Crystal structure of tabtoxin resistance protein complexed with acetyl coenzyme A reveals the mechanism for beta-lactam acetylation

Tabtoxin resistance protein (TTR) is an enzyme that renders tabtoxin-producing pathogens, such as Pseudomonas syringae, tolerant to their own phytotoxins. Here, we report the crystal structure of TTR complexed with its natural cofactor, acetyl coenzyme A (AcCoA), to 1.55A resolution. The binary complex forms a characteristic "V" shape for substrate binding and contains the four motifs conserved in the GCN5-related N-acetyltransferase (GNAT) superfamily, which also includes the histone acetyltransferases (HATs). A single-step mechanism is proposed to explain the function of three conserved residues, Glu92, Asp130 and Tyr141, in catalyzing the acetyl group transfer to its substrate. We also report that TTR possesses HAT activity and suggest an evolutionary relationship between TTR and other GNAT members.     

Low pH potentiates both capsaicin binding and channel gating of VR1 receptors

Capsaicin ion channels are highly expressed in peripheral nervous terminals and involved in pain and thermal sensations. One characteristic of the cloned VR1 receptor is its multimodal responses to various types of noxious stimuli. The channel is independently activated by capsaicin and related vanilloids at submicromolar range, by heat above 40 degrees C, and by protons at pH below 6.5. Furthermore, simultaneous applications of two or more stimuli lead to cross sensitization of the receptor, with an apparent increase in the sensitivity to any individual stimulus when applied alone. We studied here the mechanism underlying such cross-sensitization; in particular, between capsaicin and pH, two prototypical stimuli for the channel. By analyzing single-channel currents recorded from excised-patches expressing single recombinant VR1 receptors, we examined the effect of pH on burst properties of capsaicin activation at low concentrations and the effect on gating kinetics at high concentrations. Our results indicate that pH has dual effects on both capsaicin binding and channel gating. Lowering pH enhances the apparent binding affinity of capsaicin, promotes the occurrences of long openings and short closures, and stabilizes at least one of the open conformations of the channel. Our data also demonstrate that capsaicin binding and protonation of the receptor interact allosterically, where the effect of one can be offset by the effect of the other. These results provide important basis to further understand the nature of the activation pathways of the channel evoked by different stimuli as well as the general mechanism underling the cross-sensitization of pain.     

Positive and negative regulation of the gamma-secretase activity by nicastrin in a murine model

Nicastrin is a component of the gamma-secretase complex that has been shown to adhere to presenilin-1 (PS1), Notch, and APP. Here we demonstrate that Nicastrin-deficient mice showed a phenotype that is indistinguishable from PS1/PS2 double knock-out mice, whereas heterozygotes were healthy and viable. Fibroblasts derived from Nicastrin-deficient embryos were unable to generate amyloid beta-peptide and failed to release the intracellular domain of APP- or Notch1-Gal4-VP16 fusion proteins. Additionally, C- and N-terminal fragments of PS1 and the C-terminal fragments of PS2 were not detectable in Nicastrin-null fibroblasts, whereas full-length PS1 accumulated in null fibroblasts, indicating that Nicastrin is required for the endoproteolytic processing of presenilins. Interestingly, cells derived from Nicastrin heterozygotes produced relatively higher levels of amyloid beta-peptide whether the source was endogenous mouse or transfected human APP. These data demonstrate that Nicastrin is essential for the gamma-secretase cleavage of APP and Notch in mammalian cells and that Nicastrin has both positive and negative functions in the regulation of gamma-secretase activity.     

Phosphorylation screening identifies translational initiation factor 4GII as an intracellular target of Ca(2+)/calmodulin-dependent protein kinase I

CaMKI is a Ca2+/calmodulin-dependent protein kinase that is widely expressed in eukaryotic cells and tissues but for which few, if any, physiological substrates are known. We screened a human lung cDNA expression library for potential CaMKI substrates by solid phase in situ phosphorylation ("phosphorylation screening"). Multiple overlapping partial length cDNAs encoding three proteins were detected. Two of these proteins are known: 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase and eukaryotic translation initiation factor (eIF) 4GII. To determine whether CaMKI substrates identified by phosphorylation screening represent authentic physiological targets, we examined the potential for [Ca2+]i- and CaMKI-dependent phosphorylation of eIF4GII in vitro and in vivo. Endogenous eIF4GII immunoprecipitated from HEK293T cells was phosphorylated by CaMKI, in vitro as was a recombinant fragment of eIF4GII encompassing the central and C-terminal regions. The latter phosphorylation occurred with favorable kinetics (Km = 1 microm; kcat = 1.8 s-1) at a single site, Ser1156, located in a segment of eIF4GII aligning with the phosphoregion of eIF4GI. Phosphopeptide mapping and back phosphorylation experiments revealed [Ca2+]i-dependent, CaMKI site-specific, eIF4GII phosphorylation in vivo. This phosphorylation was blocked by kinase-negative CaMKI consistent with a requirement for endogenous CaMKI for in vivo eIF4GII phosphorylation. We conclude that phosphorylation screening is an effective method for searching for intracellular targets of CaMKI and may have identified a new role of Ca2+ signaling to the translation apparatus.     

Soluble human core 2 beta6-N-acetylglucosaminyltransferase C2GnT1 requires its conserved cysteine residues for full activity

Human UDP-GlcNAc: Galbeta1-3GalNAc- (GlcNAc to GalNAc) beta1,6-GlcNAc-transferase (C2GnT1) is a member of a group of beta6-GlcNAc-transferases that belongs to CAZy family 14. One of the striking features of these beta6-GlcNAc-transferases is the occurrence of nine completely conserved cysteine residues that are located throughout the catalytic domain. We have expressed the soluble catalytic domain of human C2GnT1 in insect cells, and isolated active enzyme as a secreted protein. beta-Mercaptoethanol (beta-ME) and dithiothreitol (DTT) were found to stimulate the enzyme activity up to 20-fold, indicating a requirement for a reduced sulfhydryl for activity. When the enzyme was subjected to nonreducing PAGE, the migration of the protein was identical to the migration in reducing gels, demonstrating the absence of intermolecular disulfide bonds. This suggested that the monomer is the active form of the enzyme. Sulfhydryl reagents such as 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) and N-ethylmaleimide (NEM) inactivated the enzyme, and the inactivation was partially prevented by prior addition of donor or acceptor substrate and by sulfhydryl reducing agents. We therefore investigated the role of all nine conserved cysteine residues in enzyme stability and activity by site-directed mutagenesis where individual cysteine residues were changed to serine. All of the mutants were expressed as soluble proteins. Seven of the Cys mutants were found to be inactive, while C100S and C217S mutants had 10% and 41% activity, respectively, when compared to the wild-type enzyme. Wild-type and C217S enzymes had similar K(M) and V(max) values for acceptor substrate Galbeta1-3GalNAcalpha-p-nitrophenyl (GGApnp), but the K(M) value for UDP-GlcNAc was higher for C217S than for the wild-type enzyme. In contrast to wild-type enzyme, C217S was not stimulated by reducing agents and was not inhibited by sulfhydryl specific reagents. These results suggest that Cys-217 is a free sulfhydryl in active wild-type enzyme and that Cys-217, although not required for activity, is in or near the active site of the protein. Since seven of the mutations were totally inactive, it is likely that these seven Cys residues play a role in maintaining an active conformation of soluble C2GnT1 by forming disulfide bonds. These bonds are only broken at high concentrations of disulfide reducing agents.     

BNIPL-2, a novel homologue of BNIP-2, interacts with Bcl-2 and Cdc42GAP in apoptosis

The execution phase of apoptosis is characterized by marked changes in cell morphology that include contraction and membrane blebbing. Little is known about the mechanisms underlying this process. We report here the identification of a novel member of BNIPL family, designated Bcl-2/adenovirus E1B 19kDa interacting protein 2 like-2 (BNIPL-2), which interacts with Bcl-2 and Cdc42GAP. We found that the human BNIPL-2 shares homology to human BNIP-2 and also possesses a BNIP-2 and Cdc42GAP homology (BCH) domain. Deletion experiments indicated that the BCH domain of BNIPL-2 is critical for its interactions with the Bcl-2 and Cdc42GAP and also for its cell death-inducing function. Our data showed that BNIPL-2 may be a linker protein located at the front end of Bcl-2 pathway for DNA fragmentation and Cdc42 signaling for morphological changes during apoptosis. We propose that BNIPL-2 protein may play an important role in regulation of both pathways for DNA fragmentation and for formation of membrane blebs in apoptotic cells.     

Monitoring RNA base structure and dynamics using site-directed spin labeling

Site-directed spin labeling utilizes site-specific attachment of a stable nitroxide radical to probe the structure and dynamics of macromolecules. In the present study, a 4-thiouridine base is introduced at each of six different positions in a 23-nucleotide RNA molecule. The 4-thiouridine derivatives were subsequently modified with one of three methanethiosulfonate nitroxide reagents to introduce a spin label at specific sites. The electron paramagnetic resonance spectra of the labeled RNAs were analyzed in terms of nitroxide motion and the RNA solution structure. At a base-paired site in the RNA helix, where the nitroxide has weak or no local interactions, motion of the nitroxide is apparently dominated by rotation about bonds within the probe. The motion is similar to that found for a structurally related probe on helical sites in proteins, suggesting a similar mode of motion. At other sites that are hydrogen bonded and stacked within the helix, local interactions within the RNA molecule modulate the nitroxide motion in a manner consistent with expectations based on the known structure. For a base that is not structurally constrained, the mobility is higher than at any other site, presumably due to motion of the base itself. These results demonstrate the general utility of the 4-thiouridine/methanethiosulfonate coupling method to introduce nitroxide spin labels into RNA and the ability of the resulting label to probe local structure and dynamics.     

The apoptosis-associated protein BNIPL interacts with two cell proliferation-related proteins,MIF and GFER

Bcl-2/adenovirus E1B 19 kDa interacting protein 2-like, BNIP-2-like (BNIPL) is a recently cloned and characterized apoptosis-associated protein that shares 72% homology with BNIP-2. It is highly expressed in human placenta and lung. A yeast two-hybrid system was used to obtain two BNIPL-interacting proteins, MIF (macrophage migration inhibitory factor) and GFER (growth factor erv1 (Saccharomyces cerevisiae)-like). The interactions were confirmed by glutathione S-transferase pull-down assay in vitro and co-immunoprecipitation assay in vivo. Colony formation assay and cell proliferation test suggest that overexpression of BNIPL could inhibit the growth of BEL-7402 cells. These findings suggest that BNIPL may physically bind to cell proliferation-related proteins, MIF and GFER.