A General Strategy for Discovery of Inhibitors and Activators of RING and U-box E3 Ligases with Ubiquitin Variants.

RING and U-box E3 ubiquitin ligases regulate diverse eukaryotic processes and have been implicated in numerous diseases, but targeting these enzymes remains a major challenge. We report the development of three ubiquitin variants (UbVs), each binding selectively to the RING or U-box domain of a distinct E3 ligase: monomeric UBE4B, phosphorylated active CBL, or dimeric XIAP. Structural and biochemical analyses revealed that UbVs specifically inhibited the activity of UBE4B or phosphorylated CBL by blocking the E2∼Ub binding site. Surprisingly, the UbV selective for dimeric XIAP formed a dimer to stimulate E3 activity by stabilizing the closed E2∼Ub conformation. We further verified the inhibitory and stimulatory functions of UbVs in cells. Our work provides a general strategy to inhibit or activate RING/U-box E3 ligases and provides a resource for the research community to modulate these enzymes.

Structure of UBE2K-Ub/E3/polyUb reveals mechanisms of K48-linked Ub chain extension.

Ubiquitin (Ub) chain types govern distinct biological processes. K48-linked polyUb chains target substrates for proteasomal degradation, but the mechanism of Ub chain synthesis remains elusive due to the transient nature of Ub handover. Here, we present the structure of a chemically trapped complex of the E2 UBE2K covalently linked to donor Ub and acceptor K48-linked di-Ub, primed for K48-linked Ub chain synthesis by a RING E3. The structure reveals the basis for acceptor Ub recognition by UBE2K active site residues and the C-terminal Ub-associated (UBA) domain, to impart K48-linked Ub specificity and catalysis. Furthermore, the structure unveils multiple Ub-binding surfaces on the UBA domain that allow distinct binding modes for K48- and K63-linked Ub chains. This multivalent Ub-binding feature serves to recruit UBE2K to ubiquitinated substrates to overcome weak acceptor Ub affinity and thereby promote chain elongation. These findings elucidate the mechanism of processive K48-linked polyUb chain formation by UBE2K.

Activation of a primed RING E3-E2-ubiquitin complex by non-covalent ubiquitin.

RING ubiquitin ligases (E3) recruit ubiquitin-conjugate enzymes (E2) charged with ubiquitin (Ub) to catalyze ubiquitination. Non-covalent Ub binding to the backside of certain E2s promotes processive polyUb formation, but the mechanism remains elusive. Here, we show that backside bound Ub (Ub(B)) enhances both RING-independent and RING-dependent UbcH5B-catalyzed donor Ub (Ub(D)) transfer, but with a more prominent effect in RING-dependent transfer. Ub(B) enhances RING E3s' affinities for UbcH5B-Ub, and RING E3-UbcH5B-Ub complex improves Ub(B)'s affinity for UbcH5B. A comparison of the crystal structures of a RING E3, RNF38, bound to UbcH5B-Ub in the absence and presence of Ub(B), together with molecular dynamics simulation and biochemical analyses, suggests Ub(B) restricts the flexibility of UbcH5B's α1 and α1ß1 loop. Ub(B) supports E3 function by stabilizing the RING E3-UbcH5B-Ub complex, thereby improving the catalytic efficiency of Ub transfer. Thus, Ub(B) serves as an allosteric activator of RING E3-mediated Ub transfer.

Structural insights into non-covalent ubiquitin activation of the cIAP1-UbcH5B∼ubiquitin complex.

Ubiquitin (Ub)-conjugating enzymes and Ub ligases control protein degradation and regulate many cellular processes in eukaryotes. Cellular inhibitor of apoptosis protein-1 (cIAP1) plays a central role in apoptosis and tumor necrosis factor signaling. It harbors a C-terminal RING domain that homodimerizes to recruit E2∼Ub (where ∼ denotes a thioester bond) complex to catalyze Ub transfer. Noncovalent Ub binding to the backside of the E2 Ub-conjugating enzyme UbcH5 has previously been shown to enhance RING domain activity, but the molecular basis for this enhancement is unclear. To investigate how dimeric cIAP1 RING activates E2∼Ub for Ub transfer and what role noncovalently bound Ub has in Ub transfer, here we determined the crystal structure of the cIAP1 RING dimer bound to both UbcH5B covalently linked to Ub (UbcH5B-Ub) and a noncovalent Ub to 1.7 Å resolution. The structure along with biochemical analyses revealed that the cIAP1 RING domain interacts with UbcH5B-Ub and thereby promotes the formation of a closed UbcH5B-Ub conformation that primes the thioester bond for Ub transfer. We observed that the noncovalent Ub binds to the backside of UbcH5B and abuts UbcH5B's α1ß1-loop, which, in turn, stabilizes the closed UbcH5B-Ub conformation. Our results disclose the mechanism by which cIAP1 RING dimer activates UbcH5B∼Ub and indicate that noncovalent Ub binding further stabilizes the cIAP1-UbcH5B∼Ub complex in the active conformation to stimulate Ub transfer.

Casitas B-lineage lymphoma linker helix mutations found in myeloproliferative neoplasms affect conformation.

BACKGROUND: Casitas B-lineage lymphoma (Cbl or c-Cbl) is a RING ubiquitin ligase that negatively regulates protein tyrosine kinase (PTK) signalling. Phosphorylation of a conserved residue (Tyr371) on the linker helix region (LHR) between the substrate-binding and RING domains is required to ubiquitinate PTKs, thereby flagging them for degradation. This conserved Tyr is a mutational hotspot in myeloproliferative neoplasms. Previous studies have revealed that select point mutations in Tyr371 can potentiate transformation in cells and mice but not all possible mutations do so. To trigger oncogenic potential, Cbl Tyr371 mutants must perturb the LHR-substrate-binding domain interaction and eliminate PTK ubiquitination. Although structures of native and pTyr371-Cbl are available, they do not reveal how Tyr371 mutations affect Cbl's conformation. Here, we investigate how Tyr371 mutations affect Cbl's conformation in solution and how this relates to Cbl's ability to potentiate transformation in cells. RESULTS: To explore how Tyr371 mutations affect Cbl's properties, we used surface plasmon resonance to measure Cbl mutant binding affinities for E2 conjugated with ubiquitin (E2-Ub), small angle X-ray scattering studies to investigate Cbl mutant conformation in solution and focus formation assays to assay Cbl mutant transformation potential in cells. Cbl Tyr371 mutants enhance E2-Ub binding and cause Cbl to adopt extended conformations in solution. LHR flexibility, RING domain accessibility and transformation potential are associated with the extent of LHR-substrate-binding domain perturbation affected by the chemical nature of the mutation. More disruptive mutants like Cbl Y371D or Y371S are more extended and the RING domain is more accessible, whereas Cbl Y371F mimics native Cbl in solution. Correspondingly, the only Tyr371 mutants that potentiate transformation in cells are those that perturb the LHR-substrate-binding domain interaction. CONCLUSIONS: c-Cbl's LHR mutations are only oncogenic when they disrupt the native state and fail to ubiquitinate PTKs. These findings provide new insights into how LHR mutations deregulate c-Cbl.

E3 ligase-inactivation rewires CBL interactome to elicit oncogenesis by hijacking RTK-CBL-CIN85 axis.

Casitas B-lineage lymphoma (CBL) is a ubiquitin ligase (E3) that becomes activated upon Tyr371-phosphorylation and targets receptor protein tyrosine kinases for ubiquitin-mediated degradation. Deregulation of CBL and its E3 activity is observed in myeloproliferative neoplasms and other cancers, including breast, colon, and prostate cancer. Here, we explore the oncogenic mechanism of E3-inactive CBL mutants identified in myeloproliferative neoplasms. We show that these mutants bind strongly to CIN85 under normal growth conditions and alter the CBL interactome. Lack of E3 activity deregulates CIN85 endosomal trafficking, leading to an altered transcriptome that amplifies signaling events to promote oncogenesis. Disruption of CBL mutant interactions with EGFR or CIN85 reduces oncogenic transformation. Given the importance of the CBL-CIN85 interaction in breast cancers, we examined the expression levels of CIN85, CBL, and the status of Tyr371-phosphorylated CBL (pCBL) in human breast cancer tissue microarrays. Interestingly, pCBL shows an inverse correlation with both CIN85 and CBL, suggesting that high expression of inactivated CBL could coordinate with CIN85 for breast cancer progression. Inhibition of the CBL-CIN85 interaction with a proline-rich peptide of CBL that binds CIN85 reduced the proliferation of MDA-MB-231 cells. Together, these results provide a rationale for exploring the potential of targeting the EGFR-CBL-CIN85 axis in CBL-inactivated mutant cancers.

DELTEX2 C-terminal domain recognizes and recruits ADP-ribosylated proteins for ubiquitination.

Cross-talk between ubiquitination and ADP-ribosylation regulates spatiotemporal recruitment of key players in many signaling pathways. The DELTEX family ubiquitin ligases (DTX1 to DTX4 and DTX3L) are characterized by a RING domain followed by a C-terminal domain (DTC) of hitherto unknown function. Here, we use two label-free mass spectrometry techniques to investigate the interactome and ubiquitinated substrates of human DTX2 and identify a large proportion of proteins associated with the DNA damage repair pathway. We show that DTX2-catalyzed ubiquitination of these interacting proteins requires PARP1/2-mediated ADP-ribosylation and depends on the DTC domain. Using a combination of structural, biochemical, and cell-based techniques, we show that the DTX2 DTC domain harbors an ADP-ribose-binding pocket and recruits poly-ADP-ribose (PAR)-modified proteins for ubiquitination. This PAR-binding property of DTC domain is conserved across the DELTEX family E3s. These findings uncover a new ADP-ribose-binding domain that facilitates PAR-dependent ubiquitination.

Structural insights into ADP-ribosylation of ubiquitin by Deltex family E3 ubiquitin ligases.

Cellular cross-talk between ubiquitination and other posttranslational modifications contributes to the regulation of numerous processes. One example is ADP-ribosylation of the carboxyl terminus of ubiquitin by the E3 DTX3L/ADP-ribosyltransferase PARP9 heterodimer, but the mechanism remains elusive. Here, we show that independently of PARP9, the conserved carboxyl-terminal RING and DTC (Deltex carboxyl-terminal) domains of DTX3L and other human Deltex proteins (DTX1 to DTX4) catalyze ADP-ribosylation of ubiquitin's Gly76 Structural studies reveal a hitherto unknown function of the DTC domain in binding NAD+ Deltex RING domain recruits E2 thioesterified with ubiquitin and juxtaposes it with NAD+ bound to the DTC domain to facilitate ADP-ribosylation of ubiquitin. This ubiquitin modification prevents its activation but is reversed by the linkage nonspecific deubiquitinases. Our study provides mechanistic insights into ADP-ribosylation of ubiquitin by Deltex E3s and will enable future studies directed at understanding the increasingly complex network of ubiquitin cross-talk.

Structural basis for DNA damage-induced phosphoregulation of MDM2 RING domain.

Phosphorylation of MDM2 by ATM upon DNA damage is an important mechanism for deregulating MDM2, thereby leading to p53 activation. ATM phosphorylates multiple residues near the RING domain of MDM2, but the underlying molecular basis for deregulation remains elusive. Here we show that Ser429 phosphorylation selectively enhances the ubiquitin ligase activity of MDM2 homodimer but not MDM2-MDMX heterodimer. A crystal structure of phospho-Ser429 (pS429)-MDM2 bound to E2-ubiquitin reveals a unique 310-helical feature present in MDM2 homodimer that allows pS429 to stabilize the closed E2-ubiquitin conformation and thereby enhancing ubiquitin transfer. In cells Ser429 phosphorylation increases MDM2 autoubiquitination and degradation upon DNA damage, whereas S429A substitution protects MDM2 from auto-degradation. Our results demonstrate that Ser429 phosphorylation serves as a switch to boost the activity of MDM2 homodimer and promote its self-destruction to enable rapid p53 stabilization and resolve a long-standing controversy surrounding MDM2 auto-degradation in response to DNA damage.

dDYRK2 and Minibrain interact with the chromatin remodelling factors SNR1 and TRX.

The DYRKs (dual specificity tyrosine phosphorylation-regulated kinases) are a conserved family of protein kinases that autophosphorylate a tyrosine residue in their activation loop by an intra-molecular mechanism and phosphorylate exogenous substrates on serine/threonine residues. Little is known about the identity of true substrates for DYRK family members and their binding partners. To address this question, we used full-length dDYRK2 (Drosophila DYRK2) as bait in a yeast two-hybrid screen of a Drosophila embryo cDNA library. Of 14 independent dDYRK2 interacting clones identified, three were derived from the chromatin remodelling factor, SNR1 (Snf5-related 1), and three from the essential chromatin component, TRX (trithorax). The association of dDYRK2 with SNR1 and TRX was confirmed by co-immunoprecipitation studies. Deletion analysis showed that the C-terminus of dDYRK2 modulated the interaction with SNR1 and TRX. DYRK family member MNB (Minibrain) was also found to co-precipitate with SNR1 and TRX, associations that did not require the C-terminus of the molecule. dDYRK2 and MNB were also found to phosphorylate SNR1 at Thr102 in vitro and in vivo. This phosphorylation required the highly conserved DH-box (DYRK homology box) of dDYRK2, whereas the DH-box was not essential for phosphorylation by MNB. This is the first instance of phosphorylation of SNR1 or any of its homologues and implicates the DYRK family of kinases with a role in chromatin remodelling.

Structural analysis of MDM2 RING separates degradation from regulation of p53 transcription activity.

MDM2-MDMX complexes bind the p53 tumor-suppressor protein, inhibiting p53's transcriptional activity and targeting p53 for proteasomal degradation. Inhibitors that disrupt binding between p53 and MDM2 efficiently activate a p53 response, but their use in the treatment of cancers that retain wild-type p53 may be limited by on-target toxicities due to p53 activation in normal tissue. Guided by a novel crystal structure of the MDM2-MDMX-E2(UbcH5B)-ubiquitin complex, we designed MDM2 mutants that prevent E2-ubiquitin binding without altering the RING-domain structure. These mutants lack MDM2's E3 activity but retain the ability to limit p53's transcriptional activity and allow cell proliferation. Cells expressing these mutants respond more quickly to cellular stress than cells expressing wild-type MDM2, but basal p53 control is maintained. Targeting the MDM2 E3-ligase activity could therefore widen the therapeutic window of p53 activation in tumors.

Mutational activation of BRAF confers sensitivity to transforming growth factor beta inhibitors in human cancer cells.

Recent data implicate elevated transforming growth factor-ß (TGFß) signalling in BRAF inhibitor drug-resistance mechanisms, but the potential for targeting TGFß signalling in cases of advanced melanoma has not been investigated. We show that mutant BRAFV600E confers an intrinsic dependence on TGFß/TGFß receptor 1 (TGFBR1) signalling for clonogenicity of murine melanocytes. Pharmacological inhibition of the TGFBR1 blocked the clonogenicity of human mutant BRAF melanoma cells through SMAD4-independent inhibition of mitosis, and also inhibited metastasis in xenografted zebrafish. When investigating the therapeutic potential of combining inhibitors of mutant BRAF and TGFBR1, we noted that unexpectedly, low-dose PLX-4720 (a vemurafenib analogue) promoted proliferation of drug-naïve melanoma cells. Pharmacological or pharmacogenetic inhibition of TGFBR1 blocked growth promotion and phosphorylation of SRC, which is frequently associated with vemurafenib-resistance mechanisms. Importantly, vemurafenib-resistant patient derived cells retained sensitivity to TGFBR1 inhibition, suggesting that TGFBR1 could be targeted therapeutically to combat the development of vemurafenib drug-resistance.

Down-regulation of MHC class I by bovine papillomavirus E5 oncoproteins.

The papillomavirus E5 protein is localized in the endoplasmic reticulum (ER) and Golgi apparatus (GA) of the host cell. Transformed bovine fibroblasts expressing bovine papillomavirus (BPV) E5 are highly vacuolated and have a much enlarged, distorted and fragmented GA. Major histocompatibility complex class I (MHC I) is processed and transported to the cell surface through the GA. Given the cellular localization of E5 in the GA and the morphologically abnormal GA, we investigated the expression of MHC I in cells transformed by E5 from BPV-1 and BPV-4. Two cell lines were used: bovine cells that also express E6, E7 and activated ras, and NIH3T3 cells that express only E5. In addition, PalF cells acutely infected with a recombinant retrovirus expressing E5 were also examined. In contrast to non-transformed normal cells, or transformed cells expressing other papillomavirus proteins, cells expressing E5 do not express MHC I on their surface, but retain it intracellularly, independently of the presence of other viral or cellular oncogenes, or of whether the cells are long-term transformants or acutely infected. We conclude that expression of E5 prevents expression of MHC I to the cell surface and causes its retention within the cell. In addition, lower amounts of total MHC I heavy chain and of heavy chain RNA are detected in E5-transformed cells than in control cells. As surface expression of another glycosylated membrane protein, the transferrin receptor, is not affected, it appears that E5 targets MHC I with at least a degree of specificity. In papillomavirus lesions this effect would have important implications for antigen presentation by, and immunosurveillance of, virally infected cells.

dDYRK2: a novel dual-specificity tyrosine-phosphorylation-regulated kinase in Drosophila.

Dual-specificity tyrosine-phosphorylation-regulated kinases (DYRKs) are an emerging family of protein kinases that have been identified in all eukaryotic organisms examined to date. DYRK family members are involved in regulating key developmental and cellular processes such as neurogenesis, cell proliferation, cytokinesis and cellular differentiation. Two distinct subgroups exist, nuclear and cytosolic. In Drosophila, the founding family member minibrain, whose human orthologue maps to the Down syndrome critical region, belongs to the nuclear subclass and affects post-embryonic neurogenesis. In the present paper, we report the isolation of dDYRK2, a cytosolic DYRK and the putative product of the smell-impaired smi35A gene. This is the second such kinase described in Drosophila, but the first to be characterized at the molecular and biochemical level. dDYRK2 is an 81 kDa dual-specificity kinase that autophosphorylates on tyrosine and serine/threonine residues, but appears to phosphorylate exogenous substrates only on serine/threonine residues. It contains a YXY motif in the activation loop of the kinase domain in the same location as the TXY motif in mitogen-activated protein kinases. dDYRK2 is tyrosine-phosphorylated in vivo, and mutational analysis reveals that the activation loop tyrosines are phosphorylated and are essential for kinase activity. Finally, dDYRK2 is active at all stages of fly development, with elevated levels observed during embryogenesis and pupation.

Essentiality of a non-RING element in priming donor ubiquitin for catalysis by a monomeric E3.

RING E3 ligases catalyze the transfer of ubiquitin (Ub) from E2 ubiquitin-conjugating enzyme thioesterified with Ub (E2~Ub) to substrate. For RING E3 dimers, the RING domain of one subunit and tail of the second cooperate to prime Ub, but how this is accomplished by monomeric RING E3s in the absence of a tail-like component is currently unknown. Here, we present a crystal structure of a monomeric RING E3, Tyr363-phosphorylated human CBL-B, bound to a stabilized Ub-linked E2, revealing a similar mechanism in activating E2~Ub. Both pTyr363 and the pTyr363-induced element interact directly with Ub's Ile36 surface, improving the catalytic efficiency of Ub transfer by ~200-fold. Hence, interactions outside the canonical RING domain are crucial for optimizing Ub transfer in both monomeric and dimeric RING E3s. We propose that an additional non-RING Ub-priming element may be a common RING E3 feature.

BIRC7-E2 ubiquitin conjugate structure reveals the mechanism of ubiquitin transfer by a RING dimer.

Certain RING ubiquitin ligases (E3s) dimerize to facilitate ubiquitin (Ub) transfer from ubiquitin-conjugating enzyme (E2) to substrate, but structural evidence on how this process promotes Ub transfer is lacking. Here we report the structure of the human dimeric RING domain from BIRC7 in complex with the E2 UbcH5B covalently linked to Ub (UbcH5B∼Ub). The structure reveals extensive noncovalent donor Ub interactions with UbcH5B and both subunits of the RING domain dimer that stabilize the globular body and C-terminal tail of Ub. Mutations that disrupt these noncovalent interactions or RING dimerization reduce UbcH5B∼Ub binding affinity and ubiquitination activity. Moreover, NMR analyses demonstrate that BIRC7 binding to UbcH5B∼Ub induces peak-shift perturbations in the donor Ub consistent with the crystallographically-observed Ub interactions. Our results provide structural insights into how dimeric RING E3s recruit E2∼Ub and optimize the donor Ub configuration for transfer.

Deep evolutionary conservation of an intramolecular protein kinase activation mechanism.

DYRK-family kinases employ an intramolecular mechanism to autophosphorylate a critical tyrosine residue in the activation loop. Once phosphorylated, DYRKs lose tyrosine kinase activity and function as serine/threonine kinases. DYRKs have been characterized in organisms from yeast to human; however, all entities belong to the Unikont supergroup, only one of five eukaryotic supergroups. To assess the evolutionary age and conservation of the DYRK intramolecular kinase-activation mechanism, we surveyed 21 genomes representing four of the five eukaryotic supergroups for the presence of DYRKs. We also analyzed the activation mechanism of the sole DYRK (class 2 DYRK) present in Trypanosoma brucei (TbDYRK2), a member of the excavate supergroup and separated from Drosophila by ∼850 million years. Bioinformatics showed the DYRKs clustering into five known subfamilies, class 1, class 2, Yaks, HIPKs and Prp4s. Only class 2 DYRKs were present in all four supergroups. These diverse class 2 DYRKs also exhibited conservation of N-terminal NAPA regions located outside of the kinase domain, and were shown to have an essential role in activation loop autophosphorylation of Drosophila DmDYRK2. Class 2 TbDYRK2 required the activation loop tyrosine conserved in other DYRKs, the NAPA regions were critical for this autophosphorylation event, and the NAPA-regions of Trypanosoma and human DYRK2 complemented autophosphorylation by the kinase domain of DmDYRK2 in trans. Finally, sequential deletion analysis was used to further define the minimal region required for trans-complementation. Our analysis provides strong evidence that class 2 DYRKs were present in the primordial or root eukaryote, and suggest this subgroup may be the oldest, founding member of the DYRK family. The conservation of activation loop autophosphorylation demonstrates that kinase self-activation mechanisms are also primitive.

Structural basis for autoinhibition and phosphorylation-dependent activation of c-Cbl.

Cbls are RING ubiquitin ligases that attenuate receptor tyrosine kinase (RTK) signal transduction. Cbl ubiquitination activity is stimulated by phosphorylation of a linker helix region (LHR) tyrosine residue. To elucidate the mechanism of activation, we determined the structures of human CBL, a CBL-substrate peptide complex and a phosphorylated-Tyr371-CBL-E2-substrate peptide complex, and we compared them with the known structure of a CBL-E2-substrate peptide complex. Structural and biochemical analyses show that CBL adopts an autoinhibited RING conformation, where the RING's E2-binding surface associates with CBL to reduce E2 affinity. Tyr371 phosphorylation activates CBL by inducing LHR conformational changes that eliminate autoinhibition, flip the RING domain and E2 into proximity of the substrate-binding site and transform the RING domain into an enhanced E2-binding module. This activation is required for RTK ubiquitination. Our results present a mechanism for regulation of c-Cbl's activity by autoinhibition and phosphorylation-induced activation.

Characterization of a domain that transiently converts class 2 DYRKs into intramolecular tyrosine kinases.

Dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs) autophosphorylate an essential tyrosine residue in their activation loop and phosphorylate their substrates on serine and threonine residues. Phosphorylation of the activation loop tyrosine occurs intramolecularly, is mediated by a short-lived transitional intermediate during protein maturation, and is required for functional serine-threonine kinase activity of DYRKs. The DYRK family is separated into two subclasses. Through bioinformatics and mutational analyses, we identified a conserved domain in the noncatalytic N terminus of a class 2 DYRK that was required for autophosphorylation of the activation loop tyrosine but not for the phosphorylation of serine or threonine residues in substrates. We propose that this domain, which we term the NAPA domain, provides a chaperone-like function that transiently converts class 2 DYRKs into intramolecular kinases capable of autophosphorylating the activation loop tyrosine. The conservation of the NAPA domain from trypanosomes to humans indicates that this form of intramolecular phosphorylation of the activation loop is ancient and may represent a primordial mechanism for the activation of protein kinases.

A chaperone-dependent GSK3beta transitional intermediate mediates activation-loop autophosphorylation.

Glycogen synthase kinase 3 (GSK3), a key component of the insulin and wnt signaling pathways, is unusual, as it is constitutively active and is inhibited in response to upstream signals. Kinase activity is thought to be increased by intramolecular phosphorylation of a tyrosine in the activation loop (Y216 in GSK3beta), whose timing and mechanism is undefined. We show that GSK3beta autophosphorylates Y216 as a chaperone-dependent transitional intermediate possessing intramolecular tyrosine kinase activity and displaying different sensitivity to small-molecule inhibitors compared to mature GSK3beta. After autophosphorylation, mature GSK3beta is then an intermolecular serine/threonine kinase no longer requiring a chaperone. This shows that autoactivating kinases have adopted different molecular mechanisms for autophosphorylation; and for kinases such as GSK3, inhibitors that affect only the transitional intermediate would be missed in conventional drug screens.