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Previous Article | Next Article 
Molecular and Cellular Biology, May 2007, p. 3542-3555, Vol. 27, No. 9
0270-7306/07/$08.00+0 doi:10.1128/MCB.01595-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Irena Dornreiter,2 and
Ted R. Hupp1*
University of Edinburgh, Cancer Research Centre, CRUK p53 Signal Transduction Group, South Crewe Road, Edinburgh, United Kingdom EH4 2XR,1 Heinrich-Pette-Institut fur Experimentelle Virologie und Immunologie, Universitat Hamburg, Hamburg, Germany2
Received 26 August 2006/ Returned for modification 16 October 2006/ Accepted 31 January 2007
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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p53 spliced variant lacking the BOX-V motif is refractory to Ser20 phosphorylation by CHK1. These data indicate that the BOX-V motif of p53 has evolved the capacity to bind to enzymes that mediate either p53 phosphorylation or ubiquitination, thus controlling the specific activity of p53 as a transcription factor. |
INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The N-terminal conserved BOX-I domain of p53 encompasses approximately 15 amino acids and has evolved as a multiprotein binding domain resulting in an interaction with a set of acetyltransferases, ubiquitin ligases, and protein kinases. The BOX-I domain harbors the LXXLL-type transactivation domain containing a binding site for the cofactor p300 (27). The BOX-I domain also contains the FXXWXXXL consensus MDM2 binding site that is required for MDM2-mediated inhibition of p53. DNA damage-activated protein kinases like CHK1/2 modify the BOX-I domain of p53 at Thr18 and Ser20 (46) by an allosteric mechanism (10). Phosphorylation of p53 at Thr18 and Ser20 can in turn differentially modulate the binding of the coactivator p300 or the inhibitor MDM2. Phosphorylation at Thr18 has the most striking effect on blocking MDM2 binding (13), while phosphorylation at Ser20 has no effect on MDM2 binding (45). Rather, phosphorylation at Ser20 creates a p300-phospho-consensus LXXLL binding site in the activation domain of p53 (16), within a novel phospho-LXXLL peptide-binding module in p300 (18, 21). Thus, these phosphorylation events can stimulate the p53 transcriptional response.
Mutation of Ser20 to Ala20 can reduce the specific activity of p53 in vitro (52) and in vivo, increasing cancer incidence in mice (36), suggesting that Ser20 phosphorylation is a key modifier of the p53 response in some cell types, including B cells. Consistent with this, phosphomimetic amino acid substitutions at Ser20 or Thr18 can increase the specific activity of p53 as a transcription factor (28). As such, identification of the stress-activated Ser20 kinases is an important goal in understanding the mechanisms underlying p53 activation as a tumor suppressor. The enzymes that modify p53 within the transactivation domain at Thr18 and Ser20 are beginning to be defined, although the data are relatively controversial. CHK2 and CHK1 were the original enzymes reported to modify both Thr18 and Ser20 (46), and CHK2 deletion reduces the specific activity of p53 in irradiated cells (35, 50). Furthermore, one study indicates that CHK2 phosphorylates p53 at only Thr18 and Ser20 in the activation domain (10), while another report has indicated that CHK2 modifies multiple sites in the C-terminal domain of p53 (41); thus, an understanding of where CHK2 modifies p53 is still unclear at a mechanistic level. Furthermore, additional studies indicate that neither CHK2 nor CHK1 may be a radiation-induced Ser20 kinase in vivo (2, 30); thus, it is not evident what the actual Ser20 kinase(s) for p53 protein might be.
The Ser20 site in the transactivation domain of p53 will presumably be the target of distinct stress-activated signaling pathways containing kinases recruited to phosphorylate and stimulate p53 transcriptional activity. In order to begin to identify the class of Ser20 kinases that exist for p53, we have set up a cell-free Ser20 kinase assay which demonstrated that a subset of the calcium calmodulin kinase superfamily can function as Ser20 kinases. These enzymes have the property of being activated in vitro by a docking site peptide derived from the BOX-V region in the DNA-binding domain of p53. This docking site motif overlaps with the more recently defined MDM2-binding site in the BOX-V domain of p53 that plays an additional role as a ubiquitination signal (47, 54). Furthermore, using an in vivo Ser20 kinase assay and CHK1 as a prototypical member of the calcium calmodulin kinase superfamily, we show how deletion of the region containing the BOX-V domain in the DNA-binding domain of p53 prevents Ser20 phosphorylation in vivo and that an enhanced green fluorescent protein (EGFP)-BOX-V fusion peptide can bind to and attenuate CHK1-mediated Ser20 phosphorylation in vivo. These data demonstrate a role for the conserved BOX-V domain docking site in mediating not only p53 ubiquitination, but also p53 phosphorylation. Furthermore, the data provide a framework from which to evaluate the role of a subset of calcium calmodulin kinase superfamily members as stress-induced p53 activators in vivo.
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MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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50 ng) was incubated with p53 substrate (150 ng) and with a 50 µM final concentration of peptides unless indicated otherwise. Reaction buffer contained 50 mM HEPES (pH 7.5), 10 mM MgCl2, 1 mM ATP, 1 mM dithiothreitol, and 0.8 mM EDTA with 0.1 mM ß-glycerol phosphate in a final volume of 20 µl. Reaction mixtures were incubated at 30°C for 30 min unless indicated otherwise. Reactions were terminated by addition of Laemmli sample buffer and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When radioactive kinase assays were set up, [
-32P]ATP incorporation into reaction products was visualized by autoradiography after electrophoresis.
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gene cloning was described previously (44). MDM2-mediated ubiquitin of p53 in vivo was described previously (54). Protein G beads were obtained from Pharmacia. DAPK-1, CHK1, CHK2, DAPK-3, and GST antibodies were obtained from Sigma, Santa Cruz, and BD Biosciences. GFP antibodies were obtained from Clontech. Monoclonal antibodies to phospho-BOX-I epitope peptides including FPS20, FPT18, and FPS15 (as used in Fig. 1 and 2) were developed by standard methods as described previously (12, 13). Immunoblotting for Ser20 or Thr18 site phosphorylation of p53 after kinase incubation at 30°C for 30 min was performed by immunoblotting with anti-phospho-site immunoglobulin G (IgG) at a concentration of 1 µg/ml essentially as described previously (10).
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RESULTS |
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These biochemical properties prompted us to evaluate whether key members of this kinase superfamily can function as p53-Ser20 kinases. The kinases evaluated include CHK2, CHK1, DAPK-1, DAPK-3, the DAPK-1/3 homologue DRAK-1, and AMPK. The kinase assay utilized phospho-specific monoclonal antibodies with validated specificity for phospho-sites in the p53 activation domain (Fig. 1C). When fragments of p53 harboring the activation domain alone were used as a kinase substrate, most of these enzymes exhibited little detectable activity towards p53 (data not shown). One exception was DAPK-1, which had detectable basal activity to the p53 transactivation domain fragment (as in Fig. 3G, lane 5 versus 1), which can be explained by the high degree of hydrophobic amino acid homology between p53 and the DAPK-1 consensus site peptides (Fig. 1D). The second exception was AMPK (Fig. 1F, lane 10 versus 1), which exhibits AMP-stimulated phosphorylation of p53 at Ser20 (Fig. 1F, lane 11 versus 10). The other enzymes do not have an intrinsic capacity to modify the minimal transactivation domain of p53 (Fig. 1F and G).
Although the information required for a protein kinase to phosphorylate a target is generally thought to occur within the primary phosphorylation site, growing evidence points to a second determinant of specificity involving enzyme docking (5). The role of small linear interaction motifs in driving protein-protein interactions may be more widespread than realized (38). In the case of p53, the protein kinases CHK2 and CHK1 have been shown in vitro to modify p53 at Ser20 by a docking-dependent interaction with the BOX-V domain of p53 (10). When DAPK-1, CHK2, CHK1, DRAK, and AMPK were assayed for p53 activation domain Ser20 kinase activity in the presence of the peptide fragment from the BOX-V domain of p53, significant stimulation of Ser20 kinase activity was detected (Fig. 1F, lanes 3, 5, 7, 9, and 12 versus lanes 2, 4, 6, 8, and 10, respectively). Furthermore, in a crude cell-free system, the same docking site peptide induced Ser20 site phosphorylation, highlighting an enzyme(s) in cell lysates with similar docking-dependent Ser20 kinase activity (Fig. 1F, lane 14 versus 13). We also examined DAPK-1, CHK2, CHK1, DRAK, and AMPK for p53 transactivation domain Thr18 kinase activity in the presence of the docking site peptide fragment from the BOX-V domain of p53. Of these enzymes, CHK1, DRAK-1, and AMPK exhibited the most pronounced Thr18 kinase activity in a docking-dependent manner (Fig. 1G, lanes 7, 9, and 12 versus lanes 6, 8, and 10, respectively). DAPK-1 did not exhibit same dual-site kinase activity (Fig. 1G, lane 3) and maintained an absolute specificity in targeting the Ser20 site of p53 (see Fig. 3).
Ser20 kinase activity from cell lysates coelutes with CHK1, CHK2, and/or DAPK-1. Together, these data affirm that members of the calcium calmodulin kinase superfamily have evolved a docking-dependent capacity to target the Ser20 and/or Thr18 site of p53, but whether these enzymes mediate this phosphorylation in cells is not clear. In order to first identify the major Ser20 kinases from cells, we had to develop a highly specific cell-free immunochemical assay that uses tetrameric p53 as a substrate. The identification of a cell line that produces high levels of Ser20-modified p53 in the absence of DNA damage would increase the likelihood of yielding a source of activity distinct from CHK2. Undamaged cycling A375 cells containing wild-type p53 produce an active pool of p53 that is constitutively modified at Ser20 (6) (11). Kinase activity in a cell-free system was assayed with native p53 tetramers as substrates using three phospho-specific monoclonal antibodies that bind to the phospho-Ser15, phospho-Thr18, or phospho-Ser20 p53 activation domain, respectively. Significant levels of Ser20 kinase activity can be detected in lysates from undamaged cells (Fig. 2A, lane 2 versus 1), consistent with the existence of an enzyme that can modify the Ser20 site constitutively in cycling A375 cells.
When p53 activity was measured using a reporter assay and compared to cell-free Ser20 kinase activity, a correlation was observed between p53 activity and Ser20 kinase function. Low levels of UV irradiation (1 to 2 J/m2) attenuated basal p53 activity (Fig. 2B) that correlated with loss of Ser20 kinase function (Fig. 2A, lane 3 versus 2). Progressively elevated levels of UV radiation that gave rise to increasing p53 activity also gave rise to progressively higher levels of cell-free Ser20 kinase activity (Fig. 2B versus A). This cell system was used to determine whether the major Ser20 kinase was coincident with or distinct from CHK1, CHK2, or DAPK-1. AMPK and DRAK-1 were not evaluated since there was no detectable Thr18 site kinase activity detectable in A375 cell lysates (Fig. 1G). Lysates from damaged and unirradiated cells were fractionated to determine whether the Ser20 kinase activity copurified with CHK1/2 or DAPK-1/3. The application of lysates from cycling A375 cells to an ion-exchange column results in the very broad elution of Ser20 kinase activity (Fig. 2C, fractions 12 to 20 and 21 to 26 [data not shown]). The bulk of the broadly eluting Ser20 kinase activity is separate from CHK1 (Fig. 2F, two peaks over fractions 5 to 13) and CHK2 (Fig. 2E, fractions 14 to 15). However, high levels of UV damage that results in progressively elevated levels of p53 activity (Fig. 2B) results in increasing levels of cell-free Ser20 kinase activity (Fig. 2A, lanes 4 to 6). This UV-inducible Ser20 kinase activity (Fig. 2G, lanes 9 to 11) copurifies with CHK2 (Fig. 2H, lanes 9 to 11), not CHK1, and is consistent with CHK2 being a major DNA damage-inducible p53-Ser20 kinase (46). Given the ability of recombinant DAPK-1 to function as Ser20 kinase, we next wanted to determine whether DAPK-1 protein coeluted with the Ser20 kinase activity from A375 cells. The total Ser20 kinase activity eluting from fractions 13 to 19 (Fig. 2C) is plotted as a function of the elution of DAPK-1 (Fig. 2D, fractions 15 to 19). DAPK-1 eluted as a broad peak distinct from CHK1 and CHK2, and the broad elution of DAPK-1 correlated with the broad elution of the Ser20 kinase activity, thus indicating that one possible identity of Ser20 kinase was DAPK-1. It has not been possible to purify the Ser20 kinase for identification from monolayer cycling A375 cells, as its abundance in hindsight was too low (data not shown; see Fig. 2J).
Additional cell lines were screened for those with higher levels of endogenous DAPK-1 protein to determine whether DAPK-1 elution correlated with cell-free Ser20 kinase activity. A set of cells with known targeting of the Ser20 site on p53 were evaluated, including A375, A549, and HCT116 cells (Fig. 2J). Of these, A549 cells exhibited the highest levels of DAPK-1 protein, and lysates were fractionated and analyzed for BOX-V-stimulated kinase activity. The majority of DAPK-1 protein (Fig. 2M) correlated with BOX-V stimulated kinase activity (Fig. 2P; broad elution from fractions 24 to 32) and was distinct from CHK2 (Fig. 2N, fraction 22) and DAPK-3 (Fig. 2O, fraction 30). Furthermore, the peak fraction of the Ser20 kinase activity (fraction 26) could be neutralized as a Ser20 kinase using an anti-DAPK-1 IgG, thus indicating there was a DAPK-1 component to the Ser20 kinase (Fig. 2R, lane 2 versus 1).
Recombinant DAPK-1 was characterized further in relation to the established Ser20 kinases, CHK1 and -2. The DAPK-1 purified from E. coli had an intrinsic in vitro Ser20 kinase activity (Fig. 3A, lane 4 versus 1). DAPK-1 has no intrinsic Thr18 or Ser15 kinase activity (Fig. 3B and C, lane 4), making it distinct from CHK2 or DRAK-1, which are dual-site kinases modifying both Ser20 and Thr18 (Fig. 3D and E). A kinase-dead form of DAPK-1 is not active as a Ser20 kinase (Fig. 3D, lane 3 versus lane 2). DAPK-3 was also examined in p53 kinase assays, and it also modifies only Ser20 and not Thr18 (Fig. 3A, lane 5), consistent with the 
-peptide consensus defined for DAPK-3, where the Ser20 amino acid equivalent plays a role in kinase recognition (Fig. 1E). As positive controls for the immunochemical kinase assay, DNA-PK has no detectable Ser20 kinase activity (Fig. 3A, lane 3 versus lane 1), but has relatively high levels of intrinsic Ser15- and Thr18-associated kinase activity (Fig. 3B and C, lane 3).
Similar to CHK2, the Ser20 kinase activity associated with DAPK-1 was not inhibited when the BOX-I phosphorylation site peptide was included in a Ser20 kinase assay containing tetrameric p53 as a substrate (Fig. 3E, lane 3 versus lane 2). The same BOX-I peptide can bind MDM2 (16, 54), indicating that the peptide is bioactive. The inability of the p53 transactivation domain peptide (BOX-I) to inhibit DAPK-1 suggests that DAPK-1 might have higher-affinity docking sites that mediate p53 tetramer phosphorylation. In fact, overlapping peptides from the entire coding region of p53 (Fig. 3I) were used to show that two major regions within the core domain of p53 have apparent contact sites for DAPK-1: BOX-II- and BOX-V-derived peptides (peptides 5 and 18) attenuated phosphorylation of p53 (Fig. 3E, lane 7 versus 2 and lane 22 versus 14; and Fig. 3F, peptides 5 and 18).
The highly conserved BOX-II, BOX-IV/V linker, and BOX-V domain peptides reside in the same plane that may form the basis for an anchoring site for DAPK-1 (Fig. 3H), as is the case for CHK1 and CHK2 (10). The addition in trans of the BOX-II, BOX-IV/V linker, and BOX-V domain peptides to CHK2 allosterically activates the enzyme towards a small transactivation domain fragment of p53 (p53N1-66; Fig. 3G, lanes 2 to 4 versus 1). DAPK-1 has a significantly higher basal kinase activity towards p53 than CHK2 (Fig. 3G, lane 5 versus 1), which is consistent with the fact that DAPK-1 consensus site peptides have more homology to p53 than the CHK2 consensus peptides (7, 53) (Fig. 1D). The inclusion of BOX-II, BOX-IV/V linker, and BOX-V peptides stimulated p53N1-66 substrate phosphorylation by DAPK-1 (Fig. 3G, lanes 6 to 8 versus 5). These biochemical characterizations have established that recombinant DAPK-1 is a bona fide Ser20 kinase and that its docking dependence towards p53 is similar to that of the established Ser20 kinase, CHK2. However, DAPK-1 is quite distinct in that it cannot function as a dual-site kinase like CHK1 or CHK2.
Evaluation of DAPK-1, CHK1, and CHK2 as p53-activating kinases in vivo. CHK1 and CHK2 enzymes have been validated previously in cells as p53-interacting kinases (46), and we next evaluated whether DAPK-1 similarly forms stable complexes with p53 in cells after transfection. The immunoprecipitation of DAPK-1 after transfection with the indicated DAPK-1 or p53 expression vectors shows that a relatively small proportion of transfected p53 protein is bound to endogenous DAPK-1 (Fig. 4A, lane 5 versus 1). In contrast, the immunoprecipitation of endogenous p53 after transfection with the DAPK-1 gene shows that a relatively large proportion of transfected DAPK-1 is bound to endogenous p53 (Fig. 4B, lane 3 versus 4). These data suggest that a relatively large pool of ectopically expressed DAPK-1 can interact stably with p53 in cells, while the majority of endogenous DAPK-1 in cells cannot be easily dissociated from its anchor to bind transfected p53. In order to examine whether this binding of DAPK-1 to p53 correlated with changes in p53 activity, endogenous p21 protein changes were evaluated after transfection with the indicated DAPK-1 and DAPK-3 genes into the HCT116 p53 wild-type and p53-null cell lines. DAPK-1 transfection (Fig. 4E, lanes 1 and 4) can elevate the basal levels of p21 protein in p53-containing HCT116 cells, relative to the isogenic p53-negative HCT116 cells (Fig. 4C, lane 4 versus 1). In contrast, DAPK-3 transfection (Fig. 4F, lanes 5 and 2) is unable to induce a similar p53-dependent induction of p21 (Fig. 4C, lane 5 versus 2). This is consistent with the p21-luc reporter template response (see Fig. 5K below) and indicates that p53-dependent induction of p21 is DAPK-1 specific.
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CHK1 transfection (from 1 to 5 µg of DNA) also gave rise to a dose-dependent increase in p53-dependent transcription (Fig. 5J). Similar to CHK1, transfection of DAPK-1 gave rise to elevated p53-dependent transcription (Fig. 5K), while the DAPK-1 core or full-length DAPK-1 kinase-dead expression vectors attenuated the basal p53-dependent transcription (Fig. 5L). In contrast, CHK2 transfection actually attenuated the basal Ser20 site phosphorylation (Fig. 5D, lane 6 versus 2), which is consistent with the dose-dependent attenuation of p53-dependent transcription by CHK2 (Fig. 5J). Furthermore, compared to DAPK-1 transfection, which was able to elevate basal Ser20 site phosphorylation (Fig. 5I, lanes 3 to 5 versus 1), DAPK-3 transfection also attenuated basal Ser20 site phosphorylation (Fig. 5I, lanes 6 to 8 versus 1). The latter data are also consistent with the ability of DAPK-3 to attenuate p53-dependent transcription function (Fig. 5K and 4C). Thus, by two distinct transfection-based assays (p53-dependent transcription reporter activity and Ser20 site phosphorylation), only CHK1 and DAPK-1 are able to positively affect the p53 pathway, while DAPK-3 and CHK2 attenuate p53 function.
Mutation of the Ser20 site to Ala20can attenuate p53 activity in apoptotic and transgene-based systems (36, 52), suggesting a role for phosphorylation in p53 regulation. However, in a transfection-based system, the mutation of the Ser20 residue to Asp20 only marginally elevated basal p53-dependent transcription (Fig. 5M). The kinase-dead form of DAPK-1 is unable to stimulate p53 (Fig. 5O), suggesting that the kinase activity is required to stimulate p53 function. Nevertheless, the Asp20 mutant p53 is further stimulated more strikingly by transfected DAPK-1 (Fig. 5N), suggesting that the transfection system reflects a greater contribution of kinase docking to p53 over kinase phosphorylation of p53. In fact, the Ala20 mutant version of p53 is stimulated as well as the wild-type p53 or Asp20 mutant p53 from the bax promoter (Fig. 5O), highlighting the importance of kinase docking compared to phosphorylation in this transfection assay. As such, we next evaluated whether deletion of the BOX-V region on p53 prevented kinase stimulation of p53 activity and associated Ser20 phosphorylation in cells.
Calcium calmodulin kinase interaction with p53 requires the BOX-V domain of p53.
A recent report (44) has highlighted the existence of a naturally occurring spliced form of p53 that has removed approximately 55 internal amino acids harboring the BOX-V docking region of p53 (Fig. 6A). The BOX-V docking site of p53 that is required for the calcium calmodulin kinase superfamily to modify the Ser20 site of p53 (Fig. 1) also contains the "ubiquitination signal" that is required for dual-site stimulation of the E3 ubiquitin ligase function of MDM2 (47, 54). The removal of this region within p53 in fact impairs MDM2-mediated ubiquitination of the transfected p53 isoform (Fig. 6B, lane 4 versus 3). However, the p53 protein is able to drive p53-dependent activity from the p21 reporter template (Fig. 6C), indicating that this BOX-V domain deletion does not simply unfold and inactivate p53. That this p53 spliced variant is active as a transcription factor is relatively surprising given that so many tumor-derived point mutations within this region of p53 can inactivate p53 function. Similar to the inability of p53 to be ubiquitinated by MDM2, p53 basal activity is also not stimulated by CHK1 relative to wild-type full-length p53 (Fig. 6C). Mutant p53 encoded by the HIS175 allele induced no activity from the p21 reporter, as expected (Fig. 6C). However, the p53 HIS175 protein is reactivated after CHK1 transfection (Fig. 6C). Under these conditions, the p53 HIS175 protein is hyperubiquitinated in vivo due to exposure of the BOX-V ubiquitination signal (48). The ability of CHK1 to drive the p53 HIS175 protein into an active state suggests that CHK1 and MDM2 can compete for binding to the BOX-V domain and drive p53 into either a ubiquitinated or transactivation-competent state.
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has no basal Ser20 site phosphorylation (Fig. 6F and H, lane 5 versus 3, and K and J, lane 2 versus 1). Transfected CHK1 and DAPK-1 are unable to induce Ser20 site phosphorylation on p53 (Fig. 6F and H, lane 6 versus 4). Controls measuring transfected tagged or untagged CHK1 protein levels are depicted in Fig. 6L to N. These data indicate that the BOX-V docking region of p53 is required for basal Ser20 site phosphorylation in vivo by an endogenous Ser20 site kinase, is required for both CHK1- and DAPK-1-mediated Ser20 phosphorylation in vivo, and is required for MDM2-mediated ubiquitination of p53. Thus, the BOX-V region appears to be a multiprotein docking site rivaling the N-terminal transactivation domain of p53, which also has both an MDM2 binding site, a p300 binding site, and the calcium calmodulin kinase superfamily phospho-acceptor site. Having established that CHK1 and DAPK-1 are the most potent inducers of the Ser20 site phosphorylation in H1299 cells and that this reaction is BOX-V docking dependent, we focused on determining which of the three enzymes under study—CHK2, CHK1, or DAPK-1—was the major genetic mediator of Ser20 phosphorylation of ectopically expressed p53 in H1299 cells. The use of siRNA to DAPK-1, CHK1, or CHK2 can reliably attenuate the levels of each kinase in A375 cells without affecting the levels of the others (Fig. 7A to C), and of these three manipulations, only DAPK-1 depletion reduced p53 protein levels (Fig. 7D). Using these siRNA oligonucleotides in H1299 cells, we found that only CHK1 protein depletion (Fig. 7G, lane 3 versus 1 and 2) gave rise to the most pronounced attenuation of Ser20 site phosphorylation and reduction in p53 protein levels of ectopically expressed p53 (Fig. 7E and F, lane 3 versus 1, 2, and 4). As a control, the depletion of CHK2 had no effect on the endogenous Ser20 site phosphorylation and p53 protein levels (Fig. 7E and F, lane 4 versus 1 to 3). Thus, endogenous CHK1 is required for the majority of Ser20 site phosphorylation of ectopically expressed p53 in H1299 cells. We evaluated whether a GFP-BOX-V fusion peptide could attenuate full-length p53 phosphorylation at Ser20. This would affirm that the Ser20 site kinase in cells is similar to CHK1 in that it binds to the BOX-V domain of p53.
H1299 cells were transfected with expression vectors for p53 and CHK1 along with either the EGFP control (Fig. 7H to K, lanes 2, 4, 6, and 8) or EGFP-BOX-V fusion peptide (Fig. 7H to K, lanes 3, 5, 7, and 9). This EGFP-BOX-V fusion peptide contains the ubiquitination signal for MDM2 and can inhibit MDM2-mediated ubiquitination of p53 in vitro and in cells (54). Lysates were evaluated for Ser20 site phosphorylation of p53 after transfection of the EGFP constructs (Fig. 7H), and the data demonstrate that the BOX-V peptide can attenuate the endogenous Ser20 site phosphorylation of p53 (Fig. 7H, lanes 3, 5, 7, and 9 versus 2, 4, 6, and 8), which is consistent with the inability of the p53 to function as a CHK1 substrate in cells. The p53 protein cannot be reactivated for Ser20 site phosphorylation in vivo using the GFP-BOX-V peptide fusion (Fig. 7N, lanes 6 versus 2 and 3), under conditions where full-length p53 Ser20 site phosphorylation is attenuated (Fig. 7N, lane 3 versus 2). The attenuation of p53 phosphorylation by the EGFP-BOX-V fusion peptide correlated with the stable binding of CHK1 to the EGFP-BOX-V fusion peptide in cell lysates (Fig. 7T, lane 4 versus 7R, lane 4). Together, these data indicate that the BOX-V region in p53 binds to members of the calcium calmodulin kinase superfamily and that this binding is required to stimulate p53 function. However, only CHK-1 and DAPK-1 exhibit a pronounced ability to mediate p53 protein stabilization and/or p53 activation in experimentally tractable systems.
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DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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CHK2 was the original biochemically and genetically defined Ser20 kinase for p53 (46, 50). Other protein kinases presumably have evolved to target the p53 transactivation domain and link distinct stresses to p53 activation. We have used bioinformatic and biochemical fractionation approaches to establish that members of the calcium calmodulin kinase superfamily have the common feature of being able to target the p53 activation domain at Thr18 and/or Ser20 and that they all also harbor a high-affinity interaction with the BOX-V domain of p53. The enzymes have included CHK1, CHK2, DAPK-1, DAPK-3, DRAK-1, and AMPK. Using this benchmark, we fractionated cell lysates to determine what the major Ser20 kinases are in a tumor cell used to study p53 and found that CHK1, CHK2, and DAPK-1 were the major Ser20 kinases. As such, we focused on the study of these three enzymes to determine, using transfection technologies, which of these might be the most potent mediator of Ser20 site phosphorylation of p53 in cells and also to determine whether the BOX-V docking site plays a dominant role in calcium calmodulin kinase-p53 interactions.
Five observations are notable: (i) CHK1 and DAPK-1 transfections into cells mediate the best stimulation of p53-dependent transcription reporter activity; (ii) CHK1 and DAPK-1 transfections into cells induce the most significant Ser20 site phosphorylation of p53; (iii) endogenous CHK1 is the most evident mediator of Ser20 site phosphorylation in H1299 cells, while endogenous DAPK-1 is the most effective p53 protein stabilizer in A375 cells, highlighting a possible cell-specific function for the Ser20 site kinases; (iv) CHK1 is attenuated as an inducer of Ser20 site phosphorylation by transfection of the EGFP-BOX-V peptide fusion protein; and (v) deletion of the BOX-V region of p53 prevents Ser20 site phosphorylation in vivo and prevents stimulation of p53 function by transfected kinase. Furthermore, CHK2 and DAPK-3 were distinct from CHK1 and DAPK-1 in that ectopic expression of the former two was unable to induce either Ser20 site phosphorylation or p53 transactivation. These data highlight the calcium calmodulin kinase superfamily as the most likely type of Ser20 site kinase, establish the BOX-V docking dependence in Ser20 site phosphorylation, and open the door to evaluate how distinct stresses might recruit different members of the calcium calmodulin kinase superfamily to mediate genetic activation of p53. For example, given that mutation of the Ser20-equivalent phospho-acceptor site in mice accelerates B-cell lymphoma, we are now examining in B cells which calcium calmodulin kinase superfamily plays the most dominant role in activating p53-dependent apoptosis in response to DNA damage. Such a model provides a framework for evaluating calcium calmodulin kinase superfamily members as stress-regulated activators of p53 in a cell-specific manner.
Apart from the ability of CHK1 to function as the most potent docking-dependent kinase for p53, we also report that DAPK-1 can stimulate Ser20 site phosphorylation of p53. DAPK-1 is defined as a tumor suppressor and was shown to be part of a p53-dependent cell death pathway that operates via p19ARF (42). No physiological cell cycle substrate that mediates DAPK-1-dependent activation of p53 is known, although it is involved in intriguing biological pathways, including autophagic vesicle formation (23). In our biological analysis of the DAPK family, we find that the p53-specific modification in vitro at Ser20 involves primarily DAPK-1/3 rather than DRAK-1 homologues. A p53 specificity for DAPK-1 was also observed in (i) p53-dependent reporter transcription assays; (ii) p21 protein stabilization; (iii) siRNA analysis, where p53 can be depleted upon DAPK-1 protein attenuation in A375 cells; and (iv) the inability of DAPK-3 to simulate the p53 response in vivo. Although CHK2 is held to be the radiation damage-induced p53 kinase, a recent report has shown that irradiation-induced p53 protein stabilization in vivo is attenuated in kinase domain mutants of DAPK-1 (32), suggesting that DAPK-1 can be a component of a genetic program to activate p53 in irradiated cells. It remains to be determined how DAPK-1 coordinately stimulates p53-dependent checkpoint control. The mechanism appears to be distinct from CHK2, since we have been unable to show that DAPK-1 can modify E2F-1 (data not shown); thus, DAPK-1 cannot replace CHK2 as a p53- and E2F-1-activating kinase pathway (49).
A high-resolution structure of the p53 tetramer has not been defined, but biochemical studies have shown that some of the functions of the tetramer are allosterically controlled. One pronounced allosteric effect of p53 protein is the DNA dependence in acetylation (17, 18), suggesting conformational changes mediate both binding stability of the acetyltransferase p300 and release of the acetylation motif from a cryptic state (8, 43). Kinase modification of p53 has evolved a docking-dependent interaction similar to that of the p300 acetyltransferase or the E3 ubiquitin ligase MDM2, though p300, MDM2, and protein kinases dock at distinct sites on p53: (i) P300 binds primarily at the BOX-I domain and the proline-repeat domain, (ii) MDM2 binds at the BOX-I and BOX-V domains, (iii) CDK2 modification at Ser315 requires a docking interaction within the tetramerization domain between residues 321 and 348 (20), (iv) PKC-mediated modification at Ser371 also requires distal docking motifs to mediate substrate phosphorylation that are distinct from the CDK2 interface (34), and (v) CK2 docking to the tetramerization domain of p53 tethers the kinase to the low-affinity phospho-acceptor site at Ser392 (34).
The BOX-V domain of p53 forms a docking site for the calcium calmodulin kinase superfamily, as described here, but MDM2 was the first BOX-V domain binding protein (47). The BOX-V region is now defined as a key ubiquitination signal for MDM2, which is driven by a dual-site docking mechanism by MDM2 on the p53 tetramer (54). The unfolding of BOX-V region in mutant p53 protein also enhances p53 ubiquitination in vivo (48). Thus, the BOX-V interface appears to be a conformationally flexible multiprotein binding domain that controls covalent modification of the p53 tetramer. Studies using nuclear magnetic resonance have confirmed that the binding of MDM2 to the core domain of p53 can induce chemical shifts within the BOX-V motif (56). There is potential, therefore, for cross talk through competition for the same BOX-V interface: CHK1/DAPK-1 and MDM2 could potentially antagonize each other through competition for the same multiprotein docking site. Although the exposure of this BOX-V region on mutant p53 can enhance MDM2-dependent ubiquitination of p53 in vitro and in cells (48), it is striking that CHK1 can restore partial transcriptional activity to the same mutant p53 protein in vivo (Fig. 7C). These data suggest CHK1 could overcome MDM2-mediated ubiquitination by competition for the same interface in the DNA-binding domain of p53. Furthermore, physiological deletion of this BOX-V region on p53, as exemplified in the p53 S-phase alternatively spliced variant (44), would provide a transcriptionally active form of p53 (as in Fig. 7C) that cannot be degraded well by MDM2 (Fig. 7B) and that cannot be modified by enzymes with the specificity of CHK1 (Fig. 7F). The p53 mutant may have evolved the capacity to evade these regulators during S phase, to alter cell-cycle-responsive promoters, and possibly to directly alter DNA replication (9). As an example of how different regulators of p53 may direct the protein to distinct compartments, in UV-irradiated human skin stem cells, Ser15 and Ser392 phosphorylation of p53 can occur in the same nucleus but within distinct regions of the nucleus (22), suggesting promoter-specific functions for these kinase modifications. Together, these data highlight the potential of the BOX-V domain of p53 to function as a calcium calmodulin kinase docking site, an MDM2 ubiquitination signal, and a region whose removal by splicing offers the cell a range of mechanisms to coordinate p53-dependent responses to distinct cellular stresses.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Published ahead of print on 5 March 2007.
Present address: CRUK Beatson Laboratories, Glasgow, Scotland.
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REFERENCES |
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