Role of the activation loop tyrosines in regulation of the insulin-like growth factor I receptor-tyrosine kinase

The tyrosine kinase activity of insulin-like growth factor I receptor (IGF1R) is under tight control. Ligand binding to the extracellular portion of IGF1R stimulates autophosphorylation at three sites (Tyr1131, Tyr1135, and Tyr1136) in the activation loop within the tyrosine kinase catalytic domain. Autophosphorylation at all three sites is required for maximum enzyme activity, and for IGF1-stimulated cellular activity of the receptor. Previous studies have not clarified the contributions of the individual tyrosines to enzymatic activation. Here, we produced single Tyr-to-Phe mutations at these positions, and compared activities of the purified kinase domains (unphosphorylated and phosphorylated) with wild-type IGF1R. Rates of autophosphorylation of the three mutants were more rapid than for wild-type IGF1R; this was most apparent for the Y1135F mutant. Substrate phosphorylation studies on the unphosphorylated forms of IGF1R confirmed that the value of Vmax for Y1135F was elevated relative to wild-type IGF1R, consistent with a disruption of an autoinhibitory interaction. In contrast, activity measurements on the fully phosphorylated enzymes indicated that kcat/Km values were lowered relative to wild-type IGF1R; this effect was most dramatic for Y1136F. We confirmed these findings using limited proteolysis and tryptophan fluorescence experiments. The results demonstrate that Tyr1135 plays a particularly important role in stabilizing the autoinhibited conformation of the activation loop, while Tyr1136 plays the key role in stabilizing the open, activated conformation of IGF1R.     

Multiple autophosphorylation sites of the epidermal growth factor receptor are essential for receptor kinase activity and internalization. Contrasting significance of tyrosine 992 in the native and truncated receptors.

The role of epidermal growth factor (EGF) receptor autophosphorylation sites in the regulation of receptor functions has been studied using cells transfected with mutant EGF receptors. Simultaneous point mutation of 4 tyrosines (Y1068, Y1086, Y1148, Y1173) to phenylalanine, as well as removal of these sites by truncation of the carboxyl-terminal 123 amino acid residues, resulted in reduced receptor phosphorylation of an in vivo specific substrate phospholipase C-gamma 1 to less than 50% compared to the wild-type receptor. The internalization rate constant Ke was also significantly lower in these mutants (0.15/min) compared to cells transfected with wild-type receptor (0.27/min). Additional mutation of tyrosine 992 to phenylalanine in the truncated receptor mutant (Dc-123F) further decreased the receptor internalization rate to a minimal level (ke = 0.07-0.10/min), equivalent to the ke measured for cells expressing kinase-negative receptor (A721). Moreover, tyrosine kinase activity of the Dc-123F receptor toward phospholipase C-gamma 1, compared to wild-type receptor, was reduced by 90%. Taken together, these results show that EGF receptor lacking five autophosphorylation sites functions similar to a kinase-negative receptor. Mutation of tyrosine residue Y992 alone in the context of full length EGF receptor, however, did not affect receptor internalization or kinase activity toward phospholipase C-gamma 1. These data indicate that tyrosine 992 is critical for substrate phosphorylation and internalization only in the context of the truncated receptor, and that minor autophosphorylation sites, such as Y992, may act as compensatory regulatory sties in the absence of the major EGF receptor autophosphorylation sites.     

c-Abl has high intrinsic tyrosine kinase activity that is stimulated by mutation of the Src homology 3 domain and by autophosphorylation at two distinct regulatory tyrosines

Using the specific Abl tyrosine kinase inhibitor STI 571, we purified unphosphorylated murine type IV c-Abl and measured the kinetic parameters of c-Abl tyrosine kinase activity in a solution with a peptide-based assay. Unphosphorylated c-Abl exhibited substantial peptide kinase activity with K(m) of 204 microm and V(max) of 33 pmol min(-1). Contrary to previous observations using immune complex kinase assays, we found that a transforming c-Abl mutant with a Src homology 3 domain point mutation (P131L) had significantly (about 6-fold) higher intrinsic kinase activity than wild-type c-Abl (K(m) = 91 microm, V(max) = 112 pmol min(-1)). Autophosphorylation stimulated the activity of wild-type c-Abl about 18-fold and c-Abl P131L about 3.6-fold, resulting in highly active kinases with similar catalytic rates. The autophosphorylation rate was dependent on Abl protein concentration consistent with an intermolecular reaction. A tyrosine to phenylalanine mutation (Y412F) at the c-Abl residue homologous to the c-Src catalytic domain autophosphorylation site impaired the activation of wild-type c-Abl by 90% but reduced activation of c-Abl P131L by only 45%. Mutation of a tyrosine (Tyr-245) in the linker region between the Src homology 2 and catalytic domains that is conserved among the Abl family inhibited the autophosphorylation-induced activation of wild-type c-Abl by 50%, whereas the c-Abl Y245F/Y412F double mutant was minimally activated by autophosphorylation. These results support a model where c-Abl is inhibited in part through an intramolecular Src homology 3-linker interaction and stimulated to full catalytic activity by sequential phosphorylation at Tyr-412 and Tyr-245.     

Functions of the major tyrosine phosphorylation site of the PDGF receptor beta subunit.

Two tyrosine phosphorylation sites in the human platelet-derived growth factor receptor (PDGFR) beta subunit have been mapped previously to tyrosine (Y)751, in the kinase insert, and Y857, in the kinase domain. Y857 is the major site of tyrosine phosphorylation in PDGF-stimulated cells. To evaluate the importance of these phosphorylations, we have characterized the wild-type (WT) and mutant human PDGF receptor beta subunits in dog kidney epithelial cells. Replacement of either Y751 or Y857 with phenylalanine (F) reduced PDGF-stimulated DNA synthesis to approximately 50% of the WT level. A mutant receptor with both tyrosines mutated was unable to initiate DNA synthesis, as was a kinase-inactive mutant receptor. Transmodulation of the epidermal growth factor receptor required Y857 but not Y751. We also tested the effects of phosphorylation site mutations on PDGF-stimulated receptor kinase activity. PDGF-induced tyrosine phosphorylation of two cellular proteins, phospholipase C gamma 1 (PLC gamma 1) and the GTPase activating protein of Ras (GAP), was assayed in epithelial cells expressing each of the mutant receptors. Tyrosine phosphorylation of GAP and PLC gamma 1 was reduced markedly by the F857 mutation but not significantly by the F751 mutation. Reduced kinase activity of F857 receptors was also evident in vitro. Immunoprecipitated WT receptors showed a two- to fourfold increase in specific kinase activity if immunoprecipitated from PDGF-stimulated cells. The F751 receptors showed a similar increase in activity, but F857 receptors did not. Our data suggest that phosphorylation of Y857 may be important for stimulation of kinase activity of the receptors and for downstream actions such as epidermal growth factor receptor transmodulation and mitogenesis.     

Multiple autophosphorylation site mutations of the epidermal growth factor receptor. Analysis of kinase activity and endocytosis

We have utilized site-directed mutants to study the role of autophosphorylation of the epidermal growth factor (EGF) receptor in the regulation of receptor kinase activity and ligand-induced endocytosis. A single mutation of the major autophosphorylation site, Y1173, and a double mutation of two autophosphorylation sites, Y1173 and Y1148, did not inhibit kinase activity in vivo, using PLC gamma 1 as a specific substrate for the EGF receptor kinase. The simultaneous mutation of three major autophosphorylation sites (Y1173, Y1148, Y1068), however, caused more than a 50% decrease in EGF-induced tyrosine phosphorylation of PLC gamma 1. The triple mutation also resulted in a substantial inhibition of the EGF-receptor endocytic system. We have used three types of experiments to analyze internalization, recycling, and degradation of EGF in cells with these mutants or the wild-type receptor. Using a simple mathematical model we have shown that the internalization rate constant is 2-fold lower in cells expressing the triple mutation receptor (F3 cells) than in cells expressing wild-type EGF receptor (wild-type cells). However, the rate constant for recycling was similar in both cell types. The EGF degradation rate constant was also lower in F3 cells. EGF-induced EGF receptor degradation was slower in F3 cells (t1/2 = 4 h) than in wild-type cells (t1/2 = 1 h). Therefore, our results suggest that multiple autophosphorylations of the carboxyl terminus of the EGF receptor are required for EGF receptor kinase activation, and for the internalization and intracellular processing of the EGF.receptor complex.     

Activation of Jak2 catalytic activity requires phosphorylation of Y1007 in the kinase activation loop.

The Janus protein tyrosine kinases (Jaks) play critical roles in transducing growth and differentiation signals emanating from ligand-activated cytokine receptor complexes. The activation of the Jaks is hypothesized to occur as a consequence of auto- or transphosphorylation on tyrosine residues associated with ligand-induced aggregation of the receptor chains and the associated Jaks. In many kinases, regulation of catalytic activity by phosphorylation occurs on residues within the activation loop of the kinase domain. Within the Jak2 kinase domain, there is a region that has considerable sequence homology to the regulatory region of the insulin receptor and contains two tyrosines, Y1007 and Y1008, that are potential regulatory sites. In the studies presented here, we demonstrate that among a variety of sites, Y1007 and Y1008 are sites of trans- or autophosphorylation in vivo and in in vitro kinase reactions. Mutation of Y1007, or both Y1007 and Y1008, to phenylalanine essentially eliminated kinase activity, whereas mutation of Y1008 to phenylalanine had no detectable effect on kinase activity. The mutants were also examined for the ability to reconstitute erythropoietin signaling in gamma2 cells, which lack Jak2. Consistent with the kinase activity, mutation of Y1007 to phenylalanine eliminated the ability to restore signaling. Moreover, phosphorylation of a kinase-inactive mutant (K882E) was not detected, indicating that Jak2 activation during receptor aggregation is dependent on Jak2 and not another receptor-associated kinase. The results demonstrate the critical role of phosphorylation of Y1007 in Jak2 regulation and function.     

A single substitution of the insulin receptor kinase inhibits serine autophosphorylation in vitro: evidence for an interaction between the C-terminus and the activation loop

We examined the effects of mutations of tyrosine and serine autophosphorylation sites on the dual specificity of the insulin receptor kinase (IRKD) in vitro using autophosphorylation and substrate phosphorylation and phosphopeptide mapping. For comparable studies, the recombinant kinases were overexpressed in the baculovirus system, purified, and analyzed. The phosphate incorporation into the enzymes was in the range of 3-4.5 mol/mol, and initial velocities of autophosphorylation were reduced up to 2-fold. However, the mutation Y1151F in the activation loop inhibited phosphate incorporation in the C-terminal serine residues 1275 and 1309, due to a 10-fold decrease of the initial velocity of serine autophosphorylation. Although the K(M) and V(MAX) values of this mutant were only slightly altered in substrate phosphorylation reactions using a recombinant C-terminal insulin receptor peptide (K(M): Y1151F, 9.9 +/- 0.4 microM; IRKD, 6.1 +/- 0.2 microM; V(MAX): Y1151F, 72 +/- 4 nmol min(-)(1) mg(-)(1); IRKD, 117 +/- 6 nmol min(-)(1) mg(-)(1)), diminished phosphate incorporation into serine residues of the peptide was observed. In contrast, the phosphorylation of a recombinant IRS-1 fragment, which was shown to be phosphorylated markedly on serine residues by IRKD, was not affected by any kinase mutation. These results underline that IRKD is a kinase with dual specificity. The substrate specificity toward C-terminal serine phosphorylation sites can be modified by a single amino acid substitution in the activation loop, whereas the specificity toward IRS-1 is not affected, suggesting that the C-terminus and the activation loop interact.     

Identification of two juxtamembrane autophosphorylation sites in the PDGF beta-receptor; involvement in the interaction with Src family tyrosine kinases.

Two novel sites of autophosphorylation were localized to the juxtamembrane segment of the human platelet-derived growth factor (PDGF) beta-receptor. To evaluate the importance of these phosphorylation sites, receptor mutants were made in which Tyr579, Tyr581 or both were replaced with phenylalanine residues; the receptor mutants were stably expressed in porcine aortic endothelial cells. Compared with the wild-type receptor, the Y579F and Y581F mutants were less able to mediate association with and activation of the Src family tyrosine kinases. The ability of these phosphorylation sites to mediate directly the binding of the Src family proteins was also demonstrated by using phosphotyrosine-containing synthetic peptides representing the juxtamembrane sequence of the receptor. Both the Y579F and Y581F mutants were similar to the wild-type receptor with regard to their protein tyrosine kinase activity and ability to induce mitogenicity in response to PDGF-BB. A conclusive evaluation of the role of the Src family members in signal transduction could, however, not be made since our attempt to prevent completely the association by mutation of both Tyr579 and Tyr581, resulted in loss of kinase activity and was therefore not informative. The present data, together with previous observations, demonstrate a high degree of specificity in the interaction between different autophosphorylation sites in the PDGF beta-receptor and downstream components in the signal transduction pathway.     

Modulation of the protein tyrosine kinase activity and autophosphorylation of the epidermal growth factor receptor by its juxtamembrane region

Using peptides epidermal growth factor receptor (EGFR)-13 and EGFR-14, which correspond to residues 645-657 and 679-692, respectively, in the juxtamembrane, cytosolic region of the epidermal growth factor receptor (EGFR) we have investigated the role of specific regions of the receptor in regulating its autophosphorylation and protein tyrosine kinase activity. EGFR-13, but not EGFR-14, increased autophosphorylation (by twofold) of the full-length and two truncated forms (Delta1022-1186 and a constitutively active receptor kinase domain) of the EGFR. EGFR-13 increased the stoichiometry of tyrosine phosphorylation of the full-length receptor from 4.2 to 10.1 mol Pi/mol EGFR and that of EGFRDelta1022-1186 from 1.0 to 2 mol Pi/mol receptor. Increased receptor autophosphorylation in the presence of EGFR-13 cannot solely be attributed to an increase in tyrosine kinase activity because EGFR-14 and polylysine increased tyrosine kinase activity of EGFRDelta1022-1186 and full-length EGFR, respectively, to the same extent as EGFR-13 without any effects on receptor autophosphorylation. Phosphorylation of EGFR-13 (P-EGFR-13) on the threonine residue corresponding to Thr654 in EGFR obliterated the ability of the peptide to increase autophosphorylation and markedly diminished its capacity to increase receptor tyrosine kinase activity. Additionally, EGFR-13, but not EGFR-14 or P-EGFR-13, decreased the migration of the receptor on nondenaturing gels, indicating that EGFR-13 induces some conformational change. Phosphopeptide maps of the EGFR phosphorylated in the presence of EGFR-13 or pp60(c-src) demonstrated that the additional sites phosphorylated in the presence of EGFR-13 were the same as those phosphorylated by pp60(c-src) (i.e., Y803, Y845, Y891, Y920, and Y1101). Thus, we conclude that EGFR-13, but not EGFR-14 or P-EGFR-13, competes to disrupt interactions between amino acids 645-657 and some other region(s) on the EGFR to either alleviate a conformational constraint or alter dimer conformation. This change increases the protein tyrosine kinase activity of the EGFR and provides access to additional tyrosine autophosphorylation sites in the receptor.     

A juxtamembrane tyrosine in the colony stimulating factor-1 receptor regulates ligand-induced Src association, receptor kinase function, and down-regulation

Recent literature implicates a regulatory function of the juxtamembrane domain (JMD) in receptor tyrosine kinases. Mutations in the JMD of c-Kit and Flt3 are associated with gastrointestinal stromal tumors and acute myeloid leukemias, respectively. Additionally, autophosphorylated Tyr559 in the JMD of the colony stimulating factor-1 (CSF-1) receptor (CSF-1R) binds to Src family kinases (SFKs). To investigate SFK function in CSF-1 signaling we established stable 32D myeloid cell lines expressing CSF-1Rs with mutated SFK binding sites (Tyr559-TFI). Whereas binding to I562S was not significantly perturbed, Y559F and Y559D exhibited markedly decreased CSF-1-dependent SFK association. All JMD mutants retained intrinsic kinase activity, but Y559F, and less so Y559D, showed dramatically reduced CSF-1-induced autophosphorylation. CSF-1-mediated wild-type (WT)-CSF-1R phosphorylation was not markedly affected by SFK inhibition, indicating that lack of SFK binding is not responsible for diminished Y559F phosphorylation. Unexpectedly, cells expressing Y559F were hyperproliferative in response to CSF-1. Hyperproliferation correlated with prolonged activation of Akt, ERK, and Stat5 in the Y559F mutant. Consistent with a defect in receptor negative regulation, c-Cbl tyrosine phosphorylation and CSF-1R/c-Cbl co-association were almost undetectable in the Y559F mutant. Furthermore, Y559F underwent reduced multiubiquitination and delayed receptor internalization and degradation. In conclusion, we propose that Tyr559 is a switch residue that functions in kinase regulation, signal transduction and, indirectly, receptor down-regulation. These findings may have implications for the oncogenic conversion of c-Kit and Flt3 with JMD mutations.     

Identification of RET autophosphorylation sites by mass spectrometry

The catalytic and signaling activities of RET, a receptor-type tyrosine kinase, are regulated by the autophosphorylation of several tyrosine residues in the cytoplasmic region of RET. Some studies have revealed a few possible autophosphorylation sites of RET by [(32)P]phosphopeptide mapping or by using specific anti-phosphotyrosine antibodies. To ultimately identify these and other autophosphorylation sites of RET, we performed mass spectrometry analysis of an originally prepared RET recombinant protein. Both the autophosphorylation and kinase activity of myelin basic protein as an external substrate of the recombinant RET protein were substantially elevated in the presence of ATP without stimulation by a glial cell line-derived neurotrophic factor, a natural ligand for RET. Mass spectrometric analysis revealed that RET Tyr(806), Tyr(809), Tyr(900), Tyr(905), Tyr(981), Tyr(1062), Tyr(1090), and Tyr(1096) were autophosphorylation sites. Levels of autophosphorylation and kinase activity of RET-MEN2A (multiple endocrine neoplasia 2A), a constitutively active form of RET with substitution of Tyr(900) by phenylalanine (Y900F), were comparable with those of original RET-MEN2A, whereas those of the mutant Y905F were greatly decreased. Interestingly, those of a double mutant, Y900F/Y905F, were completely abolished. Both the kinase activity and transforming activity were impaired in the mutants Y806F and Y809F. These results provide convincing evidence for both previously suggested and new tyrosine autophosphorylation sites of RET as well as for novel functions of Tyr(806), Tyr(809), and Tyr(900) phosphorylation in both catalytic kinase activities and cell growth. The significance of the identified autophosphorylation sites in various protein-tyrosine kinases registered in a data base is discussed in this paper.     

Mechanism and functional significance of Itk autophosphorylation

Tec family non-receptor tyrosine kinases (Itk, Btk, Tec, Rlk and Bmx) are characterized by the presence of an autophosphorylation site within the non-catalytic Src homology 3 (SH3) domain. The full-length Itk mutant containing phenylalanine in place of the autophosphorylated tyrosine has been studied in Itk-deficient primary T cells. These studies revealed that the non-phosphorylated enzyme restores Itk mediated signaling only partially. In spite of these insights, the precise role of the Tec kinase autophosphorylation site is unclear and the mechanism of the autophosphorylation reaction within the Tec kinases is not known. Here, we show both in vitro and in vivo that Itk autophosphorylation on Y180 within the SH3 domain occurs exclusively via an intramolecular, in cis mechanism. Using an in vitro kinase assay, we show that mutation of the Itk autophosphorylation site Y180 to Phe decreases kinase activity of the full-length enzyme by increasing Km for a peptide substrate. Moreover, mutation of Y180 to Glu, a residue chosen to mimic the phosphorylated tyrosine, alters the ligand-binding capability of the Itk SH3 domain in a ligand-dependent fashion. NMR chemical shift mapping gives residue-specific structural insight into the effect of the Y180E mutation on ligand binding. These data provide a molecular level context with which to interpret in vivo functional data and allow development of a structural model for Itk autophosphorylation.     

Two signaling molecules share a phosphotyrosine-containing binding site in the platelet-derived growth factor receptor.

Autophosphorylation sites of growth factor receptors with tyrosine kinase activity function as specific binding sites for Src homology 2 (SH2) domains of signaling molecules. This interaction appears to be a crucial step in a mechanism by which receptor tyrosine kinases relay signals to downstream signaling pathways. Nck is a widely expressed protein consisting exclusively of SH2 and SH3 domains, the overexpression of which causes cell transformation. It has been shown that various growth factors stimulate the phosphorylation of Nck and its association with autophosphorylated growth factor receptors. A panel of platelet-derived growth factor (PDGF) receptor mutations at tyrosine residues has been used to identify the Nck binding site. Here we show that mutation at Tyr-751 of the PDGF beta-receptor eliminates Nck binding both in vitro and in living cells. Moreover, the Y751F PDGF receptor mutant failed to mediate PDGF-stimulated phosphorylation of Nck in intact cells. A phosphorylated Tyr-751 is also required for binding of phosphatidylinositol-3 kinase to the PDGF receptor. Hence, the SH2 domains of p85 and Nck share a binding site in the PDGF receptor. Competition experiments with different phosphopeptides derived from the PDGF receptor suggest that binding of Nck and p85 is influenced by different residues around Tyr-751. Thus, a single tyrosine autophosphorylation site is able to link the PDGF receptor to two distinct SH2 domain-containing signaling molecules.     

MET(PRC) mutations in the Ron receptor result in upregulation of tyrosine kinase activity and acquisition of oncogenic potential.

Ron and Met are structurally related receptor tyrosine kinases that elicit a complex biological response leading to invasive growth. Naturally occurring point mutations activate the Met kinase in papillary renal carcinomas (MET(PRC) mutations). By site-directed mutagenesis, we generated homologous amino acid substitutions in the Ron kinase domain and analyzed the biochemical and biological properties of the mutant receptors. Among the mutations studied, D(1232)H and M(1254)T displayed transforming activity in NIH3T3 cells, inducing focus formation and anchorage-independent growth. The D(1232)H and M(1254)T substitutions resulted in increased Ron autophosphorylation both in vivo and in vitro and constitutive binding to intracellular signal transducers. Both mutations yielded a dramatic increase in catalytic efficiency, indicating a direct correlation between kinase activity and oncogenic potential. Molecular modeling of the Ron D(1232)H mutation suggests that this single amino acid substitution favors the transition of the kinase from the inactive to the active state. These data demonstrate that point mutations can confer transforming activity to the Ron receptor and show that RON is a potential oncogene.     

Intrasteric inhibition of ATP binding is not required to prevent unregulated autophosphorylation or signaling by the insulin receptor

Receptor tyrosine kinases may use intrasteric inhibition to suppress autophosphorylation prior to growth factor stimulation. To test this hypothesis we made an Asp1161Ala mutant in the activation loop that relieved intrasteric inhibition of the unphosphorylated insulin receptor (IR) and its recombinant cytoplasmic kinase domain (IRKD) without affecting the activated state. Solution studies with the unphosphorylated mutant IRKD demonstrated conformational changes and greater catalytic efficiency from a 10-fold increase in k(cat) and a 15-fold-lower K(m ATP) although K(m peptide) was unchanged. Kinetic parameters of the autophosphorylated mutant and wild-type kinase domains were virtually identical. The Asp1161Ala mutation increased the rate of in vitro autophosphorylation of the IRKD or IR at low ATP concentrations and in the absence of insulin. However, saturation with ATP (for the IRKD) or the presence of insulin (for the IR) yielded equivalent rates of autophosphorylation for mutant versus wild-type kinases. Despite a biochemically more active kinase domain, the mutant IR expressed in C2C12 myoblasts was not constitutively autophosphorylated. However, it displayed a 2.5-fold-lower 50% effective concentration for insulin stimulation of autophosphorylation and was dephosphorylated more slowly following withdrawal of insulin than wild-type IR. In tests of the regulation of the unphosphorylated basal state, these results demonstrate that neither intrasteric inhibition against ATP binding nor suppression of kinase activity is required to prevent premature autophosphorylation of the IR. Finally, the lower rate of dephosphorylation suggests invariant residues of the activation loop such as Asp1161 may function at multiple junctures in cellular regulation of receptor tyrosine kinases.     

Profound ligand-independent kinase activation of fibroblast growth factor receptor 3 by the activation loop mutation responsible for a lethal skeletal dysplasia, thanatophoric dysplasia type II.

Thanatophoric dysplasia type II (TDII) is a neonatal lethal skeletal dysplasia caused by a recurrent Lys-650-->Glu mutation within the highly conserved activation loop of the kinase domain of fibroblast growth factor receptor 3 (FGFR3). We demonstrate here that this mutation results in profound constitutive activation of the FGFR3 tyrosine kinase, approximately 100-fold above that of wild-type FGFR3. The mechanism of FGFR3 activation in TDII was probed by constructing various point mutations in the activation loop. Substitutions at position 650 indicated that not only Glu but also Asp and, to a lesser extent, Gln and Leu result in pronounced constitutive activation of FGFR3. Additional mutagenesis within the beta10-beta11 loop region (amino acids Tyr-647 to Leu-656) demonstrated that amino acid 650 is the only residue which can activate the receptor when changed to a Glu, indicating a specificity of position as well as charge for mutations which can give rise to kinase activation. Furthermore, when predicted sites of autophosphorylation at Tyr-647 and Tyr-648 were mutated to Phe, either singly or in combination, constitutive kinase activity was still observed in response to the Lys-650-->Glu mutation, although the effect of these mutations on downstream signalling was not investigated. Our data suggest that the molecular effect of the TDII activation loop mutation is to mimic the conformational changes that activate the tyrosine kinase domain, which are normally initiated by ligand binding and autophosphorylation. These results have broad implications for understanding the molecular basis of other human developmental syndromes that involve mutations in members of the FGFR family. Moreover, these findings are relevant to the study of kinase regulation and the design of activating mutations in related tyrosine kinases.     

Identification of two C-terminal autophosphorylation sites in the PDGF beta-receptor: involvement in the interaction with phospholipase C-gamma.

Two novel sites of autophosphorylation were localized to the C-terminal tail of the PDGF beta-receptor. To evaluate the importance of these phosphorylation sites, receptor mutants in which Tyr1009, Tyr1021 or both were replaced with phenylalanine residues, were expressed in porcine aortic endothelial (PAE) cells. These mutants were similar to the wild type receptor with regard to protein tyrosine kinase activity and ability to induce mitogenicity in response to PDGF-BB. However, both the Y1009F and Y1021F mutants showed a decreased ability to mediate association with and the tyrosine phosphorylation of phospholipase C-gamma (PLC-gamma) compared to the wild type PDGF beta-receptor; in the case of the Y1009F/Y1021F double mutant, no association or phosphorylation of PLC-gamma could be detected. These data show that tyrosine phosphorylation of PLC-gamma is dependent on autophosphorylation of the PDGF beta-receptor at Tyr1009 and Tyr1021.     

Modulation of the eukaryotic initiation factor 2 alpha-subunit kinase PERK by tyrosine phosphorylation

The endoplasmic reticulum (ER)-resident protein kinase PERK attenuates protein synthesis in response to ER stress through the phosphorylation of translation initiation factor eIF2alpha at serine 51. ER stress induces PERK autophosphorylation at several serine/threonine residues, a process that is required for kinase activation and phosphorylation of eIF2alpha. Herein, we demonstrate that PERK also possesses tyrosine kinase activity. Specifically, we show that PERK is capable of autophosphorylating on tyrosine residues in vitro and in vivo. We further show that tyrosine 615, which is embedded in a highly conserved region of the kinase domain of PERK, is essential for autocatalytic activity. That is, mutation of Tyr-615 to phenylalanine compromises the autophosphorylation capacity of PERK and the phosphorylation of eIF2alpha in vitro and in vivo. The Y615F mutation also impairs the ability of PERK to induce translation of ATF4. Immunoblot analyses with a phosphospecific antibody confirm the phosphorylation of PERK at Tyr-615 both in vitro and in vivo. Thus, our data classify PERK as a dual specificity kinase whose regulation by tyrosine phosphorylation contributes to its optimal activation in response to ER stress.     

Autophosphorylation of the PDGF receptor in the kinase insert region regulates interactions with cell proteins

We have identified two platelet-derived growth factor (PDGF)-dependent autophosphorylation sites in the beta subunit of the human PDGF receptor (PDGF-R). The major site of phosphorylation (Tyr-857) corresponds to the major autophosphorylation site in many other tyrosine kinases. Tyr-751, which lies within the kinase insert region, is a second in vivo site and the major in vitro site. Immunoprecipitates of wild-type PDGF-Rs prepared from PDGF-treated cells contained a phosphatidylinositol (PI) 3 kinase activity and three specific polypeptides as well as the PDGF-R. Mutation of Tyr-751 to Phe or Gly, or mutation of the catalytic domain to abolish kinase activity, blocked association of the PDGF-R with the PI kinase and the three proteins. These results suggest that autophosphorylation in the kinase insert region triggers the binding of the activated PDGF-R to specific cellular proteins, including a PI kinase whose activity is known to be stimulated by PDGF. Thus autophosphorylation may play a novel role in signal transduction via the PDGF-R.