Tomas Obsil

Cyclin-dependent protein kinase 16 (CDK16) regulates both physiological and pathological processes, including autophagy, spermatogenesis and cancer. Unlike other CDKs, CDK16 is regulated by an unclear mechanism involving phosphorylated cyclin Y (CCNY) in complex with 14-3-3 proteins rather than CCNY alone. The present study aims at elucidating this mechanism by structurally characterizing CDK16 in complex with CCNY and 14-3-3 using several biophysical techniques. As shown by cryo-EM analysis and hydrogen/deuterium exchange coupled to mass spectrometry, 14-3-3 binding modulates the conformation of a key moiety of the CDK binding surface of CCNY, thereby enabling CDK16 activation. CDK16 interacts with the cyclin box of CCNY, while 14-3-3 provides additional contacts, including with the activation segment of CDK16. CDK16 activation also requires interactions of CCNY with the N-terminal extension of CDK16. These findings not only clarify the role of CCNY and 14-3-3 in CDK16 activation but also highlight the potential of targeting CDK16 protein-protein interactions for cancer therapy.

The Nedd4 subfamily of HECT E3 ligases is a ubiquitous group of 10 enzymes that share the same domain structure, consisting of a C2 domain, several WW domains and a catalytic HECT domain. Over the past decade, significant progress has been made in characterizing the molecular details of their activity and regulation. Studies have shown that, in the inactive state, the HECT domain is shielded by its N‐terminal domains, thereby blocking access to the active site. The catalytic functions of Nedd4 enzymes include accepting ubiquitin molecules from ubiquitin‐conjugating enzymes, transferring them to substrates, and generating diverse polyubiquitin chains. The modulation of Nedd4 enzyme activity involves mechanisms that facilitate enzymatic activation, relay binding to components of the enzymatic cascade, and enable (auto)ubiquitination. This minireview provides a comprehensive overview of the structural features distinguishing the inactive and active conformations in this group of E3 ligases, while underscoring the need for further research necessary to develop pharmaceutical solutions targeting pathological conditions rooted in Nedd4 dysfunction.

The transcription factors FOXO4 and p53 regulate aging, and their deregulation has been linked to several diseases, including cancer. Under stress conditions, cellular senescence is promoted by p53 sequestration and senescence-associated protein p21 transcriptional upregulation induced by interactions between the FOXO4 Forkhead DNA-binding domain and the p53 transactivation domain. However, the molecular details of these interactions remain unclear. Here, we report that these interactions between p53 and FOXO4 domains are highly heterogeneous. The p53 transactivation domain primarily interacts with the region formed by the N-terminal helical bundle of the FOXO4 Forkhead domain but retains a substantial degree of flexibility in the complex. In addition, NMR data-driven molecular simulations suggest that p53 interacts with FOXO4 through multiple binding modes. Overall, our findings not only provide the structural insights into interactions between FOXO4 and p53 but also highlight their potential as targets for developing senolytic compounds.

Nedd4-2 E3 ligase regulates Na⁺ homeostasis by ubiquitinating various channels and membrane transporters, including the epithelial sodium channel ENaC. In turn, Nedd4-2 dysregulation leads to various conditions, including electrolytic imbalance, respiratory distress, hypertension, and kidney diseases. However, Nedd4-2 regulation remains mostly unclear. The present study aims at elucidating Nedd4-2 regulation by structurally characterizing Nedd4-2 and its complexes using several biophysical techniques. Our cryo-EM reconstruction shows that the C2 domain blocks the E2-binding surface of the HECT domain. This blockage, ubiquitin-binding exosite masking by the WW1 domain, catalytic C922 blockage and HECT domain stabilization provide the structural basis for Nedd4-2 autoinhibition. Furthermore, Ca²⁺-dependent C2 membrane binding disrupts C2/HECT interactions, but not Ca²⁺ alone, whereas 14-3-3 protein binds to a flexible region of Nedd4-2 containing the WW2 and WW3 domains, thereby inhibiting its catalytic activity and membrane binding. Overall, our data provide key mechanistic insights into Nedd4-2 regulation toward fostering the development of strategies targeting Nedd4-2 function.

Enzyme activity is regulated by several mechanisms, including phosphorylation. Phosphorylation is a key signal transduction process in all eukaryotic cells and is thus crucial for virtually all cellular processes. In addition to its direct effect on protein structure, phosphorylation also affects protein-protein interactions, such as binding to scaffolding 14-3-3 proteins, which selectively recognize phosphorylated motifs. These interactions then modulate the catalytic activity, cellular localisation and interactions of phosphorylated enzymes through different mechanisms. The aim of this mini-review is to highlight several examples of 14-3-3 protein-dependent mechanisms of enzyme regulation previously studied in our laboratory over the past decade. More specifically, we address here the regulation of the human enzymes ubiquitin ligase Nedd4-2, procaspase-2, calcium-calmodulin dependent kinases CaMKK1/2, and death-associated protein kinase 2 (DAPK2) and yeast neutral trehalase Nth1.

Apoptosis signal-regulating kinase 1 (ASK1) is a crucial stress sensor, directing cells toward apoptosis, differentiation, and senescence via the p38 and JNK signaling pathways. ASK1 dysregulation has been associated with cancer and inflammatory, cardiovascular, and neurodegenerative diseases, among others. However, our limited knowledge of the underlying structural mechanism of ASK1 regulation hampers our ability to target this member of the MAP3K protein family towards developing therapeutic interventions for these disorders. Nevertheless, as a multidomain Ser/Thr protein kinase, ASK1 is regulated by a complex mechanism involving dimerization and interactions with several other proteins, including thioredoxin 1 (TRX1). Thus, the present study aims at structurally characterizing ASK1 and its complex with TRX1 using several biophysical techniques. As shown by cryo-EM analysis, in a state close to its active form, ASK1 is a compact and asymmetric dimer, which enables extensive interdomain and interchain interactions. These interactions stabilize the active conformation of the ASK1 kinase domain. In turn, TRX1 functions as a negative allosteric effector of ASK1, modifying the structure of the TRX1-binding domain and changing its interaction with the tetratricopeptide repeats domain. Consequently, TRX1 reduces access to the activation segment of the kinase domain. Overall, our findings not only clarify the role of ASK1 dimerization and inter-domain contacts but also provide key mechanistic insights into its regulation, thereby highlighting the potential of ASK1 protein-protein interactions as targets for anti-inflammatory therapy.

Apoptosis signal-regulating kinase 1 (ASK1) is a crucial stress sensor, directing cells towards apoptosis, differentiation and senescence via the p38 and JNK signaling pathways. ASK1 dysregulation has been associated with cancer and inflammatory, cardiovascular and neurodegenerative diseases, among others. However, our limited knowledge of the underlying structural mechanism of ASK1 regulation hampers our ability to target this member of the MAP3K protein family towards developing therapeutic interventions for these disorders. Nevertheless, as a multidomain Ser/Thr protein kinase, ASK1 is regulated by a complex mechanism involving dimerization and interactions with several other proteins, including thioredoxin 1 (TRX1). Thus, the present study aims at structurally characterizing ASK1 and its complex with TRX1 using several biophysical techniques. As shown by cryo-EM analysis, in a state close to its active form, ASK1 is a compact and asymmetric dimer, which enables extensive interdomain and interchain interactions. These interactions stabilize the active conformation of the ASK1 kinase domain. In turn, TRX1 functions as a negative allosteric effector of ASK1, modifying the structure of the TRX1-binding domain and changing its interaction with the tetratricopeptide repeats domain. Consequently, TRX1 reduces access to the activation segment of the kinase domain. Overall, our findings not only clarify the role of ASK1 dimerization and inter-domain contacts but also provide key mechanistic insights into its regulation, thereby highlighting the potential of ASK1 protein-protein interactions as targets for antiinflammatory therapy.

Cell signaling regulates several physiological processes by receiving, processing, and transmitting signals between the extracellular and intracellular environments. In signal transduction, phosphorylation is a crucial effector as the most common posttranslational modification. Selectively recognizing specific phosphorylated motifs of target proteins and modulating their functions through binding interactions, the yeast 14-3-3 proteins Bmh1 and Bmh2 are involved in catabolite repression, carbon metabolism, endocytosis, and mitochondrial retrograde signaling, among other key cellular processes. These conserved scaffolding molecules also mediate crosstalk between ubiquitination and phosphorylation, the spatiotemporal control of meiosis, and the activity of ion transporters Trk1 and Nha1. In humans, deregulation of analogous processes triggers the development of serious diseases, such as diabetes, cancer, viral infections, microbial conditions and neuronal and age-related diseases. Accordingly, the aim of this review article is to provide a brief overview of the latest findings on the functions of yeast 14-3-3 proteins, focusing on their role in modulating the aforementioned processes.

Apoptosis signal-regulating kinase 1 (ASK1) is a crucial stress sensor, directing cells towards apoptosis, differentiation and senescence via the p38 and JNK signaling pathways. ASK1 dysregulation has been associated with cancer and inflammatory, cardiovascular and neurodegenerative diseases, among others. However, our limited knowledge of the underlying structural mechanism of ASK1 regulation hampers our ability to target this member of the MAP3K protein family towards developing therapeutic interventions for these disorders. Nevertheless, as a multidomain Ser/Thr protein kinase, ASK1 is regulated by a complex mechanism involving dimerization and interactions with several other proteins, including thioredoxin 1 (TRX1). Thus, the present study aims at structurally characterizing ASK1 and its complex with TRX1 using several biophysical techniques. As shown by cryo-EM analysis, in a state close to its active form, ASK1 is a compact and asymmetric dimer, which enables extensive interdomain and interchain interactions. These interactions stabilize the active conformation of the ASK1 kinase domain. In turn, TRX1 functions as a negative allosteric effector of ASK1, modifying the structure of the TRX1-binding domain and changing its interaction with the tetratricopeptide repeats domain. Consequently, TRX1 reduces access to the activation segment of the kinase domain. Overall, our findings not only clarify the role of ASK1 dimerization and inter-domain contacts but also provide key mechanistic insights into its regulation, thereby highlighting the potential of ASK1 protein-protein interactions as targets for antiinflammatory therapy.

Ca ²⁺ /CaM‐dependent protein kinase kinases 1 and 2 (CaMKK1 and CaMKK2) phosphorylate and enhance the catalytic activity of downstream kinases CaMKI, CaMKIV, and protein kinase B. Accordingly, CaMKK1 and CaMKK2 regulate key physiological and pathological processes, such as tumorigenesis, neuronal morphogenesis, synaptic plasticity, transcription factor activation, and cellular energy homeostasis, and promote cell survival. Both CaMKKs are partly inhibited by phosphorylation, which in turn triggers adaptor and scaffolding protein 14‐3‐3 binding. However, 14‐3‐3 binding only significantly affects CaMKK1 function. CaMKK2 activity remains almost unchanged after complex formation for reasons still unclear. Here, we aim at structurally characterizing CaMKK1:14‐3‐3 and CaMKK2:14‐3‐3 complexes by SAXS, H/D exchange coupled to MS, and fluorescence spectroscopy. The results revealed that complex formation suppresses the interaction of both phosphorylated CaMKKs with Ca ²⁺ /CaM and affects the structure of their kinase domains and autoinhibitory segments. But these effects are much stronger on CaMKK1 than on CaMKK2 because the CaMKK1:14‐3‐3γ complex has a more compact and rigid structure in which the active site of the kinase domain directly interacts with the last two C‐terminal helices of the 14‐3‐3γ protein, thereby inhibiting CaMKK1. In contrast, the CaMKK2:14‐3‐3 complex has a looser and more flexible structure, so 14‐3‐3 binding only negligibly affects the catalytic activity of CaMKK2. Therefore, Ca ²⁺ /CaM binding suppression and the interaction of the kinase active site of CaMKK1 with the last two C‐terminal helices of 14‐3‐3γ protein provide the structural basis for 14‐3‐3‐mediated CaMKK1 inhibition.