Human herpesvirus 8 molecular mimicry of ephrin ligands facilitates cell entry and triggers EphA2 signaling.

Human herpesvirus 8 (HHV-8) is an oncogenic virus that enters cells by fusion of the viral and endosomal cellular membranes in a process mediated by viral surface glycoproteins. One of the cellular receptors hijacked by HHV-8 to gain access to cells is the EphA2 tyrosine kinase receptor, and the mechanistic basis of EphA2-mediated viral entry remains unclear. Using X-ray structure analysis, targeted mutagenesis, and binding studies, we here show that the HHV-8 envelope glycoprotein complex H and L (gH/gL) binds with subnanomolar affinity to EphA2 via molecular mimicry of the receptor's cellular ligands, ephrins (Eph family receptor interacting proteins), revealing a pivotal role for the conserved gH residue E52 and the amino-terminal peptide of gL. Using FSI-FRET and cell contraction assays, we further demonstrate that the gH/gL complex also functionally mimics ephrin ligand by inducing EphA2 receptor association via its dimerization interface, thus triggering receptor signaling for cytoskeleton remodeling. These results now provide novel insight into the entry mechanism of HHV-8, opening avenues for the search of therapeutic agents that could interfere with HHV-8-related diseases.

A cancer mutation promotes EphA4 oligomerization and signaling by altering the conformation of the SAM domain.

The Eph receptor tyrosine kinases and their ephrin ligands regulate many physiological and pathological processes. EphA4 plays important roles in nervous system development and adult homeostasis, while aberrant EphA4 signaling has been implicated in neurodegeneration. EphA4 may also affect cancer malignancy, but the regulation and effects of EphA4 signaling in cancer are poorly understood. A correlation between decreased patient survival and high EphA4 mRNA expression in melanoma tumors that also highly express ephrinA ligands suggests that enhanced EphA4 signaling may contribute to melanoma progression. A search for EphA4 gain-of-function mutations in melanoma uncovered a mutation of the highly conserved leucine 920 in the EphA4 sterile alpha motif (SAM) domain. We found that mutation of L920 to phenylalanine (L920F) potentiates EphA4 autophosphorylation and signaling, making it the first documented EphA4 cancer mutation that increases kinase activity. Quantitative Föster resonance energy transfer and fluorescence intensity fluctuation (FIF) analyses revealed that the L920F mutation induces a switch in EphA4 oligomer size, from a dimer to a trimer. We propose this switch in oligomer size as a novel mechanism underlying EphA4-linked tumorigenesis. Molecular dynamics simulations suggest that the L920F mutation alters EphA4 SAM domain conformation, leading to the formation of EphA4 trimers that assemble through two aberrant SAM domain interfaces. Accordingly, EphA4 wild-type and the L920F mutant are affected differently by the SAM domain and are differentially regulated by ephrin ligand stimulation. The increased EphA4 activation induced by the L920F mutation, through the novel mechanism we uncovered, supports a functional role for EphA4 in promoting pathogenesis.

Interaction between the transmembrane domains of neurotrophin receptors p75 and TrkA mediates their reciprocal activation.

The neurotrophin receptors p75 and tyrosine protein kinase receptor A (TrkA) play important roles in the development and survival of the nervous system. Biochemical data suggest that p75 and TrkA reciprocally regulate the activities of each other. For instance, p75 is able to regulate the response of TrkA to lower concentrations of nerve growth factor (NGF), and TrkA promotes shedding of the extracellular domain of p75 by α-secretases in a ligand-dependent manner. The current model suggests that p75 and TrkA are regulated by means of a direct physical interaction; however, the nature of such interaction has been elusive thus far. Here, using NMR in micelles, multiscale molecular dynamics, FRET, and functional studies, we identified and characterized the direct interaction between TrkA and p75 through their respective transmembrane domains (TMDs). Molecular dynamics of p75-TMD mutants suggests that although the interaction between TrkA and p75 TMDs is maintained upon mutation, a specific protein interface is required to facilitate TrkA active homodimerization in the presence of NGF. The same mutations in the TMD protein interface of p75 reduced the activation of TrkA by NGF as well as reducing cell differentiation. In summary, we provide a structural model of the p75-TrkA receptor complex necessary for neuronal development stabilized by TMD interactions.

Hofmeister Ion-Induced Changes in Water Structure Correlate with Changes in Solvation of an Aggregated Protein Complex.

RecA is a naturally aggregating Escherichia coli protein that catalyzes the strand exchange reaction utilized in DNA repair. Previous studies have shown that the presence of salts influence RecA activity, aggregation, and stability and that salts stabilize RecA in an inverse-anionic Hofmeister series. Here we utilized attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and circular dichroism (CD) to investigate how various Hofmeister salts alter the water structure and RecA solvation and aggregation. Spectroscopic studies performed in water and deuterium oxide suggest that salts alter water O-(1)H and O-(2)H stretch and bend vibrations as well as protein amide I (or I') and amide II (or II') vibrations. Anions have a much larger influence on water vibrations than cations. Water studies also show increased water-water and/or water-ion interactions in the presence of strongly hydrated SO4(2-) salts and evidence for decreased interactions with weakly hydrated Cl(-) and ClO4(-) salts. Salt-water difference infrared spectra show that kosmotropic salts are more hydrated than chaotropic salts. Interestingly, this is the opposite trend to the changes in protein solvation. Infrared spectra of RecA show that vibrations associated with protein desolvation were observed in the presence of SO4(2-) salts. Conversely, vibrations associated with protein solvation were observed in the presence of Cl(-) and ClO4(-) salts. Difference infrared studies on the dehydration of model proteins aided in identifying changes in RecA-solvent interactions. This study provides evidence that salt-induced changes in water vibrations correlate to changes in protein solvent interactions and thermal stability.

Regulation of the EphA2 receptor intracellular region by phosphomimetic negative charges in the kinase-SAM linker.

Eph receptor tyrosine kinases play a key role in cell-cell communication. Lack of structural information on the entire multi-domain intracellular region of any Eph receptor has hindered understanding of their signaling mechanisms. Here, we use integrative structural biology to investigate the structure and dynamics of the EphA2 intracellular region. EphA2 promotes cancer malignancy through a poorly understood non-canonical form of signaling involving serine/threonine phosphorylation of the linker connecting its kinase and SAM domains. We show that accumulation of multiple linker negative charges, mimicking phosphorylation, induces cooperative changes in the EphA2 intracellular region from more closed to more extended conformations and perturbs the EphA2 juxtamembrane segment and kinase domain. In cells, linker negative charges promote EphA2 oligomerization. We also identify multiple kinases catalyzing linker phosphorylation. Our findings suggest multiple effects of linker phosphorylation on EphA2 signaling and imply that coordination of different kinases is necessary to promote EphA2 non-canonical signaling.

Anion-Caffeine Interactions Studied by 13C and 1H NMR and ATR-FTIR Spectroscopy.

This work investigates the interactions of a series of 11 anions with caffeine by utilizing 13C and 1H NMR and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The aim of this study is to elucidate the molecular mechanisms of ion interactions with caffeine and to study how these interactions affect caffeine aggregation in aqueous solution. The chemical shift changes of caffeine 13C and 1H in the presence of salts provide a measure for anions' salting-out/salting-in abilities on individual carbon and hydrogen atoms in caffeine. The relative influences of anions on the chemical shift of individual atoms in the caffeine molecule are quantified. It is observed that strongly hydrated anions are excluded from the carbons on the six-member ring in caffeine and promote caffeine aggregation. On the other hand, weakly hydrated anions decrease caffeine aggregation by accumulating around the periphery of the caffeine molecule and binding to the ring structure. The ATR-FTIR results demonstrate that strongly hydrated anions desolvate the caffeine molecule and increase aggregation, while weakly hydrated anions have the opposite effects and salt caffeine into solution.