The role of calcium in the interaction between calmodulin and a minimal functional construct of eukaryotic elongation factor 2 kinase

Revealing eEF-2 kinase: recent structural insights into function

The α-kinase eukaryotic elongation factor 2 kinase (eEF-2K) regulates translational elongation by phosphorylating its ribosome-associated substrate, the GTPase eEF-2. eEF-2K is activated by calmodulin (CaM) through a distinctive mechanism unlike that in other CaM-dependent kinases (CAMK). We describe recent structural insights into this unique activation process and examine the effects of specific regulatory signals on this mechanism. We also highlight key unanswered questions to guide future structure-function studies. These include structural mechanisms which enable eEF-2K to interact with upstream/downstream partners and facilitate its integration of diverse inputs, including Ca2+ transients, phosphorylation mediated by energy/nutrient-sensing pathways, pH changes, and metabolites. Answering these questions is key to establishing how eEF-2K harmonizes translation with cellular requirements within the boundaries of its molecular landscape.

eEF2K Inhibitor Design: The Progression of Exemplary Structure-Based Drug Design

The α-kinase, eEF2K, phosphorylates the threonine 56 residue of eEF2 to inhibit global peptide elongation (protein translation). As a master regulator of protein synthesis, in combination with its unique atypical kinase active site, investigations into the targeting of eEF2K represents a case of intense structure-based drug design that includes the use of modern computational techniques. The role of eEF2K is incredibly diverse and has been scrutinized in several different diseases including cancer and neurological disorders-with numerous studies inhibiting eEF2K as a potential treatment option, as described in this paper. Using available crystal structures of related α-kinases, particularly MHCKA, we report how homology modeling has been used to improve inhibitor design and efficacy. This review presents an overview of eEF2K related drug discovery efforts predating from the 1990's, to more recent in vivo studies in rat models. We also provide the reader with a basic introduction to several approaches and software programs used to undertake such drug discovery campaigns. With the recent exciting publication of an eEF2K crystal structure, we present our view regarding the future of eEF2K drug discovery.

Structural basis for the calmodulin-mediated activation of eukaryotic elongation factor 2 kinase

Translation is a tightly regulated process that ensures optimal protein quality and enables adaptation to energy/nutrient availability. The α-kinase eukaryotic elongation factor 2 kinase (eEF-2K), a key regulator of translation, specifically phosphorylates the guanosine triphosphatase eEF-2, thereby reducing its affinity for the ribosome and suppressing the elongation phase of protein synthesis. eEF-2K activation requires calmodulin binding and autophosphorylation at the primary stimulatory site, T348. Biochemical studies predict a calmodulin-mediated activation mechanism for eEF-2K distinct from other calmodulin-dependent kinases. Here, we resolve the atomic details of this mechanism through a 2.3-Å crystal structure of the heterodimeric complex of calmodulin and the functional core of eEF-2K (eEF-2KTR). This structure, which represents the activated T348-phosphorylated state of eEF-2KTR, highlights an intimate association of the kinase with the calmodulin C-lobe, creating an "activation spine" that connects its amino-terminal calmodulin-targeting motif to its active site through a conserved regulatory element.

Structural dynamics of the complex of calmodulin with a minimal functional construct of eukaryotic elongation factor 2 kinase and the role of Thr348 autophosphorylation

The calmodulin (CaM) activated α-kinase, eukaryotic elongation factor 2 kinase (eEF-2K), plays a central role in regulating translational elongation by phosphorylating eukaryotic elongation factor 2 (eEF-2), thereby reducing its ability to associate with the ribosome and suppressing global protein synthesis. Using TR (for truncated), a minimal functional construct of eEF-2K, and utilizing hydrogen/deuterium exchange mass spectrometry (HXMS), solution-state nuclear magnetic resonance (NMR) and biochemical approaches, we investigate the conformational changes accompanying complex formation between Ca²⁺ -CaM and TR and the effects of autophosphorylation of the latter at Thr348, its primary regulatory site. Our results suggest that a CaM C-lobe surface, complementary to the one involved in recognizing the calmodulin-binding domain (CBD) of TR, provides a secondary TR-interaction platform. CaM helix F, which is part of this secondary surface, responds to both Thr348 phosphorylation and pH changes, indicating its integration into an allosteric network that encompasses both components of the Ca²⁺ -CaM•TR complex. Solution NMR data suggest that CaM_H107K , which carries a helix F mutation, is compromised in its ability to drive the conformational changes in TR necessary to enable efficient Thr348 phosphorylation. Biochemical studies confirm the diminished capacity of CaM_H107K to induce TR autophosphorylation compared to wild-type CaM.

Dynamic Na+/H+ exchanger 1 (NHE1) - calmodulin complexes of varying stoichiometry and structure regulate Ca2+-dependent NHE1 activation

Calmodulin (CaM) engages in Ca²⁺-dependent interactions with numerous proteins, including a still incompletely understood physical and functional interaction with the human Na⁺/H⁺-exchanger NHE1. Using nuclear magnetic resonance (NMR) spectroscopy, isothermal titration calorimetry, and fibroblasts stably expressing wildtype and mutant NHE1, we discovered multiple accessible states of this functionally important complex existing in different NHE1:CaM stoichiometries and structures. We determined the NMR solution structure of a ternary complex in which CaM links two NHE1 cytosolic tails. In vitro, stoichiometries and affinities could be tuned by variations in NHE1:CaM ratio and calcium ([Ca²⁺]) and by phosphorylation of S648 in the first CaM-binding α-helix. In cells, Ca²⁺-CaM-induced NHE1 activity was reduced by mimicking S648 phosphorylation and by mutation of the first CaM-binding α-helix, whereas it was unaffected by inhibition of Akt, one of several kinases phosphorylating S648. Our results demonstrate a diversity of NHE1:CaM interaction modes and suggest that CaM may contribute to NHE1 dimerization and thereby augment NHE1 regulation. We propose that a similar structural diversity is of relevance to many other CaM complexes.

The role of calcium in the interaction between calmodulin and a minimal functional construct of eukaryotic elongation factor 2 kinase

Eukaryotic elongation factor 2 kinase (eEF-2K) regulates protein synthesis by phosphorylating eukaryotic elongation factor 2 (eEF-2), thereby reducing its affinity for the ribosome and suppressing global translational elongation rates. eEF-2K is regulated by calmodulin (CaM) through a mechanism that is distinct from that of other CaM-regulated kinases. We had previously identified a minimal construct of eEF-2K (TR) that is activated similarly to the wild-type enzyme by CaM in vitro and retains its ability to phosphorylate eEF-2 efficiently in cells. Here, we employ solution nuclear magnetic resonance techniques relying on Ile δ1-methyls of TR and Ile δ1- and Met ε-methyls of CaM, as probes of their mutual interaction and the influence of Ca²⁺ thereon. We find that in the absence of Ca²⁺ , CaM exclusively utilizes its C-terminal lobe (CaM_C ) to engage the N-terminal CaM-binding domain (CBD) of TR in a high-affinity interaction. Avidity resulting from additional weak interactions of TR with the Ca²⁺ -loaded N-terminal lobe of CaM (CaM_N ) at increased Ca²⁺ levels serves to enhance the affinity further. These latter interactions under Ca²⁺ saturation result in minimal perturbations in the spectra of TR in the context of its complex with CaM, suggesting that the latter is capable of driving TR to its final, presumably active conformation, in the Ca²⁺ -free state. Our data are consistent with a scenario in which Ca²⁺ enhances the affinity of the TR/CaM interactions, resulting in the increased effective concentration of the CaM-bound species without significantly modifying the conformation of TR within the final, active complex.