Zhi-Xin Wang

Structural insight into the autoinhibition mechanism of AMP-activated protein kinase

The AMP-activated protein kinase (AMPK) is characterized by its ability to bind to AMP, which enables it to adjust enzymatic activity by sensing the cellular energy status and maintain the balance between ATP production and consumption in eukaryotic cells. It also has important roles in the regulation of cell growth and proliferation, and in the establishment and maintenance of cell polarity. These important functions have rendered AMPK an important drug target for obesity, type 2 diabetes and cancer treatments. However, the regulatory mechanism of AMPK activity by AMP binding remains unsolved. Here we report the crystal structures of an unphosphorylated fragment of the AMPK α-subunit (KD-AID) from Schizosaccharomyces pombe that contains both the catalytic kinase domain and an autoinhibitory domain (AID), and of a phosphorylated kinase domain from Saccharomyces cerevisiae (Snf1-pKD). The AID binds, from the ‘backside’, to the hinge region of its kinase domain, forming contacts with both amino-terminal and carboxy-terminal lobes. Structural analyses indicate that AID binding might constrain the mobility of helix αC, hence resulting in an autoinhibited KD-AID with much lower kinase activity than that of the kinase domain alone. AMP activates AMPK both allosterically and by inhibiting dephosphorylation. Further in vitro kinetic studies demonstrate that disruption of the KD-AID interface reverses the autoinhibition and these AMPK heterotrimeric mutants no longer respond to the change in AMP concentration. The structural and biochemical data have shown the primary mechanism of AMPK autoinhibition and suggest a conformational switch model for AMPK activation by AMP.

Conserved regulatory elements in AMPK

arising from B. Xiao et al. 472, 230–233 (2011)10.1038/nature09932The AMP-activated protein kinase (AMPK), an αβγ heterotrimeric enzyme, has a central role in regulating cellular metabolism and energy homeostasis. The α-subunit of AMPK possesses the catalytic kinase domain, followed by a regulatory region comprising the autoinhibitory domain (AID) and α-linker. Structural and biochemical studies suggested that AID is central to mammalian AMPK regulation; however, this notion has been challenged recently by Xiao et al. on the basis of their active AMPK structure (Protein Data Bank accession 2Y94). On close inspection, however, we found that the α-subunit regulatory region was incorrectly built in their model, and our rebuilt model suggests a universal occurrence of the AID domain in AMPKs; we have also identified a novel regulatory motif that is essential for AMPK regulation.

Enzymatic Activity and Substrate Specificity of Mitogen-activated Protein Kinase p38α in Different Phosphorylation States

The mitogen-activated protein (MAP) kinases are essential signaling molecules that mediate many cellular effects of growth factors, cytokines, and stress stimuli. Full activation of the MAP kinases requires dual phosphorylation of the Thr and Tyr residues in the TXY motif of the activation loop by MAP kinase kinases. Down-regulation of MAP kinase activity can be initiated by multiple serine/threonine phosphatases, tyrosine-specific phosphatases, and dual specificity phosphatases (MAP kinase phosphatases). This would inevitably lead to the formation of monophosphorylated MAP kinases. However, the biological functions of these monophosphorylated MAP kinases are currently not clear. In this study, we have prepared MAP kinase p38α, a member of the MAP kinase family, in all phosphorylated forms and characterized their biochemical properties. Our results indicated the following: (i) p38α phosphorylated at both Thr-180 and Tyr-182 was 10–20-fold more active than p38α phosphorylated at Thr-180 only, whereas p38α phosphorylated at Tyr-182 alone was inactive; (ii) the dual-specific MKP5, the tyrosine-specific hematopoietic protein-tyrosine phosphatase, and the serine/threonine-specific PP2Cα are all highly specific for the dephosphorylation of p38α, and the dephosphorylation rates were significantly affected by different phosphorylated states of p38α; (iii) the N-terminal domain of MPK5 has no effect on enzyme catalysis, whereas deletion of the MAP kinase-binding domain in MKP5 leads to a 370-fold decrease in kcat/Km for the dephosphorylation of p38α. This study has thus revealed the quantitative contributions of phosphorylation of Thr, Tyr, or both to the activation of p38α and to the substrate specificity for various phosphatases.

Molecular Mechanism of the Negative Regulation of Smad1/5 Protein by Carboxyl Terminus of Hsc70-interacting Protein (CHIP)

The transforming growth factor-β (TGF-β) superfamily of ligands signals along two intracellular pathways, Smad2/3-mediated TGF-β/activin pathway and Smad1/5/8-mediated bone morphogenetic protein pathway. The C terminus of Hsc70-interacting protein (CHIP) serves as an E3 ubiquitin ligase to mediate the degradation of Smad proteins and many other signaling proteins. However, the molecular mechanism for CHIP-mediated down-regulation of TGF-β signaling remains unclear. Here we show that the extreme C-terminal sequence of Smad1 plays an indispensable role in its direct association with the tetratricopeptide repeat (TPR) domain of CHIP. Interestingly, Smad1 undergoes CHIP-mediated polyubiquitination in the absence of molecular chaperones, and phosphorylation of the C-terminal SXS motif of Smad1 enhances the interaction and ubiquitination. We also found that CHIP preferentially binds to Smad1/5 and specifically disrupts the core signaling complex of Smad1/5 and Smad4. We determined the crystal structures of CHIP-TPR in complex with the phosphorylated/pseudophosphorylated Smad1 peptides and with an Hsp70/Hsc70 C-terminal peptide. Structural analyses and subsequent biochemical studies revealed that the distinct CHIP binding affinities of Smad1/5 or Smad2/3 result from the nonconservative hydrophobic residues at R-Smad C termini. Unexpectedly, the C-terminal peptides from Smad1 and Hsp70/Hsc70 bind in the same groove of CHIP-TPR, and heat shock proteins compete with Smad1/5 for CHIP interaction and concomitantly suppress, rather than facilitate, CHIP-mediated Smad ubiquitination. Thus, we conclude that CHIP inhibits the signaling activities of Smad1/5 by recruiting Smad1/5 from the functional R-/Co-Smad complex and further promoting the ubiquitination/degradation of Smad1/5 in a chaperone-independent manner.

Mitogen-activated Protein Kinase (MAPK) Phosphatase 3-mediated Cross-talk between MAPKs ERK2 and p38α

MAPK phosphatase 3 (MKP3) is highly specific for ERK1/2 inactivation via dephosphorylation of both phosphotyrosine and phosphothreonine critical for enzymatic activation. Here, we show that MKP3 is able to effectively dephosphorylate the phosphotyrosine, but not phosphothreonine, in the activation loop of p38α in vitro and in intact cells. The catalytic constant of the MKP3 reaction for p38α is comparable with that for ERK2. Remarkably, MKP3, ERK2, and phosphorylated p38α can form a stable ternary complex in solution, and the phosphatase activity of MKP3 toward p38α substrate is allosterically regulated by ERK2-MKP3 interaction. This suggests that MKP3 not only controls the activities of ERK2 and p38α but also mediates cross-talk between these two MAPK pathways. The crystal structure of bisphosphorylated p38α has been determined at 2.1 Å resolution. Comparisons between the phosphorylated MAPK structures reveal the molecular basis of MKP3 substrate specificity.

Molecular Mechanism for Inhibition of a Critical Component in the Arabidopsis thaliana Abscisic Acid Signal Transduction Pathways, SnRK2.6, by Protein Phosphatase ABI1

Background: The antagonistic complex of SnRK2.6 and ABI1 regulates abscisic acid (ABA) signaling in plants. Results: Presented here are the structure of SnRK2.6 kinase domain, and biochemical and computational characterizations of the ABI1-SnRK2.6 complex. Conclusion: Our studies revealed the molecular basis for ABI1-mediated inhibition of SnRK2.6. Significance: The studies advanced our understanding of the downstream signal transduction of ABA through SnRK2s and protein phosphatase type 2Cs. Subclass III SnRK2s (SnRK2.6/2.3/2.2) are the key positive regulators of abscisic acid (ABA) signal transduction in Arabidopsis thaliana. The kinases, activated by ABA or osmotic stress, phosphorylate stress-related transcription factors and ion channels, which ultimately leads to the protection of plants from dehydration or high salinity. In the absence of stressors, SnRK2s are subject to negative regulation by group A protein phosphatase type 2Cs (PP2C), whereas the underlying molecular mechanism remains to be elucidated. Here we report the crystal structure of the kinase domain of SnRK2.6 at 2.6-Å resolution. Structure-guided biochemical analyses identified two distinct interfaces between SnRK2.6 and ABI1, a member of group A PP2Cs. Structural modeling suggested that the two interfaces lock SnRK2.6 and ABI1 in an orientation such that the activation loop of SnRK2.6 is posited to the catalytic site of ABI1 for dephosphorylation. These studies revealed the molecular basis for PP2Cs-mediated inhibition of SnRK2s and provided important insights into the downstream signal transduction of ABA.

Structural basis for the autoprocessing of zinc metalloproteases in the thermolysin family

Thermolysin-like proteases (TLPs), a large group of zinc metalloproteases, are synthesized as inactive precursors. TLPs with a long propeptide (∼200 residues) undergo maturation following autoprocessing through an elusive molecular mechanism. We report the first two crystal structures for the autoprocessed complexes of a typical TLP, MCP-02. In the autoprocessed complex, Ala205 shifts upward by 33 Å from the previously covalently linked residue, His204, indicating that, following autocleavage of the peptide bond between His204 and Ala205, a large conformational change from the zymogen to the autoprocessed complex occurs. The eight N-terminal residues (residues Ala205-Gly212) of the catalytic domain form a new β-strand, nestling into two other β-strands. Simultaneously, the apparent Tm of the autoprocessed complex increases 20u2009°C compared to that of the zymogen. The stepwise degradation of the propeptide begins with two sequential cuttings at Ser49-Val50 and Gly57-Leu58, which lead to the disassembly of the propeptide and the formation of mature MCP-02. Our findings give new insights into the molecular mechanism of TLP maturation.

Structural Insights into the Autoactivation Mechanism of p21-Activated Protein Kinase

p21-activated kinases (PAKs) play an important role in diverse cellular processes. Full activation of PAKs requires autophosphorylation of a critical threonine/serine located in the activation loop of the kinase domain. Here we report crystal structures of the phosphorylated and unphosphorylated PAK1 kinase domain. The phosphorylated PAK1 kinase domain has a conformation typical of all active protein kinases. Interestingly, the structure of the unphosphorylated PAK1 kinase domain reveals an unusual dimeric arrangement expected in an authentic enzyme-substrate complex, in which the activation loop of the putative substrate is projected into the active site of the enzyme. The enzyme is bound to AMP-PNP and has an active conformation, whereas the substrate is empty and adopts an inactive conformation. Thus, the structure of the asymmetric homodimer mimics a trans-autophosphorylation complex, and suggests that unphosphorylated PAK1 could dynamically adopt both the active and inactive conformations in solution.

A Distinct Interaction Mode Revealed by the Crystal Structure of the Kinase p38α with the MAPK Binding Domain of the Phosphatase MKP5

Identification of the mechanism through which the kinase p38α interacts with the phosphatase that inactivates it could lead to the development of more specific anti-inflammatory drugs. Docking the Kinase to the Phosphatase The mitogen-activated protein kinase (MAPK) p38α promotes inflammation in diseases such as psoriasis, rheumatoid arthritis, and chronic obstructive pulmonary disease. Drugs that decrease the activity of p38α have been developed to treat these diseases; however, these compounds tend to have side effects that limit their clinical utility because they target a site in p38α that is conserved in other kinases. Phosphorylated or active p38α is dephosphorylated by the MAPK phosphatase 5 (MKP5). Zhang et al. (see also the Perspective by Goldsmith) report the crystal structure of the p38α-binding domain of MKP5 with p38α and show that these two proteins dock in a way that is distinct from that seen for other MAPKs and their phosphatases. Thus, the unconventional interaction between p38α and MKP5 could lead to the development of drugs that specifically act on p38α. The mitogen-activated protein kinase (MAPK) cascades play a pivotal role in a myriad of cellular functions. The specificity and efficiency of MAPK signaling are controlled by docking interactions between MAPKs and their cognate proteins. Many MAPK-interacting partners, including substrates, MAPK kinases, phosphatases, and scaffolding proteins, have linear sequence motifs that mediate the interaction with the common docking site on MAPKs. We report the crystal structure of p38α in complex with the MAPK binding domain (KBD) from MAPK phosphatase 5 (MKP5) at 2.7 Å resolution. In contrast to the well-known docking mode, the KBD binds p38α in a bipartite manner, in which two distinct helical regions of KBD engage the p38α docking site, which is situated on the back of the p38α active site. We also determined the crystal structure of the KBD of MKP7, which closely resembles the MKP5 KBD, suggesting that the mechanism of molecular recognition by the KBD of MKP5 is conserved in the cytoplasmic p38- and c-Jun N-terminal kinase–specific MKP subgroup. This previously unknown binding mode provides new insights into how MAPKs interact with their binding partners to achieve functional specificity.

Structural insights into the negative regulation of BRI1 signaling by BRI1-interacting protein BKI1

Brassinosteroids (BRs) are essential steroid hormones that have crucial roles in plant growth and development. BRs are perceived by the cell-surface receptor-like kinase brassinosteroid insensitive 1 (BRI1). In the absence of BRs, the cytosolic kinase domain (KD) of BRI1 is inhibited by its auto-inhibitory carboxyl terminus, as well as by interacting with an inhibitor protein, BRI1 kinase inhibitor 1 (BKI1). How BR binding to the extracellular domain of BRI1 leads to activation of the KD and dissociation of BKI1 into the cytosol remains unclear. Here we report the crystal structure of BRI1 KD in complex with the interacting peptide derived from BKI1. We also provide biochemical evidence that BRI1-associated kinase 1 (BAK1) plays an essential role in initiating BR signaling. Steroid-dependent heterodimerization of BRI1 and BAK1 ectodomains brings their cytoplasmic KDs in the right orientation for competing with BKI1 and transphosphorylation.