Binding of phosphatidylinositol 3,4,5-trisphosphate to the pleckstrin homology domain of protein kinase B induces a conformational change

Protein kinase B (PKB/Akt) is a key regulator of cell growth, proliferation and metabolism. It possesses an N-terminal pleckstrin homology (PH) domain that interacts with equal affinity with the second messengers PtdIns(3,4,5)P3 and PtdIns(3,4)P2, generated through insulin and growth factor-mediated activation of phosphoinositide 3-kinase (PI3K). The binding of PKB to PtdIns(3,4,5)P3/PtdIns(3,4)P2 recruits PKB from the cytosol to the plasma membrane and is also thought to induce a conformational change that converts PKB into a substrate that can be activated by the phosphoinositide-dependent kinase 1 (PDK1). In this study we describe two high-resolution crystal structures of the PH domain of PKBalpha in a noncomplexed form and compare this to a new atomic resolution (0.98 A, where 1 A=0.1 nm) structure of the PH domain of PKBalpha complexed to Ins(1,3,4,5)P4, the head group of PtdIns(3,4,5)P3. Remarkably, in contrast to all other PH domains crystallized so far, our data suggest that binding of Ins(1,3,4,5)P4 to the PH domain of PKB, induces a large conformational change. This is characterized by marked changes in certain residues making up the phosphoinositide-binding site, formation of a short a-helix in variable loop 2, and a movement of variable loop 3 away from the lipid-binding site. Solution studies with CD also provided evidence of conformational changes taking place upon binding of Ins(1,3,4,5)P4 to the PH domain of PKB. Our data provides the first structural insight into the mechanism by which the interaction of PKB with PtdIns(3,4,5)P3/PtdIns(3,4)P2 induces conformational changes that could enable PKB to be activated by PDK1.

Insulin-induced Drosophila S6 kinase activation requires phosphoinositide 3-kinase and protein kinase B

An important mechanism by which insulin regulates cell growth and protein synthesis is through activation of the p70 ribosomal S6 protein kinase (S6K). In mammalian cells, insulin-induced PI3K (phosphoinositide 3-kinase) activation, generates the lipid second messenger PtdIns(3,4,5) P (3), which is thought to play a key role in triggering the activation of S6K. Although the major components of the insulin-signalling pathway are conserved in Drosophila, recent studies suggested that S6K activation does not require PI3K in this system. To investigate further the role of dPI3K (Drosophila PI3K) in dS6K (Drosophila S6K) activation, we examined the effect of two structurally distinct PI3K inhibitors on insulin-induced dS6K activation in Kc167 and S2 Drosophila cell lines. We found that both inhibitors prevented insulin-stimulated phosphorylation and activation of dS6K. To investigate further the role of the dPI3K pathway in regulating dS6K activation, we also used dsRNAi (double-stranded RNA-mediated interference) to decrease expression of dPI3K and the PtdIns(3,4,5) P (3) phosphatase dPTEN ( Drosophila phosphatase and tensin homologue deleted on chromosome 10) in Kc167 and S2 cells. Knock-down of dPI3K prevented dS6K activation, whereas knock-down of dPTEN, which would be expected to increase PtdIns(3,4,5) P (3) levels, stimulated dS6K activity. Moreover, when the expression of the dPI3K target, dPKB (Drosophila protein kinase B), was decreased to undetectable levels, we found that insulin could no longer trigger dS6K activation. This observation provides the first direct demonstration that dPKB is required for insulin-stimulated dS6K activation. We also present evidence that the amino-acid-induced activation of dS6K in the absence of insulin, thought to be mediated by dTOR (Drosophila target of rapamycin), which is unaffected by the inhibition of dPI3K by wortmannin. The results of the present study support the view that, in Drosophila cells, dPI3K and dPKB, as well dTOR, are required for the activation of dS6K by insulin.

In vivo role of the PIF-binding docking site of PDK1 defined by knock-in mutation

PKB/Akt, S6K, SGK and RSK are mediators of responses triggered by insulin and growth factors and are activated following phosphorylation by 3-phosphoinositide-dependent protein kinase-1 (PDK1). To investigate the importance of a substrate-docking site in the kinase domain of PDK1 termed the 'PIF-pocket', we generated embryonic stem (ES) cells in which both copies of the PDK1 gene were altered by knock-in mutation to express a form of PDK1 retaining catalytic activity, in which the PIF-pocket site was disrupted. The knock-in ES cells were viable, mutant PDK1 was expressed at normal levels and insulin-like growth factor 1 induced normal activation of PKB and phosphorylation of the PKB substrates GSK3 and FKHR. In contrast, S6K, RSK and SGK were not activated, nor were physiological substrates of S6K and RSK phosphorylated. These experiments establish the importance of the PIF-pocket in governing the activation of S6K, RSK, SGK, but not PKB, in vivo. They also illustrate the power of knock-in technology to probe the physiological roles of docking interactions in regulating the specificity of signal transduction pathways.

Heat-shock protein 90 and Cdc37 interact with LKB1 and regulate its stability

LKB1 is a widely expressed serine/threonine protein kinase that is mutated in the inherited Peutz-Jeghers cancer syndrome. Recent findings indicate that LKB1 functions as a tumour suppressor, but little is known regarding the detailed mechanism by which LKB1 regulates cell growth. In this study we have purified LKB1 from cells and establish that it is associated with the heat-shock protein 90 (Hsp90) chaperone and the Cdc37 kinase-specific targetting subunit for Hsp90. We demonstrate that Cdc37 and Hsp90 bind specifically to the kinase domain of LKB1. We also perform experiments using Hsp90 inhibitors, which indicate that the association of Hsp90 and Cdc37 with LKB1 regulates LKB1 stability and prevents its degradation by the proteasome. Hsp90 inhibitors are being considered as potential anti-cancer agents. However, our observations indicate that prolonged usage of these drugs could possibly lead to tumour development by decreasing cellular levels of LKB1.

Ionizing radiation induces ataxia telangiectasia mutated kinase (ATM)-mediated phosphorylation of LKB1/STK11 at Thr-366

The serine/threonine protein kinase LKB1 functions as a tumour suppressor, and mutations in this enzyme lead to the inherited Peutz-Jeghers cancer syndrome. We previously found that LKB1 was phosphorylated at Thr-366 in vivo, a residue conserved in mammalian, Xenopus and Drosophila LKB1, located on a C-terminal non-catalytic moiety of the enzyme. Mutation of Thr-366 to Ala or Asp partially inhibited the ability of LKB1 to suppress growth of G361 melanoma cells, but did not affect LKB1 activity in vitro or LKB1 localization in vivo. As a first step in exploring the role of this phosphorylation further, we have generated a phosphospecific antibody specifically recognizing LKB1 phosphorylated at Thr-366 and demonstrate that exposure of cells to ionizing radiation (IR) induced a marked phosphorylation of LKB1 at Thr-366 in the nucleus. Thr-366 lies in an optimal phosphorylation motif for the phosphoinositide 3-kinase-like kinases DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated kinase (ATM) and ataxia telangiectasia-related kinase (ATR), which function as sensors for DNA damage in cells and mediate cellular responses to DNA damage. We demonstrate that both DNA-PK and ATM efficiently phosphorylate LKB1 at Thr-366 in vitro and provide evidence that ATM mediates this phosphorylation in vivo. This is based on the finding that LKB1 is not phosphorylated in a cell line lacking ATM in response to IR, and that agents which induce cellular responses via ATR in preference to ATM poorly induce phosphorylation of LKB1 at Thr-366. These observations provide the first link between ATM and LKB1 and suggest that ATM could regulate LKB1.

A phosphoserine/threonine-binding pocket in AGC kinases and PDK1 mediates activation by hydrophobic motif phosphorylation

The growth factor-activated AGC protein kinases RSK, S6K, PKB, MSK and SGK are activated by serine/threonine phosphorylation in the activation loop and in the hydrophobic motif, C-terminal to the kinase domain. In some of these kinases, phosphorylation of the hydrophobic motif creates a specific docking site that recruits and activates PDK1, which then phosphorylates the activation loop. Here, we discover a pocket in the kinase domain of PDK1 that recognizes the phosphoserine/phosphothreonine in the hydrophobic motif by identifying two oppositely positioned arginine and lysine residues that bind the phosphate. Moreover, we demonstrate that RSK2, S6K1, PKBalpha, MSK1 and SGK1 contain a similar phosphate-binding pocket, which they use for intramolecular interaction with their own phosphorylated hydrophobic motif. Molecular modelling and experimental data provide evidence for a common activation mechanism in which the phosphorylated hydrophobic motif and activation loop act on the alphaC-helix of the kinase structure to induce synergistic stimulation of catalytic activity. Sequence conservation suggests that this mechanism is a key feature in activation of >40 human AGC kinases.

High resolution crystal structure of the human PDK1 catalytic domain defines the regulatory phosphopeptide docking site

3-phosphoinositide dependent protein kinase-1 (PDK1) plays a key role in regulating signalling pathways by activating AGC kinases such as PKB/Akt and S6K. Here we describe the 2.0 A crystal structure of the PDK1 kinase domain in complex with ATP. The structure defines the hydrophobic pocket termed the "PIF-pocket", which plays a key role in mediating the interaction and phosphorylation of certain substrates such as S6K1. Phosphorylation of S6K1 at its C-terminal PIF-pocket-interacting motif promotes the binding of S6K1 with PDK1. In the PDK1 structure, this pocket is occupied by a crystallographic contact with another molecule of PDK1. Interestingly, close to the PIF-pocket in PDK1, there is an ordered sulfate ion, interacting tightly with four surrounding side chains. The roles of these residues were investigated through a combination of site-directed mutagenesis and kinetic studies, the results of which confirm that this region of PDK1 represents a phosphate-dependent docking site. We discuss the possibility that an analogous phosphate-binding regulatory motif may participate in the activation of other AGC kinases. Furthermore, the structure of PDK1 provides a scaffold for the design of specific PDK1 inhibitors.

Molecular basis for the substrate specificity of NIMA-related kinase-6 (NEK6). Evidence that NEK6 does not phosphorylate the hydrophobic motif of ribosomal S6 protein kinase and serum- and glucocorticoid-induced protein kinase in vivo

The AGC family of protein kinases, which includes isoforms of protein kinase B (also known as Akt), ribosomal S6 protein kinase (S6K), and serum- and glucocorticoid-induced protein kinase (SGK) are activated in response to many extracellular signals and play key roles in regulating diverse cellular processes. They are activated by the phosphorylation of the T loop of their kinase domain by the 3-phosphoinositide-dependent protein kinase-1 and by phosphorylation of a residue located C-terminal to the kinase domain in a region termed the hydrophobic motif. Recent work has implicated the NIMA (never in mitosis, gene A)-related kinase-6 (NEK6) as the enzyme that phosphorylates the hydrophobic motif of S6K1 in vivo. Here we demonstrate that in addition to phosphorylating S6K1 and SGK1 at their hydrophobic motif, NEK6 also phosphorylates S6K1 at two other sites and phosphorylates SGK1 at one other site in vitro. Employing the Jerini pepSTAR method in combination with kinetic analysis of phosphorylation of variant peptides, we establish the key substrate specificity determinants for NEK6. Our analysis indicates that NEK6 has a strong preference for Leu 3 residues N-terminal to the site of phosphorylation. Its mutation to either Ile or Val severely reduced the efficacy with which NEK6-phosphorylated peptide substrates, and moreover, mutation of the equivalent Leu residue in S6K1 or SGK1 prevented phosphorylation of their hydrophobic motifs by NEK6 in vitro. However, these mutants of S6K1 or SGK1 still became phosphorylated at their hydrophobic motif following insulin-like growth factor-1 stimulation of transfected 293 cells. This study provides the first description of the basis for the substrate specificity of NEK6 and indicates that NEK6 is unlikely to be responsible for the IGF1-induced phosphorylation of the hydrophobic motif of S6K, SGK, and protein kinase B isoforms in vivo.

High-resolution structure of the pleckstrin homology domain of protein kinase b/akt bound to phosphatidylinositol (3,4,5)-trisphosphate

The products of PI 3-kinase activation, PtdIns(3,4,5)P3 and its immediate breakdown product PtdIns(3,4)P2, trigger physiological processes, by interacting with proteins possessing pleckstrin homology (PH) domains. One of the best characterized PtdIns(3,4,5)P3/PtdIns(3,4)P2 effector proteins is protein kinase B (PKB), also known as Akt. PKB possesses a PH domain located at its N terminus, and this domain binds specifically to PtdIns(3,4,5)P3 and PtdIns(3,4)P2 with similar affinity. Following activation of PI 3-kinase, PKB is recruited to the plasma membrane by virtue of its interaction with PtdIns(3,4,5)P3/PtdIns(3,4)P2. PKB is then activated by the 3-phosphoinositide-dependent pro-tein kinase-1 (PDK1), which like PKB, possesses a PtdIns(3,4,5)P3/PtdIns(3,4)P2 binding PH domain. Here, we describe the high-resolution crystal structure of the isolated PH domain of PKB(alpha) in complex with the head group of PtdIns(3,4,5)P3. The head group has a significantly different orientation and location compared to other Ins(1,3,4,5)P4 binding PH domains. Mutagenesis of the basic residues that form ionic interactions with the D3 and D4 phosphate groups reduces or abolishes the ability of PKB to interact with PtdIns(3,4,5)P3 and PtdIns(3,4)P2. The D5 phosphate faces the solvent and forms no significant interactions with any residue on the PH domain, and this explains why PKB interacts with similar affinity with both PtdIns(3,4,5)P3 and PtdIns(3,4)P2.

ACK family tyrosine kinase activity is a component of Dcdc42 signaling during dorsal closure in Drosophila melanogaster

We have characterized Drosophila melanogaster ACK (DACK), one of two members of the ACK family of nonreceptor tyrosine kinases in Drosophila. The ACKs are likely effectors for the small GTPase Cdc42, but signaling by these proteins remains poorly defined. ACK family tyrosine kinase activity functions downstream of Drosophila Cdc42 during dorsal closure of the embryo, as overexpression of DACK can rescue the dorsal closure defects caused by dominant-negative Dcdc42. Similar to known participants in dorsal closure, DACK is enriched in the leading edge cells of the advancing epidermis, but it does not signal through activation of the Jun amino-terminal kinase cascade operating in these cells. Transcription of DACK is responsive to changes in Dcdc42 signaling specifically at the leading edge and in the amnioserosa, two tissues involved in dorsal closure. Unlike other members of the ACK family, DACK does not contain a conserved Cdc42-binding motif, and transcriptional regulation may be one route by which Dcdc42 can affect DACK function. Expression of wild-type and kinase-dead DACK transgenes in embryos, and in the developing wing and eye, reveals that ACK family tyrosine kinase activity is involved in a range of developmental events similar to that of Dcdc42.

Identification and characterization of four novel phosphorylation sites (Ser31, Ser325, Thr336 and Thr366) on LKB1/STK11, the protein kinase mutated in Peutz-Jeghers cancer syndrome

Peutz-Jeghers syndrome is an inherited cancer syndrome, which results in a greatly increased risk of developing tumours in those affected. The causative gene encodes a nuclear-localized protein kinase, termed LKB1, which is predicted to function as a tumour suppressor. The mechanism by which LKB1 is regulated in cells is not known, and nor have any of its physiological substrates been identified. Recent studies have demonstrated that LKB1 is phosphorylated in cells. As a first step towards identifying the roles that phosphorylation of LKB1 play, we have mapped the residues that are phosphorylated in human embryonic kidney (HEK)-293 cells, as well as the major in vitro autophosphorylation sites. We demonstrate that LKB1 expressed in HEK-293 cells, in addition to being phosphorylated at Ser(431), a previously characterized phosphorylation site, is also phosphorylated at Ser(31), Ser(325) and Thr(366). Incubation of wild-type LKB1, but not a catalytically inactive mutant, with manganese-ATP in vitro resulted in the phosphorylation of LKB1 at Thr(336) as well as at Thr(366). We were unable to detect autophosphorylation at Thr(189), a site previously claimed to be an LKB1 autophosphorylation site. A catalytically inactive mutant of LKB1 was phosphorylated at Ser(31) and Ser(325) in HEK-293 cells to the same extent as the wild-type enzyme, indicating that LKB1 does not phosphorylate itself at these residues. We show that phosphorylation of LKB1 does not directly affect its nuclear localization or its catalytic activity in vitro, but that its phosphorylation at Thr(336), and perhaps to a lesser extent at Thr(366), inhibits LKB1 from suppressing cell growth.

Evidence that the tandem-pleckstrin-homology-domain-containing protein TAPP1 interacts with Ptd(3,4)P2 and the multi-PDZ-domain-containing protein MUPP1 in vivo

PtdIns(3,4,5)P3 is an established second messenger of growth-factor and insulin-induced signalling pathways. There is increasing evidence that one of the immediate breakdown products of PtdIns(3,4,5)P3, namely PtdIns(3,4)P2, whose levels are elevated by numerous extracellular agonists, might also function as a signalling molecule. Recently, we identified two related pleckstrin-homology (PH)-domain-containing proteins, termed 'tandem-PH-domain-containing protein-1' (TAPP1) and TAPP2, which interacted in vitro with high affinity with PtdIns(3,4)P2, but did not bind PtdIns(3,4,5)P3 or other phosphoinositides. In the present study we demonstrate that stimulation of Swiss 3T3 or 293 cells with agonists that stimulate PtdIns(3,4)P2 production results in the marked translocation of TAPP1 to the plasma membrane. This recruitment is dependent on a functional PtdIns(3,4)P2-binding PH domain and is inhibited by wortmannin, a phosphoinositide 3-kinase inhibitor that prevents PtdIns(3,4)P2 generation. A search for proteins that interact with TAPP1 identified the multi-PDZ-containing protein termed 'MUPP1', a protein possessing 13 PDZ domains and no other known modular or catalytic domains [PDZ is postsynaptic density protein (PSD-95)/Drosophila disc large tumour suppressor (dlg)/tight junction protein (ZO1)]. We demonstrate that immunoprecipitation of endogenously expressed TAPP1 from 293-cell lysates results in the co-immunoprecipitation of endogenous MUPP1, indicating that these proteins are likely to interact with each other physiologically. We show that TAPP1 and TAPP2 interact with the 10th and 13th PDZ domain of MUPP1 through their C-terminal amino acids. The results of the present study suggest that TAPP1 and TAPP2 could function in cells as adapter proteins to recruit MUPP1, or other proteins that they may interact with, to the plasma membrane in response to signals that elevate PtdIns(3,4)P2.

Crystal structure of the phosphatidylinositol 3,4-bisphosphate-binding pleckstrin homology (PH) domain of tandem PH-domain-containing protein 1 (TAPP1): molecular basis of lipid specificity

Phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] and its immediate breakdown product PtdIns(3,4)P(2) function as second messengers in growth factor- and insulin-induced signalling pathways. One of the ways that these 3-phosphoinositides are known to regulate downstream signalling events is by attracting proteins that possess specific PtdIns-binding pleckstrin homology (PH) domains to the plasma membrane. Many of these proteins, such as protein kinase B, phosphoinositide-dependent kinase 1 and the dual adaptor for phosphotyrosine and 3-phosphoinositides (DAPP1) interact with both PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) with similar affinity. Recently, a new PH-domain-containing protein, termed tandem PH-domain-containing protein (TAPP) 1, was described which is the first protein reported to bind PtdIns(3,4)P(2) specifically. Here we describe the crystal structure of the PtdIns(3,4)P(2)-binding PH domain of TAPP1 at 1.4 A (1 A=0.1 nm) resolution in complex with an ordered citrate molecule. The structure is similar to the known structure of the PH domain of DAPP1 around the D-3 and D-4 inositol-phosphate-binding sites. However, a glycine residue adjacent to the D-5 inositol-phosphate-binding site in DAPP1 is substituted for a larger alanine residue in TAPP1, which also induces a conformational change in the neighbouring residues. We show that mutation of this glycine to alanine in DAPP1 converts DAPP1 into a TAPP1-like PH domain that only interacts with PtdIns(3,4)P(2), whereas the alanine to glycine mutation in TAPP1 permits the TAPP1 PH domain to interact with PtdIns(3,4,5)P(3).

The PIF-binding pocket in PDK1 is essential for activation of S6K and SGK, but not PKB

PKB/Akt, S6K1 and SGK are related protein kinases activated in a PI 3-kinase-dependent manner in response to insulin/growth factors signalling. Activation entails phosphorylation of these kinases at two residues, the T-loop and the hydrophobic motif. PDK1 activates S6K, SGK and PKB isoforms by phosphorylating these kinases at their T-loop. We demonstrate that a pocket in the kinase domain of PDK1, termed the 'PIF-binding pocket', plays a key role in mediating the interaction and phosphorylation of S6K1 and SGK1 at their T-loop motif by PDK1. Our data indicate that prior phosphorylation of S6K1 and SGK1 at their hydrophobic motif promotes their interaction with the PIF-binding pocket of PDK1 and their T-loop phosphorylation. Thus, the hydrophobic motif phosphorylation of S6K and SGK converts them into substrates that can be activated by PDK1. In contrast, the PIF-binding pocket of PDK1 is not required for the phosphorylation of PKBalpha by PDK1. The PIF-binding pocket represents a substrate recognition site on a protein kinase that is only required for the phosphorylation of a subset of its physiological substrates.

Phosphorylation of the protein kinase mutated in Peutz-Jeghers cancer syndrome, LKB1/STK11, at Ser431 by p90(RSK) and cAMP-dependent protein kinase, but not its farnesylation at Cys(433), is essential for LKB1 to suppress cell vrowth

Peutz-Jeghers syndrome is an inherited cancer syndrome that results in a greatly increased risk of developing tumors in those affected. The causative gene is a protein kinase termed LKB1, predicted to function as a tumor suppressor. The mechanism by which LKB1 is regulated in cells is not known. Here, we demonstrate that stimulation of Rat-2 or embryonic stem cells with activators of ERK1/2 or of cAMP-dependent protein kinase induced phosphorylation of endogenously expressed LKB1 at Ser(431). We present pharmacological and genetic evidence that p90(RSK) mediated this phosphorylation in response to agonists that activate ERK1/2 and that cAMP-dependent protein kinase mediated this phosphorylation in response to agonists that activate adenylate cyclase. Ser(431) of LKB1 lies adjacent to a putative prenylation motif, and we demonstrate that full-length LKB1 expressed in 293 cells was prenylated by addition of a farnesyl group to Cys(433). Our data suggest that phosphorylation of LKB1 at Ser(431) does not affect farnesylation and that farnesylation does not affect phosphorylation at Ser(431). Phosphorylation of LKB1 at Ser(431) did not alter the activity of LKB1 to phosphorylate itself or the tumor suppressor protein p53 or alter the amount of LKB1 associated with cell membranes. The reintroduction of wild-type LKB1 into a cancer cell line that lacks LKB1 suppressed growth, but mutants of LKB1 in which Ser(431) was mutated to Ala to prevent phosphorylation of LKB1 were ineffective in inhibiting growth. In contrast, a mutant of LKB1 that cannot be prenylated was still able to suppress the growth of cells.

Identification of pleckstrin-homology-domain-containing proteins with novel phosphoinositide-binding specificities

The second messenger phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] is generated by the action of phosphoinositide 3-kinase (PI 3-kinase), and regulates a plethora of cellular processes. An approach for dissecting the mechanisms by which these processes are regulated is to identify proteins that interact specifically with PtdIns(3,4,5)P(3). The pleckstrin homology (PH) domain has become recognized as the specialized module used by many proteins to interact with PtdIns(3,4,5)P(3). Recent work has led to the identification of a putative phosphatidylinositol 3,4,5-trisphosphate-binding motif (PPBM) at the N-terminal regions of PH domains that interact with this lipid. We have searched expressed sequence tag databases for novel proteins containing PH domains possessing a PPBM. Surprisingly, many of the PH domains that we identified do not bind PtdIns(3,4,5)P(3), but instead possess unexpected and novel phosphoinositide-binding specificities in vitro. These include proteins possessing PH domains that interact specifically with PtdIns(3,4)P(2) [TAPP1 (tandem PH-domain-containing protein-1) and TAPP2], PtdIns4P [FAPP1 (phosphatidylinositol-four-phosphate adaptor protein-1)], PtdIns3P [PEPP1 (phosphatidylinositol-three-phosphate-binding PH-domain protein-1) and AtPH1] and PtdIns(3,5)P(2) (centaurin-beta2). We have also identified two related homologues of PEPP1, termed PEPP2 and PEPP3, that may also interact with PtdIns3P. This study lays the foundation for future work to establish the phospholipid-binding specificities of these proteins in vivo, and their physiological role(s).

A 3-phosphoinositide-dependent protein kinase-1 (PDK1) docking site is required for the phosphorylation of protein kinase Czeta (PKCzeta ) and PKC-related kinase 2 by PDK1

Members of the AGC subfamily of protein kinases including protein kinase B, p70 S6 kinase, and protein kinase C (PKC) isoforms are activated and/or stabilized by phosphorylation of two residues, one that resides in the T-loop of the kinase domain and the other that is located C-terminal to the kinase domain in a region known as the hydrophobic motif. Atypical PKC isoforms, such as PKCzeta, and the PKC-related kinases, like PRK2, are also activated by phosphorylation of their T-loop site but, instead of possessing a phosphorylatable Ser/Thr in their hydrophobic motif, contain an acidic residue. The 3-phosphoinositide-dependent protein kinase (PDK1) activates many members of the AGC subfamily of kinases in vitro, including PKCzeta and PRK2 by phosphorylating the T-loop residue. In the present study we demonstrate that the hydrophobic motifs of PKCzeta and PKCiota, as well as PRK1 and PRK2, interact with the kinase domain of PDK1. Mutation of the conserved residues of the hydrophobic motif of full-length PKCzeta, full-length PRK2, or PRK2 lacking its N-terminal regulatory domain abolishes or significantly reduces the ability of these kinases to interact with PDK1 and to become phosphorylated at their T-loop sites in vivo. Furthermore, overexpression of the hydrophobic motif of PRK2 in cells prevents the T-loop phosphorylation and thus inhibits the activation of PRK2 and PKCzeta. These findings indicate that the hydrophobic motif of PRK2 and PKCzeta acts as a "docking site" enabling the recruitment of PDK1 to these substrates. This is essential for their phosphorylation by PDK1 in cells.

Identification of a pocket in the PDK1 kinase domain that interacts with PIF and the C-terminal residues of PKA

The 3-phosphoinositide-dependent protein kinase-1 (PDK1) phosphorylates and activates a number of protein kinases of the AGC subfamily. The kinase domain of PDK1 interacts with a region of protein kinase C-related kinase-2 (PRK2), termed the PDK1-interacting fragment (PIF), through a hydrophobic motif. Here we identify a hydrophobic pocket in the small lobe of the PDK1 kinase domain, separate from the ATP- and substrate-binding sites, that interacts with PIF. Mutation of residues predicted to form part of this hydrophobic pocket either abolished or significantly diminished the affinity of PDK1 for PIF. PIF increased the rate at which PDK1 phosphorylated a synthetic dodecapeptide (T308tide), corresponding to the sequences surrounding the PDK1 phosphorylation site of PKB. This peptide is a poor substrate for PDK1, but a peptide comprising T308tide fused to the PDK1-binding motif of PIF was a vastly superior substrate for PDK1. Our results suggest that the PIF-binding pocket on the kinase domain of PDK1 acts as a 'docking site', enabling it to interact with and enhance the phosphorylation of its substrates.

Characterization of the structure and regulation of two novel isoforms of serum- and glucocorticoid-induced protein kinase

The catalytic domain of serum- and glucocorticoid-induced protein kinase (SGK) is 54% identical with protein kinase B (PKB) and, like PKB, is activated in vitro by 3-phosphoinositide-dependent protein kinase-1 (PDK1) and in vivo in response to signals that activate phosphatidylinositol (PI) 3-kinase. Here we identify two novel isoforms of SGK, termed SGK2 and SGK3, whose catalytic domains share 80% amino acid sequence identity with each other and with SGK (renamed SGK1). Like SGK1, the mRNA encoding SGK3 is expressed in all tissues examined, but SGK2 mRNA is only present at significant levels in liver, kidney and pancreas and, at lower levels, in the brain. The levels of SGK2 mRNA in H4IIE cells and SGK3 mRNA in Rat2 fibroblasts are not increased by stimulation with serum or dexamethasone, whereas the level of SGK1 mRNA is increased greatly. SGK2 and SGK3 are activated in vitro by PDK1, albeit more slowly than SGK1, and their activation is accompanied by the phosphorylation of Thr(193) and Thr(253) respectively, the residues equivalent to the Thr in the 'activation loop' of PKB that is targeted by PDK1. The PDK1-catalysed phosphorylation and activation of SGK2 and SGK3, like SGK1, is greatly potentiated by mutating Ser(356) and Ser(419) respectively to Asp, these residues being equivalent to the C-terminal phosphorylation site of PKB. Like SGK1, SGK2 and SGK3 are activated 5-fold via a phosphorylation mechanism when cells are exposed to H(2)O(2) but, in contrast with SGK1, activation is only suppressed partially by inhibitors of PI 3-kinase. SGK2 and SGK3 are activated to a smaller extent by insulin-like growth factor-1 (2-fold) than SGK1 (5-fold). Like PKB and SGK1, SGK2 and SGK3 preferentially phosphorylate Ser and Thr residues that lie in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr motifs.

Heart 6-phosphofructo-2-kinase activation by insulin results from Ser-466 and Ser-483 phosphorylation and requires 3-phosphoinositide-dependent kinase-1, but not protein kinase B

Previous studies have shown that (i) the insulin-induced activation of heart 6-phosphofructo-2-kinase (PFK-2) is wortmannin-sensitive, but is insensitive to rapamycin, suggesting the involvement of phosphatidylinositol 3-kinase; and (ii) protein kinase B (PKB) activates PFK-2 in vitro by phosphorylating Ser-466 and Ser-483. In this work, we have studied the effects of phosphorylation of these residues on PFK-2 activity by replacing each or both residues with glutamate. Mutation of Ser-466 increased the V(max) of PFK-2, whereas mutation of Ser-483 decreased citrate inhibition. Mutation of both residues was required to decrease the K(m) for fructose 6-phosphate. We also studied the insulin-induced activation of heart PFK-2 in transfection experiments performed in human embryonic kidney 293 cells. Insulin activated transfected PFK-2 by phosphorylating Ser-466 and Ser-483. Kinase-dead (KD) PKB and KD 3-phosphoinositide-dependent kinase-1 (PDK-1) cotransfectants acted as dominant negatives because both prevented the insulin-induced activation of PKB as well as the inactivation of glycogen-synthase kinase-3, an established substrate of PKB. However, the insulin-induced activation of PFK-2 was prevented only by KD PDK-1, but not by KD PKB. These results indicate that the insulin-induced activation of heart PFK-2 is mediated by a PDK-1-activated protein kinase other than PKB.

Characterisation of a plant 3-phosphoinositide-dependent protein kinase-1 homologue which contains a pleckstrin homology domain

A plant homologue of mammalian 3-phosphoinositide-dependent protein kinase-1 (PDK1) has been identified in Arabidopsis and rice which displays 40% overall identity with human 3-phosphoinositide-dependent protein kinase-1. Like the mammalian 3-phosphoinositide-dependent protein kinase-1, Arabidopsis 3-phosphoinositide-dependent protein kinase-1 and rice 3-phosphoinositide-dependent protein kinase-1 possess a kinase domain at N-termini and a pleckstrin homology domain at their C-termini. Arabidopsis 3-phosphoinositide-dependent protein kinase-1 can rescue lethality in Saccharomyces cerevisiae caused by disruption of the genes encoding yeast 3-phosphoinositide-dependent protein kinase-1 homologues. Arabidopsis 3-phosphoinositide-dependent protein kinase-1 interacts via its pleckstrin homology domain with phosphatidic acid, PtdIns3P, PtdIns(3,4,5)P3 and PtdIns(3,4)P2 and to a lesser extent with PtdIns(4,5)P2 and PtdIns4P. Arabidopsis 3-phosphoinositide-dependent protein kinase-1 is able to activate human protein kinase B alpha (PKB/AKT) in the presence of PtdIns(3,4,5)P3. Arabidopsis 3-phosphoinositide-dependent protein kinase-1 is only the second plant protein reported to possess a pleckstrin homology domain and the first plant protein shown to bind 3-phosphoinositides.

PDK1 acquires PDK2 activity in the presence of a synthetic peptide derived from the carboxyl terminus of PRK2

BACKGROUND

Protein kinase B (PKB) is activated by phosphorylation of Thr308 and of Ser473. Thr308 is phosphorylated by the 3-phosphoinositide-dependent protein kinase-1 (PDK1) but the identity of the kinase that phosphorylates Ser473 (provisionally termed PDK2) is unknown.

RESULTS

The kinase domain of PDK1 interacts with a region of protein kinase C-related kinase-2 (PRK2), termed the PDK1-interacting fragment (PIF). PIF is situated carboxy-terminal to the kinase domain of PRK2, and contains a consensus motif for phosphorylation by PDK2 similar to that found in PKBalpha, except that the residue equivalent to Ser473 is aspartic acid. Mutation of any of the conserved residues in the PDK2 motif of PIF prevented interaction of PIF with PDK1. Remarkably, interaction of PDK1 with PIF, or with a synthetic peptide encompassing the PDK2 consensus sequence of PIF, converted PDK1 from an enzyme that could phosphorylate only Thr308 of PKBalpha to one that phosphorylates both Thr308 and Ser473 of PKBalpha in a manner dependent on phosphatidylinositol (3,4,5) trisphosphate (PtdIns(3,4,5)P3). Furthermore, the interaction of PIF with PDK1 converted the PDK1 from a form that is not directly activated by PtdIns(3,4,5)P3 to a form that is activated threefold by PtdIns(3,4,5)P3. We have partially purified a kinase from brain extract that phosphorylates Ser473 of PKBalpha in a PtdIns(3,4,5)P3-dependent manner and that is immunoprecipitated with PDK1 antibodies.

CONCLUSIONS

PDK1 and PDK2 might be the same enzyme, the substrate specificity and activity of PDK1 being regulated through its interaction with another protein(s). PRK2 is a probable substrate for PDK1.

Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1

3-Phosphoinositide-dependent protein kinase-1 (PDK1) interacts stereoselectively with the d-enantiomer of PtdIns(3,4,5)P3 (KD 1.6 nM) and PtdIns(3,4)P2 (KD 5.2 nM), but binds with lower affinity to PtdIns3P or PtdIns(4,5)P2. The binding of PtdIns(3,4,5)P3 to PDK1 was greatly decreased by making specific mutations in the pleckstrin homology (PH) domain of PDK1 or by deleting it. The same mutations also greatly decreased the rate at which PDK1 activated protein kinase Balpha (PKBalpha) in vitro in the presence of lipid vesicles containing PtdIns(3,4,5)P3, but did not affect the rate at which PDK1 activated a PKBalpha mutant lacking the PH domain in the absence of PtdIns(3,4,5)P3. When overexpressed in 293 or PAE cells, PDK1 was located at the plasma membrane and in the cytosol, but was excluded from the nucleus. Mutations that disrupted the interaction of PtdIns(3,4,5)P3 or PtdIns(4,5)P2 with PDK1 abolished the association of PDK1 with the plasma membrane. Growth-factor stimulation promoted the translocation of transfected PKBalpha to the plasma membrane, but had no effect on the subcellular distribution of PDK1 as judged by immunoelectron microscopy of fixed cells. This conclusion was also supported by confocal microscopy of green fluorescent protein-PDK1 in live cells. These results, together with previous observations, indicate that PtdIns(3,4,5)P3 plays several roles in the PDK1-induced activation of PKBalpha. First, it binds to the PH domain of PKB, altering its conformation so that it can be activated by PDK1. Secondly, interaction with PtdIns(3,4,5)P3 recruits PKB to the plasma membrane with which PDK1 is localized constitutively by virtue of its much stronger interaction with PtdIns(3,4,5)P3 or PtdIns(4,5)P2. Thirdly, the interaction of PDK1 with PtdIns(3,4,5)P3 facilitates the rate at which it can activate PKB.

Mitogen- and stress-activated protein kinase-1 (MSK1) is directly activated by MAPK and SAPK2/p38, and may mediate activation of CREB

We have identified a novel mitogen- and stress-activated protein kinase (MSK1) that contains two protein kinase domains in a single polypeptide. MSK1 is activated in vitro by MAPK2/ERK2 or SAPK2/p38. Endogenous MSK1 is activated in 293 cells by either growth factor/phorbol ester stimulation, or by exposure to UV radiation, and oxidative and chemical stress. The activation of MSK1 by growth factors/phorbol esters is prevented by PD 98059, which suppresses activation of the MAPK cascade, while the activation of MSK1 by stress stimuli is prevented by SB 203580, a specific inhibitor of SAPK2/p38. In HeLa, PC12 and SK-N-MC cells, PD 98059 and SB 203580 are both required to suppress the activation of MSK1 by TNF, NGF and FGF, respectively, because these agonists activate both the MAPK/ERK and SAPK2/p38 cascades. MSK1 is localized in the nucleus of unstimulated or stimulated cells, and phosphorylates CREB at Ser133 with a Km value far lower than PKA, MAPKAP-K1(p90Rsk) and MAPKAP-K2. The effects of SB 203580, PD 98059 and Ro 318220 on agonist-induced activation of CREB and ATF1 in four cell-lines mirror the effects of these inhibitors on MSK1 activation, and exclude a role for MAPKAP-K1 and MAPKAP-K2/3 in this process. These findings, together with other observations, suggest that MSK1 may mediate the growth-factor and stress-induced activation of CREB.

Activation of protein kinase B beta and gamma isoforms by insulin in vivo and by 3-phosphoinositide-dependent protein kinase-1 in vitro: comparison with protein kinase B alpha

The regulatory and catalytic properties of the three mammalian isoforms of protein kinase B (PKB) have been compared. All three isoforms (PKBalpha, PKBbeta and PKBgamma) were phosphorylated at similar rates and activated to similar extents by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Phosphorylation and activation of each enzyme required the presence of PtdIns(3,4,5)P3 or PtdIns(3,4)P2, as well as PDK1. The activation of PKBbeta and PKBgamma by PDK1 was accompanied by the phosphorylation of the residues equivalent to Thr308 in PKBalpha, namely Thr309 (PKBbeta) and Thr305 (PKBgamma). PKBgamma which had been activated by PDK1 possessed a substrate specificity identical with that of PKBalpha and PKBbeta towards a range of peptides. The activation of PKBgamma and its phosphorylation at Thr305 was triggered by insulin-like growth factor-1 in 293 cells. Stimulation of rat adipocytes or rat hepatocytes with insulin induced the activation of PKBalpha and PKBbeta with similar kinetics. After stimulation of adipocytes, the activity of PKBbeta was twice that of PKBalpha, but in hepatocytes PKBalpha activity was four-fold higher than PKBbeta. Insulin induced the activation of PKBalpha in rat skeletal muscle in vivo, with little activation of PKBbeta. Insulin did not induce PKBgamma activity in adipocytes, hepatocytes or skeletal muscle, but PKBgamma was the major isoform activated by insulin in rat L6 myotubes (a skeletal-muscle cell line).

3-Phosphoinositide-dependent protein kinase-1 (PDK1): structural and functional homology with the Drosophila DSTPK61 kinase

BACKGROUND

The activation of protein kinase B (PKB, also known as c-Akt) is stimulated by insulin or growth factors and results from its phosphorylation at Thr308 and Ser473. We recently identified a protein kinase, termed PDK1, that phosphorylates PKB at Thr308 only in the presence of lipid vesicles containing phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P3) or phosphatidylinositol 3,4-bisphosphate (Ptdlns(3,4)P2).

RESULTS

We have cloned and sequenced human PDK1. The 556-residue monomeric enzyme comprises a catalytic domain that is most similar to the PKA, PKB and PKC subfamily of protein kinases and a carboxy-terminal pleckstrin homology (PH) domain. The PDK1 gene is located on human chromosome 16p13.3 and is expressed ubiquitously in human tissues. Human PDK1 is homologous to the Drosophila protein kinase DSTPK61, which has been implicated in the regulation of sex differentiation, oogenesis and spermatogenesis. Expressed PDK1 and DSTPK61 phosphorylated Thr308 of PKB alpha only in the presence of Ptdlns(3,4,5)P3 or Ptdlns(3,4)P2. Overexpression of PDK1 in 293 cells activated PKB alpha and potentiated the IGF1-induced phosphorylation of PKB alpha at Thr308. Experiments in which the PH domains of either PDK1 or PKB alpha were deleted indicated that the binding of Ptdlns(3,4,5)P3 or Ptdlns(3,4)P2 to PKB alpha is required for phosphorylation and activation by PDK1. IGF1 stimulation of 293 cells did not affect the activity or phosphorylation of PDK1.

CONCLUSIONS

PDK1 is likely to mediate the activation of PKB by insulin or growth factors. DSTPK61 is a Drosophila homologue of PDK1. The effect of Ptdlns(3,4,5)P3/Ptdlns(3,4)P2 in the activation of PKB alpha is at least partly substrate directed.

A cis-acting region required for the regulated expression of grg-1, a Neurospora glucose-repressible gene. Two regulatory sites (CRE and NRS) are required to repress grg-1 expression

Grg-1 is a Neurospora gene which was identified as being a highly expressed glucose-repressible gene. A cis-acting regulatory region required for the regulated expression of grg-1 has been characterized. The regulatory region is found between 440 and 500 nucleotides upstream of the first major grg-1 start of transcription site and contains two distinct cis-acting regulatory elements. The upstream element consists of the sequence GTGACGTCAC, which is identical to the previously identified CRE (Cyclic AMP-Responsive Element). The second element is a newly defined cis-acting regulatory site. The element has the sequence TTGCTAGCAA and has been named NRS (Neurospora Repressor Site). DNA binding proteins can be shown to bind to both of these cis-acting regulatory elements. Experiments in which these sites were deleted demonstrate that both cis-acting regulatory elements are required to turn off the in vitro expression of the grg-1 gene under conditions of glucose sufficiency.

Rice type I phytochrome regulates hypocotyl elongation in transgenic tobacco seedlings

We have examined the biological activity of rice type I phytochrome (PI) in transgenic tobacco seedlings. The progeny of four independent transformants that expressed the rice PI gene segregated 3:1 for shorter hypocotyl length under dim white light (0.04 W/m2). By contrast, this phenotype was not observed either in the dark or under white light at higher intensity (6.0 W/m2). This suggests that the phenotype is dependent not only on light but also on light intensity. The increased light sensitivity cosegregated with the kanamycin-resistance marker as well as with the rice PI polypeptides, indicating that this phenotype is directly related to the expression of the transgene. The transgenic plants showing short hypocotyls exhibited a reduced growth rate throughout the elongation period, and the resulting shorter hypocotyl length was attributable to shorter epidermal cell length but not to reduced cell number. Furthermore, successive pulse irradiations with red light elicited short hypocotyls similar to those obtained under dim white light, and the effect was reversed by immediate far-red light treatment, providing a direct indication that the phenotype is caused by biologically active rice PI. Therefore, the far-red-absorbing form of the introduced rice PI appears to regulate the hypocotyl length of the transgenic tobacco plants through endogenous signal-transduction pathways. This assay system will be a powerful tool for testing the biological activity of introduced phytochrome molecules.

Rice Phytochrome Is Biologically Active in Transgenic Tobacco

To investigate the mechanisms of phytochrome action in vivo, we have overexpressed rice phytochrome in transgenic tobacco plants. A full-length rice phytochrome cDNA was fused to the cauliflower mosaic virus 35S promoter and transferred to tobacco. The progeny of some of the transgenic plants contain large amounts of rice phytochrome mRNA in green leaves. Extracts prepared from overexpressing plants contain twofold to fivefold more spectrophotometrically detectable phytochrome than extracts from control plants. Species-specific, anti-phytochrome monoclonal antibodies were used in immunoblots to discriminate between rice and tobacco phytochrome apoproteins in fractions eluted from a DEAE-Sepharose column. Red minus far-red difference spectra of the partially purified rice phytochrome from the transgenic plants indicate that the rice phytochrome assembles with chromophore and is photoreversible. Analysis of the circadian pattern of Cab mRNA levels in transgenic plants versus controls demonstrates that the overproduction of rice phytochrome extends the duration of the free-running rhythm of Cab gene expression. The rice phytochrome is, therefore, biologically active in the transgenic tobacco plant, which establishes a system for in vivo functional analysis of phytochrome.

Dominant expression of a gene amplification-related herbicide resistance in medicago cell hybrids

Protoplasts from phosphinotricin resistant M. sativa and M. varia cell lines carrying an amplified glutamine synthethase gene were fused with leaf protoplasts of kanamycin resistant M. varia transformants. The dominant nature of both PPT and kanamycin resistant traits was shown by the double resistant phenotype of the intra- and interspecific cell hybrids obtained. The presence of amplified GS gene in the hybrid genomes and the expression of chimeric neomycin phosphotransferase II gene was detected. The highly embryogenic character of the M. varia parent was not expressed after cell fusion. All hybrid cell lines with the double resistant phenotype showed non-morphogenic growth similarly to the PPT resistant parent. The possible role of GS gene amplification and other factors in the dominant behaviour of unorganized cell growth in alfalfa somatic hybrids is discussed.

Transformation of Medicago by Agrobacterium mediated gene transfer

Shoot segments of Medicago varia genotype A2 were co-cultivated with Agrobacterium tumefaciens strain bo42 carrying pGA471, a plasmid coding for the kanamycin resistant determinant as transferable positive selection marker in plant cells (An et al., 1985). Resistant plants were regenerated at high frequency from green calli developed on inoculated stem cuttings under kanamycin selection. DNA-DNA hybridization analysis showed the presence of the structural gene of the kanamycin resistant determinant in total DNA isolated from several independent transformants. All data presented clearly demonstrate the transfer, stable maintenance and functional expression of the kanamycin resistance marker in Medicago varia cells which retain their morphogenic property.