be protonated for maximal activity. The group of pK(a) value 5.1 is the second ionization of the substrate phosphate moiety. Selective thiolate anion inactivation indicates the residue with pK(a) value of 5.44 is C215. The pH-dependent profiles of the D181N mutant during establish the residue with pK(a) value of 4.93 to be Asp-181 and suggest that it functions as a general acid phosphoryl transfer to the enzyme. Rapid reaction kinetics of wild-type and D181N mutant enzymes indicate that the formation of the phospho-enzyme intermediate is rate-limiting at pH 7.0 and 30 degrees C. Enzymes containing the S222A mutation exhibited rapid reaction burst kinetics, strongly suggesting that phospho-enzyme intermediate hydrolysis is fully rate-limiting. The role of the active site S222 is to accelerate the rate of phospho-enzyme intermediate hydrolysis. The kinetic analysis of a third mutant, containing both the D181N and S222A mutations, suggests that D181 also serves as a general base in the breakdown of the phospho-enzyme intermediate.

The PPS1 gene of Saccharomyces cerevisiae codes for a dual specificity protein phosphatase with a role in the DNA synthesis phase of the cell cycle

We report the identification of the PPS1 gene of Saccharomyces cerevisiae. The deduced amino acid sequence of PPS1p shows similarity with protein-tyrosine phosphatases (PTPases) and is most closely related to a subfamily of PTPases that are capable of dephosphorylating phosphoseryl and phosphothreonyl residues as well as phosphotyrosyl residues. Analysis of the predicted amino acid sequence suggests that the protein consists of an active phosphatase domain, an inactive phosphatase-like domain, and an NH2-terminal extension. Mutation of the catalytic cysteinyl residue in the active phosphatase domain reduced the in vitro activity of the mutant protein to less than 0.5% of wild type activity, while mutation of the corresponding cysteinyl residue of the inactive phosphatase-like domain had no effect on in vitro activity. The PPS1 protein was expressed in Escherichia coli, and the protein was shown to catalyze the hydrolysis of p-nitrophenyl phosphate, dephosphorylate phosphotyrosyl, and phosphothreonyl residues in synthetic diphosphorylated peptides and to inactivate the human ERK1 protein. PPS1 transcript abundance is coregulated with that of the divergently transcribed DPB3 gene, which codes for a subunit of DNA polymerase II, with both transcripts showing peak abundance in S phase. pps1Delta mutant strains did not differ from PPS1 strains under any of the conditions tested, but overexpression of the PPS1 protein in S. cerevisiae led to synchronous growth arrest and to aberrant DNA synthesis. A screen for suppressors of this growth arrest identified the RAS2 gene as a multicopy suppressor of the PPS1p overexpression arrest. The arrest was not suppressed by the presence of multicopy RAS1, TPK2, or TPK3 genes or by the presence of 5 mM cAMP in the growth medium, suggesting that PPS1 functions in a pathway involving RAS2, but not TPK kinases or adenylate cyclase.

Thyroid hormone regulates postnatal expression of transient K+ channel isoforms in rat ventricle

1. The ability of thyroid hormone to regulate the postnatal changes of the Ca2+-independent transient outward K+ current (It) was studied in rat ventricular myocytes. 2. In rat ventricle, It is very small at birth and then increases markedly between postnatal days 8 and 20. The time course of this increase in current density is similar to that of a significant rise in plasma thyroid hormone (T3) levels. 3. During early development, the density of expression of It can be altered by changes in thyroid hormone levels. Eight days after birth the density of It measured at +50 mV in control animals is 2.2 +/- 0.4 pA pF(-1). This value is about 3-fold larger (6.5 +/- 0.8 pA pF(-1)) in myocytes from age-matched hyperthyroid animals. When the plasma T3 level in newborn rats is not allowed to increase, or is decreased by making animals hypothyroid, this age-dependent increase in It fails to occur. 4. Using RNase protection assays, Kv4.2 and Kv4.3 mRNA levels were measured in ventricular tissues obtained from age-matched 8-day-old control and hyperthyroid rats. In hyperthyroid animals, where an approximately 3-fold increase in It was identified, increases in the mRNA levels for Kv4.2 and Kv4.3 were 1.6-fold and 2.6-fold, respectively. 5. These results show that thyroid hormone can regulate the development of It in rat ventricle. Direct measurements of It density and mRNA levels as a function of development and thyroid hormone levels also strongly suggest that the Kv4.2 and Kv4.3 channels are essential components of It in rat ventricular cells.

Topological analysis of the role of homology in Flp-mediated recombination

Recombination by the Flp recombinase of Saccharomyces cerevisiae is known to be inhibited by heterology of the overlap regions of the two recombining DNA targets (FRT sites). We have used topological analysis to show that Flp can promote two rounds of intramolecular recombination between heterologous FRT sites contained within the same supercoiled plasmid. The products are in parental nonrecombinant configuration. Thus, heterology may appear to "block" recombination by rendering the heteroduplex recombinant products unstable, thus favoring a second round of recombination to homoduplex (but parental) products. Hence, homology in the core region is not a requirement for the recombination reaction by Flp but for the formation of recombinant products.

Tissue and species distribution of mRNA for the IKr-like K+ channel, erg

The human K+ channel gene, HERG, has been linked to the type 2 form of the autosomal dominant long-QT syndrome and has been suggested to encode the fast component of the delayed rectifier K+ current (IKr) found in heart. To date, the published electrophysiological and pharmacological data on the Xenopus-expressed HERG are very similar but are not identical to those of the endogenous IKr. In an effort to provide a different type of correlative data on the relationship between erg and IKr. cDNA fragments of erg homologues from guinea pig, rabbit, human, dog, and rat were cloned and used to test for the presence of erg mRNA in cardiac tissue. RNase protection assays reveal that erg message is found in the hearts of all five species and that it is expressed uniformly throughout the heart. The erg transcript is expressed at relatively high levels, being approximately 50% more abundant than the most prevalent Kv-class K+ channel transcript in canine ventricle (Kv4.3) erg transcripts were found to have a wide tissue distribution in rat and are abundant in the brain, retina, thymus, and adrenal gland and are also found in skeletal muscle, lung, and cornea. Since there were no published reports of an IKr-like current in the rat heart, electrophysiological studies were performed to test whether the significant level of erg message in rat heart was correlated with the presence of an IKr-like current in rat. In isolated rat ventricular myocytes, an E-4031-sensitive current was observed, which is consistent with the presence of IKr. These results strengthen the link between erg and the native IKr in heart and suggest that erg may play an important role in other noncardiac tissues.

Identification of His141 in the active site of Bacillus subtilis adenylosuccinate lyase by affinity labeling with 6-(4-bromo-2,3-dioxobutyl)thioadenosine 5'-monophosphate

Adenylosuccinate lyase of Bacillus subtilis is inactivated by 25-400 microM 6-(4-bromo-2,3-dioxobutyl)thioadenosine 5'-monophosphate (6-BDB-TAMP) at pH 7.0 and 25 degrees C. The initial inactivation rate constant exhibits nonlinear dependence on the concentration of 6-BDB-TAMP, implying there is reversible formation of enzyme-reagent complex (K(I) = 30 +/- 4 microM) prior to irreversible modification (kmax = 0.139 +/- 0.005 min(-1)). The tetrameric enzyme incorporates about 1 mol of 6-BDB-[32P]TAMP per mol of enzyme subunit concomitant with complete inactivation. Protection against inactivation and incorporation of [32P]reagent is provided by adenylosuccinate or a combination of AMP and fumarate, whereas either AMP or fumarate alone is much less effective. These observations suggest that 6-BDB-TAMP targets the adenylosuccinate-binding site. Hydrolyzed 6-BDB-TAMP is a competitive inhibitor with respect to adenylosuccinate in the catalytic reaction and also decreases the rate of inactivation by 6-BDB-TAMP. These results account for the decrease in the inactivation rate as the reaction of 6-BDB-TAMP with the enzyme proceeds. Purification by chromatography on dihydroxyboryl-agarose and high performance liquid chromatography of the tryptic digest of inactivated enzyme yields a single radioactive peptide, Thr140-Phe150, as determined by gas-phase sequencing. Modified His141 is the reaction product of 6-BDB-TAMP and adenylosuccinate lyase. We conclude that 6-BDB-TAMP functions as a reactive adenylosuccinate analog in modifying His141 in the substrate-binding site of adenylosuccinate lyase, where it may serve as a general base accepting a proton from the succinyl group during catalysis.

Purification and kinetic characterization of the mitogen-activated protein kinase phosphatase rVH6

The dual specificity protein-tyrosine phosphatase rVH6 belongs to a subfamily of enzymes that have in vivo and in vitro catalytic activity against mitogen-activated protein kinases. A method was developed for the expression and efficient purification of recombinant rVH6 in quantities sufficient for physical and kinetic characterization of the enzyme. Matrix-assisted laser desorption mass spectrometry verified the mass of purified rVH6 to be 43,500 +/- 150, and NH2-terminal sequence analysis confirmed the predicted amino acid sequence. Kinetic characterization of full-length rVH6 identified the critical ionizations involved in the kcat/Km parameter (apparent pKa values 5.1 and 6.6) and revealed a pH-independent kcat value of 0.014 s-1. In an attempt to define the essential catalytic core of this enzyme, amino acids 134-381 of rVH6 were expressed, purified, and characterized enzymatically. Kinetic analysis revealed that the truncated enzyme exhibited a turnover value similar to that of the full-length enzyme (kcat = 0.017 s-1), with p-nitrophenyl phosphate as substrate. Secondary structure prediction and molecular modeling of rVH6 based on the x-ray structure of the dual specificity protein tyrosine phosphatase, VHR, further supported the assignment of residues 134-381 to the core catalytic domain of rVH6. These results demonstrate that the NH2 terminus of rVH6 (residues 1-133) is not required for full enzyme activity and comprises a separate domain that may play a distinct physiological function.

Developmental analysis reveals mismatches in the expression of K+ channel alpha subunits and voltage-gated K+ channel currents in rat ventricular myocytes

In the experiments here, the developmental expression of the functional Ca(2+)-independent, depolarization-activated K+ channel currents, Ito and IK, and of the voltage-gated K+ channel (Kv) alpha subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2 in rat ventricular myocytes were examined quantitatively. Using the whole-cell patch clamp recording method, the properties and the densities of Ito and IK in ventricular myocytes isolated from postnatal day 5 (P5), 10 (P10), 15 (P15), 20 (P20), 25 (P25), 30 (P30), and adult (8-12 wk) rats were characterized and compared. These experiments revealed that mean Ito densities increase fourfold between birth and P30, whereas IK densities vary only slightly. Neither the time- nor the voltage-dependent properties of the currents vary measurably, suggesting that the subunits underlying functional Ito and IK channels are the same throughout postnatal development. In parallel experiments, the developmental expression of each of the voltage-gated K+ channel alpha subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2, was examined quantitatively at the mRNA and protein levels using subunit-specific probes. RNase protection assays revealed that Kv1.4 message levels are high at birth, increase between P0 and P10, and subsequently decrease to very low levels in adult rat ventricles. The decrease in message is accompanied by a marked reduction in Kv1.4 protein, consistent with our previous suggestion that Kv1.4 does not contribute to the formation of functional K+ channels in adult rat ventricular myocytes. In contrast to Kv1.4, the mRNA levels of Kv1.2, Kv1.5, Kv2.1, and Kv4.2 increase (three- to five-fold) between birth and adult. Western analyses, however, revealed that the expression patterns of these subunits proteins vary in distinct ways: Kv1.2 and Kv4.2, for example, increase between P5 and adult, whereas Kv1.5 remains constant and Kv2.1 decreases. Throughout development, therefore, there is a mismatch between the numbers of Kv alpha subunits expressed and the functional voltage-gated K+ channel currents distinguished electrophysiologically in rat ventricular myocytes. Alternative experimental approaches will be required to define directly the Kv alpha subunits that underlie functional voltage-gated K+ channels in these (and other) cells. In addition, the finding that Kv alpha subunit protein expression levels do not necessarily mirror mRNA levels suggests that caution should be exercised in attempting functional interpretations of observed changes in mRNA levels alone.

Kinetic analysis of the catalytic domain of human cdc25B

The Cdc25 cell cycle regulator is a member of the dual-specificity class of protein-tyrosine phosphatases that hydrolyze phosphotyrosine- and phosphothreonine-containing substrates. To study the mechanism of Cdc25B, we have overexpressed and purified the catalytic domain of human Cdc25B (Xu, X., and Burke, S. P. (1996) J. Biol. Chem. 271, 5118-5124). In the present work, we have analyzed the kinetic properties of the Cdc25B catalytic domain using the artificial substrate 3-O-methylfluorescein phosphate (OMFP). Steady-state kinetic analysis indicated that the kcat/Km for OMFP hydrolysis is almost 3 orders of magnitude greater than that for p-nitrophenyl phosphate hydrolysis. Like other dual-specificity phosphatases, Cdc25 exhibits a two-step catalytic mechanism, characterized by formation and breakdown of a phosphoenzyme intermediate. Pre-steady-state kinetic analysis of OMFP hydrolysis indicated that formation of the phosphoenzyme intermediate is approximately 20 times faster than subsequent phosphoenzyme breakdown. The resulting burst pattern of product formation allowed us to derive rate constants for enzyme phosphorylation (26 s-1) and dephosphorylation (1.5 s-1) as well as the dissociation constant for OMFP (0.3 mM). Calculations suggest that OMFP binds with higher affinity and reacts faster with Cdc25B than does p-nitrophenyl phosphate. OMFP is a highly efficient substrate for the dual-specificity protein-tyrosine phosphatases VHR and rVH6, but not for two protein-tyrosine phosphatases, PTP1 and YOP. The ability to observe distinct phases of the reaction mechanism during OMFP hydrolysis will facilitate future analysis of critical catalytic residues in Cdc25 and other dual-specificity phosphatases.

Role of the Kv4.3 K+ channel in ventricular muscle. A molecular correlate for the transient outward current

The expression of 15 different K+ channels in canine heart was examined, and a new K+ channel gene (Kv4.3), which encodes a rapidly inactivating K+ current, is described. The Kv4.3 channel was found to have biophysical and pharmacological properties similar to the native canine transient outward current (I(to)). The Kv4.3 gene is also expressed in human and rat heart. It is concluded that the Kv4.3 channel underlies the bulk of the I(to) in canine ventricular myocytes, and probably in human myocytes. Both the Kv4.3 and Kv4.2 channels are likely to contribute to the I(to) in rat heart, and differential expression of these two channels can account for observed differences in the kinetic properties of the I(to) in different regions of rat ventricle. There are significant differences in the pattern of K+ channel expression in canine heart, compared with rat heart, and these differences may be an adaptation to the different requirements for cardiac function in mammals of markedly different sizes. It is possible that the much longer ventricular action potential duration observed in canine heart compared with rat heart is due, in part, to the lower levels of Kv1.2, Kv2.1, and Kv4.2 gene expression in canine heart.

Glial cell line-derived neurotrophic factor activates the receptor tyrosine kinase RET and promotes kidney morphogenesis

The receptor tyrosine kinase RET functions during the development of the kidney and the enteric nervous system, yet no ligand has been identified to date. This report demonstrates that the glial cell line-derived neurotrophic factor (GDNF) activates RET, as measured by tyrosine phosphorylation of the intracellular catalytic domain. GDNF also binds RET with a dissociation constant of 8 nM, and 125I-labeled GDNF can be coimmunoprecipitated with anti-RET antibodies. In addition, exogenous GDNF stimulates both branching and proliferation of embryonic kidneys in organ culture, whereas neutralizing antibodies against GDNF inhibit branching morphogenesis. These data indicate that RET and GDNF are components of a common signaling pathway and point to a role for GDNF in kidney development.

Glial cell line-derived neurotrophic factor signals through the RET receptor and activates mitogen-activated protein kinase

Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-beta family of growth factors, was first identified by its ability to promote the survival of midbrain dopaminergic neurons in culture. We demonstrate that GDNF treatment of several neuroblastoma cell lines leads to dose-dependent tyrosine phosphorylation of the RET receptor and that other transforming growth factor-beta family members are not able to activate the RET receptor. GDNF treatment of neuroblastoma cells also results in increased transcription of an Elk luciferase reporter gene, suggesting that GDNF activates the mitogen-activated protein kinase signal transduction pathway.

A Drosophila protein-tyrosine phosphatase associates with an adapter protein required for axonal guidance

We have used the yeast two-hybrid system to isolate a novel Drosophila adapter protein, which interacts with the Drosophila protein-tyrosine phosphatase (PTP) dPTP61F. Absence of this protein in Drosophila causes the mutant photoreceptor axon phenotype dreadlocks (dock) (Garrity, P. A., Rao, Y., Salecker, I., and Zipursky, S. L.(1996) Cell 85, 639-650). Dock is similar to the mammalian oncoprotein Nck and contains three Src homology 3 (SH3) domains and one Src homology 2 (SH2) domain. The interaction of dPTP61F with Dock was confirmed in vivo by immune precipitation experiments. A sequence containing five PXXP motifs from the non-catalytic domain of the PTP is sufficient for interaction with Dock. This suggests that binding to the PTP is mediated by one or more of the SH3 domains of Dock. Immune precipitations of Dock also co-precipitate two tyrosine-phosphorylated proteins having molecular masses of 190 and 145 kDa. Interactions between Dock and these tyrosine-phosphorylated proteins are likely mediated by the Dock SH2 domain. These findings identify potential signal-transducing partners of Dock and propose a role for dPTP61F and the unidentified phosphoproteins in axonal guidance.

The SH2 domain-containing tyrosine phosphatase PTP1D is required for interferon alpha/beta-induced gene expression

Interferons (IFNs) induce early response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat (signal transducer and activator of transcription) proteins. Previous studies demonstrated that a protein-tyrosine phosphatase (PTP) is required for activation of the ISGF3 transcription complex by IFNalpha/beta, but the specific PTP responsible remained unidentified. We now show that the SH2 domain containing tyrosine phosphatase PTP1D (also designated as SHPTP2, SHPTP3, PTP2C, or Syp) is constitutively associated with the IFNalpha/beta receptor and becomes tyrosine-phosphorylated in response to ligand. Furthermore, transient expression of a phosphatase-inactive mutant or the COOH-terminal SH2 domain of PTP1D causes a dominant negative effect on IFNalpha/beta-induced early response gene expression. These results provide strong evidence that PTP1D functions as a positive regulator of the IFNalpha/beta-induced Jak/Stat signal transduction pathway.

Transition-state structures for the native dual-specific phosphatase VHR and D92N and S131A mutants. Contributions to the driving force for catalysis

Isotope effects have been measured for the reaction of the human dual-specific phosphatase VHR with p-nitrophenyl phosphate (pNPP). Isotope effects in the nonbridge oxygen atoms, in the bridge oxygen atom, and in the nitrogen atom were measured by the competitive method using an isotope ratio mass spectrometer. These are isotope effects on V/K, and give information on the chemical step of phosphoryl transfer from substrate to the enzymatic nucleophile Cys-124. With native VHR, 18(V/K)nonbridge = 1.0003 +/- 0.0003, 18(V/K)bridge = 1.0118 +/- 0.0020, and 15(V/K) = 0.9999 +/- 0.0004. The values are similar to the intrinsic isotope effects for the uncatalyzed reaction, indicating that the chemical step is rate-limiting with the pNPP substrate. The transition-state structure resembles that for the uncatalyzed reaction and those previously found for the protein-tyrosine phosphatases YOP51 and PTP1, and is highly dissociative with P-O bond cleavage and protonation of the leaving group by the general acid Asp-92 both well advanced. The D92N mutant exhibits a transition state similar to that of the uncatalyzed reaction of the pNPP dianion, dissociative and with the leaving group departing as the nitrophenolate anion. The S131A mutation causes an increase in the pKa of the nucleophilic Cys, but the isotope effect data are unchanged from those for the native enzyme, indicating no effects of this increase in nucleophilicity on transition-state structure. The double mutant D92N/S131A manifests both the increase in pKa of the nucleophilic Cys and the loss of general acid assistance to the leaving group. In the absence of the general acid, the change in nucleophile pKa results in an increase in 18(V/K)nonbridge from 1.0019 (with D92N) to 1.0031 (with D92N/S131A), indicating loss of P-O nonbridge bond order in the transition state. It is concluded that this is more likely caused by electrostatic effects rather than resulting from increased nucleophile-phosphorus bonding in a less dissociative transition state, although the latter explanation cannot be excluded on the basis of the present data. Electrostatic effects between the thiolate anion nucleophile and the phosphoryl group may be an important part of the driving force for catalysis in this family of enzymes.

Crystal structure of the dual specificity protein phosphatase VHR

Dual specificity protein phosphatases (DSPs) regulate mitogenic signal transduction and control the cell cycle. Here, the crystal structure of a human DSP, vaccinia H1-related phosphatase (or VHR), was determined at 2.1 angstrom resolution. A shallow active site pocket in VHR allows for the hydrolysis of phosphorylated serine, threonine, or tyrosine protein residues, whereas the deeper active site of protein tyrosine phosphatases (PTPs) restricts substrate specificity to only phosphotyrosine. Positively charged crevices near the active site may explain the enzyme's preference for substrates with two phosphorylated residues. The VHR structure defines a conserved structural scaffold for both DSPs and PTPs. A "recognition region," connecting helix alpha1 to strand beta1, may determine differences in substrate specificity between VHR, the PTPs, and other DSPs.

Direct association between the Ret receptor tyrosine kinase and the Src homology 2-containing adapter protein Grb7

Adapter proteins containing Src homology 2 (SH2) domains link transmembrane receptor protein-tyrosine kinases to downstream signal transducing molecules. A family of SH2 containing adapter proteins including Grb7 and Grb10 has been recently identified. We had previously shown that Grb10 associates with Ret via its SH2 domain in an activation-dependent manner (Pandey, A., Duan, H., Di Fiore, P.P., and Dixit, V.M. (1995) J. Biol, Chem. 270, 21461-21463). We now demonstrate that the related adapter molecule Grb7 also associates with Ret in vitro and in vivo, and that the binding of the SH2 domain of Grb7 to Ret is direct. This binding is dependent upon Ret autophosphorylation since Grb7 is incapable of binding a kinase-defective mutant of Ret. Thus two members of the Grb family, Grb7 and Grb10, likely relay signals emanating from Ret to other, as yet, unidentified targets within the cell.

Crystallization and preliminary structural analysis of Bacillus subtilis adenylosuccinate lyase, an enzyme implicated in infantile autism

Adenylosuccinate lyase (ASL) from Bacillus subtilis has been crystallized and structural analysis by X-ray diffraction is in progress. ASL is a 200-kDa homotetramer that catalyzes two distinct steps of de novo purine biosynthesis leading to the formation of AMP and IMP; both steps involve the beta-elimination of fumarate. A single point mutation in the human ASL gene has been linked to mental retardation with autistic features. In addition, ASL plays an important role in the bioprocessing of anti-HIV therapeutics. B subtilis ASL, which shares 30% sequence identity and 70% sequence similarity with human ASL, has been crystallized and data to 3.3 A have been collected at 100 K. The space group is P6(1)22 or P6(5)22 with a = b = 129.4 A; the length of the c-axis varies between 275 and 290 A, depending on the crystal. An analysis of solvent content indicates a dimer in the asymmetric unit, although a self-rotation function and an analysis of native Pattersons failed to identify unambiguously the location of any noncrystallographic symmetry axes. Structure determination by isomorphous replacement is in progress.

Visualization of intermediate and transition-state structures in protein-tyrosine phosphatase catalysis

Engineering site-specific amino acid substitutions into the protein-tyrosine phosphatase (PTPase) PTP1 and the dual-specific vaccinia H1-related phosphatase (VHR), has kinetically isolated the two chemical steps of the reaction and provided a rare opportunity for examining transition states and directly observing the phosphoenzyme intermediate. Changing serine to alanine in the active-site sequence motif HCXXGXXRS shifted the rate-limiting step from intermediate formation to intermediate hydrolysis. Using phosphorus 31P NMR, the covalent thiol-phosphate intermediate was directly observed during catalytic turnover. The importance of the conserved aspartic acid (D92 in VHR and D181 in PTP1) in both chemical steps was established. Kinetic analysis of D92N and D181N mutants indicated that aspartic acid acts as a general acid by protonating the leaving-group phenolic oxygen. Structure-reactivity experiments with native and aspartate mutant enzymes established that proton transfer is concomitant with P-O cleavage, such that no charge develops on the phenolic oxygen. Steady- and presteady-state kinetics, as well as NMR analysis of the double mutant D92N/S131A (VHR), suggested that the conserved aspartic acid functions as a general base during intermediate hydrolysis. As a general base, aspartate would activate a water molecule to facilitate nucleophilic attack. The amino acids involved in transition-state stabilization for cysteinylphosphate hydrolysis were confirmed by the x-ray structure of the Yersinia PTPase complexed with vanadate, a transition-state mimic that binds covalently to the active-site cysteine. Consistent with the NMR, x-ray, biochemical, and kinetic data, a unifying mechanism for catalysis is proposed.

Oncogenic RET receptors display different autophosphorylation sites and substrate binding specificities

The c-ret proto-oncogene encodes a receptor tyrosine kinase which plays an important role in neural crest as well as kidney development. Genetic studies have demonstrated that germ line mutations in the ret oncogene are the direct cause of multiple endocrine neoplasia (MEN) 2A and 2B, familial medullary thyroid carcinoma (FMTC), and Hirschsprung's disease. However, despite the large body of genetic and biological evidence suggesting the importance of RET in development and neoplastic processes, the signal transduction mechanisms of RET remain unknown. To begin to understand the molecular mechanisms of the disease states caused by mutations in RET, the patterns of autophosphorylation of the wild-type RET and the MEN mutants were studied using site-directed mutagenesis and phosphopeptide mapping. Among the 6 autophosphorylation sites found in the wild-type RET receptor, the MEN2B mutant lacked phosphorylation at Tyr-1096, leading to decreased Grb2 binding, while simultaneously creating a new phosphorylation site. These changes in autophosphorylation suggest that the MEN2B mutation may result in the more aggressive MEN2B phenotype by altering the receptor's signaling capabilities.

Parathyroid hormone regulates the expression of the receptor protein tyrosine phosphatase, OST-PTP, in rat osteoblast-like cells

Bone remodeling requires regulated tyrosine phosphorylation mediated by specific protein tyrosine kinases, such as c-src and c-fms, and to date, unknown protein tyrosine phosphatases (PTPs). We previously reported the isolation of a novel bone-specific receptor PTP, named osteotesticular PTP (OST-PTP), which is regulated during osteoblast differentiation and after exposure to PTH. To determine the relevance of this PTH regulation, we characterized the PTH-induced increase in OST-PTP messenger RNA (mRNA) in UMR 106 cells in comparison with PTH effects on a related receptor PTP and a PTH regulated gene, rat collagenase. Treatment of cells with rat PTH 1-34 (rPTH) resulted in a dramatic concentration and time-dependent increase in OST-PTP mRNA with a threshold at 4 h (= or < 1nM rPTH) and maximal response of 6- 10-fold above control levels at 8 h (100 nM rPTH). An increase in collagenase mRNA was detectable 2 h earlier at 100 pM rPTH with a maximal response at least 5-fold greater than that observed for OST- PTP. Levels of mRNA for the structurally similar PTP, rat leucocyte antigen-related molecule, were unaffected by rPTH treatment. Administration of cycloheximide (5-100 microM) abolished the OST-PTP and collagenase responses to PTH. The cAMP analogs, CPT-cAMP (0.01-1mM; 8 h) or Sp-cAMP (0.1 and 0.5 mM) were equal or greater in their effectiveness to enhance both OST-PTP and collagenase mRNA as compared with rPTH. In contrast, phorbol esters, calcium ionophore, bovine PTH (3-34), or human PTHrP (7-34) had no effect on either transcript. Interestingly, 36 h of pretreatment of cells with epidermal growth factor (10 ng/ml), a growth factor known to modulate PTH's actions, resulted in a significant decrease in the abundance of OST-PTP mRNA after rPTH exposure. These studies suggest that regulation of OST-PTP mRNA is a secondary response to PTH stimulation that is dependent on protein synthesis and that may be primarily by activation of the protein kinase A pathway. This specific modulation of a bone receptor PTP may prove to be a critical component in the PTH modulation of osteoblast function.