The protein Keap1 is central to the regulation of the Nrf2-mediated cytoprotective response, and is increasingly recognized as an important target for therapeutic intervention in a range of diseases involving excessive oxidative stress and inflammation. The BTB domain of Keap1 plays key roles in sensing environmental electrophiles and in mediating interactions with the Cul3/Rbx1 E3 ubiquitin ligase system, and is believed to be the target for several small molecule covalent activators of the Nrf2 pathway. However, despite structural information being available for several BTB domains from related proteins, there have been no reported crystal structures of Keap1 BTB, and this has precluded a detailed understanding of its mechanism of action and interaction with antagonists. We report here the first structure of the BTB domain of Keap1, which is thought to contain the key cysteine residue responsible for interaction with electrophiles, as well as structures of the covalent complex with the antagonist CDDO/bardoxolone, and of the constitutively inactive C151W BTB mutant. In addition to providing the first structural confirmation of antagonist binding to Keap1 BTB, we also present biochemical evidence that adduction of Cys 151 by CDDO is capable of inhibiting the binding of Cul3 to Keap1, and discuss how this class of compound might exert Nrf2 activation through disruption of the BTB-Cul3 interface.

Vascular immunotargeting may facilitate the rapid and specific delivery of therapeutic agents to endothelial cells. We investigated whether targeting of an antioxidant enzyme, catalase, to the pulmonary endothelium alleviates oxidative stress in an in vivo model of lung transplantation. Intravenously injected enzymes, conjugated with an antibody to platelet-endothelial cell adhesion molecule-1, accumulate in the pulmonary vasculature and retain their activity during prolonged cold storage and transplantation. Immunotargeting of catalase to donor rats augments the antioxidant capacity of the pulmonary endothelium, reduces oxidative stress, ameliorates ischemia-reperfusion injury, prolongs the acceptable cold ischemia period of lung grafts, and improves the function of transplanted lung grafts. These findings validate the therapeutic potential of vascular immunotargeting as a drug delivery strategy to reduce endothelial injury. Potential applications of this strategy include improving the outcome of clinical lung transplantation and treating a wide variety of endothelial disorders.

The Rev protein of HIV-1 actively shuttles between nucleus and cytoplasm and mediates the export of unspliced retroviral RNAs. The localization of shuttling proteins such as Rev is controlled by the relative rates of nuclear import and export. To study nuclear export in isolation, we generated cell lines expressing a green f luorescent proteinlabeled chimeric protein consisting of HIV-1 Rev and a hormone-inducible nuclear localization sequence. Steroid removal switches off import thus allowing direct visualization of the Rev export pathway in living cells. After digitonin permeabilization of these cells, we found that a functional nuclear export sequence (NES), ATP, and fractionated cytosol were sufficient for nuclear export in vitro. Nuclear pore-specific lectins and leptomycin B were potent export inhibitors. Nuclear export was not inhibited by antagonists of calcium metabolism that block nuclear import. These data further suggest that nuclear pores do not functionally close when luminal calcium stores are depleted. The distinct requirements for nuclear import and export argue that these competing processes may be regulated independently. This system should have wide applicability for the analysis of nuclear import and export.The bidirectional exchange of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (1). Generally, the cut-off between facilitated transport and diffusion across the nuclear pore complex is 40-50 kDa. However, some smaller molecules such as histone H1 also are transported actively (2). Active transport is a highly regulated process requiring specific targeting sequences, receptors, regulatory proteins, and energy (reviewed in refs. 3 and 4). Nuclear import also requires the small GTPase Ran, p10, and GTP hydrolysis (5, 6). In addition, alternate pathways for nuclear import exist (7,8). Although many of the requirements of import have been identified, the exact mechanism of translocation across the pore is poorly understood.Nuclear export, like import, is also initiated by a specific sequence. One of the first nuclear export sequences (NESs) was identified in the HIV-1 transactivating protein Rev (9). This hydrophobic sequence, LPPLERLTLD, is characterized by critical leucine residues; removal of one of these residues inactivates this sequence and prevents export (10). Similar NESs have been identified in several other shuttling proteins (11)(12)(13)(14)(15)(16)(17)(18)(19). When the NES is placed on a heterologous protein, it is sufficient to initiate export. An excess amount of the Rev NES peptide competes efficiently with some RNA species including 5S rRNA and spliceosomal U snRNAs for export out of the nucleus (9, 20). These competition studies indicate that Rev utilizes an established nuclear export pathway within the cell.NES-containing proteins are exported in an active and saturable manner, indicating that the process is receptor-mediated. However, identification of export receptors has proven more difficult than the isolation of import r...

Targeted delivery of drugs to vascular endothelium promises more effective and specific therapies in many disease conditions, including acute lung injury (ALI). This study evaluates the therapeutic effect of drug targeting to PECAM (platelet/endothelial cell adhesion molecule-1) in vivo in the context of pulmonary oxidative stress. Endothelial injury by reactive oxygen species (e.g., H2O2) is involved in many disease conditions, including ALI/acute respiratory distress syndrome and ischemia-reperfusion. To optimize delivery of antioxidant therapeutics, we conjugated catalase with PECAM antibodies and tested properties of anti-PECAM/catalase conjugates in cell culture and mice. Anti-PECAM/catalase, but not an IgG/catalase counterpart, bound specifically to PECAM-expressing cells, augmented their H2O2-degrading capacity, and protected them against H2O2 toxicity. Anti-PECAM/catalase, but not IgG/catalase, rapidly accumulated in the lungs after intravenous injection in mice, where it was confined to the pulmonary endothelium. To test its protective effect, we employed a murine model of oxidative lung injury induced by glucose oxidase coupled with thrombomodulin antibody (anti-TM/GOX). After intravenous injection in mice, anti-TM/GOX binds to pulmonary endothelium and produces H2O2, which causes lung injury and 100% lethality within 7 h. Coinjection of anti-PECAM/catalase protected against anti-TM/GOX-induced pulmonary oxidative stress, injury, and lethality, whereas polyethylene glycol catalase or IgG/catalase conjugates afforded only marginal protective effects. This result validates vascular immunotargeting as a prospective strategy for therapeutic interventions aimed at immediate protective effects, e.g., for augmentation of antioxidant defense in the pulmonary endothelium and treatment of ALI.

In addition to the well-characterized GTPdependent nuclear transport observed in permeabilized cells, we detected a mode of nuclear transport that was GTPindependent at elevated cytoplasmic calcium concentrations. Nuclear transport under these conditions was blocked by calmodulin inhibitors. Recombinant calmodulin restored ATP-dependent nuclear transport in the absence of cytosol. Calmodulin-dependent transport was inhibited by wheat germ agglutinin consistent with transport proceeding through nuclear pores. We propose that release of intracellular calcium stores upon cell activation inhibits GTP-dependent nuclear transport; the elevated cytosolic calcium then acts through calmodulin to stimulate the novel GTP-independent mode of import.Protein transport across the nuclear pore is a highly regulated process that is just beginning to be understood (1, 2). Proteins destined for the nucleus contain a basic sequence of amino acids called a nuclear localization sequence (NLS; ref.3). The NLS is recognized and bound in the cytoplasm by karyopherin-␣, † which forms a complex with karyopherin-␤ and routes the substrate to the nuclear pore (reviewed in refs. 17 and 18). The substrate-karyopherin complex binds to RanBP2, a 360-kDa protein component of fibrils that extend from the surface of the nuclear pore into the cytoplasm of the cell. Ran͞TC4 (a small GTP-binding protein of the Ras superfamily) and p10 bind to the transport complex, and the complex is delivered to the center of the pore in a mechanism that appears to involve GTP hydrolysis. At present, the mechanism of transit across the pore itself is unclear but it is known to require nucleotide triphosphate and is sensitive to incubation at 4ЊC. Soluble components of the transport machinery are then recycled to the cytoplasm for a new round of transport.Indirect evidence suggests that the nuclear envelope must remain intact for active nuclear transport (19). In addition to the functional barrier that the nuclear envelope provides, recent evidence suggests that the calcium pools in the lumen of the nuclear envelope must also be intact for nuclear transport to occur. Greber and Gerace (20) have observed a requirement for intact intracellular calcium stores for active nuclear transport, as well as passive diffusion of 10-kDa dextrans across the pore. Nuclear import of microinjected fluorescent substrates in a low calcium buffer is reduced by pre-incubation of the cells with thapsigargin, which causes the release of calcium from intracellular stores. also observed that there is an inhibition of diffusion of 10-kDa dextrans across the pore when isolated Xenopus laevis nuclei are treated with inositol trisphosphate to release calcium stores from the lumen of the nuclear envelope. In the only structural study of the effect of calcium on the nuclear pore, Jarnik and Aebi (22) found that the removal of calcium by chelation with EGTA led to an ''open'' appearance of the nuclear pores of X. laevis germinal vesicles as assessed by electron microscopy. The pores were r...

The strain 58 Chinese hamster ovary (CHO) mutant defective in CTP:phosphocholine cytidylyltransferase was characterized as an expression system for exogenous cytidylyltransferase. Strain 58 cells express less than 5% of the wild-type level of cytidylyltransferase protein at the permissive temperature even though the steady-state messenger RNA levels were found to be similar to those in the parental CHO-K1 cell line. A point mutation from arginine to histidine at amino acid 140 was identified in the strain 58 protein. Rat liver cytidylyltransferase was stably expressed in strain 58 cells and shown to be active, targeted to the nucleus, phosphorylated, and activated by methylethanolamine supplementation or phospholipase C treatment. Thus, the mechanisms by which cytidylyltransferase is processed and regulated in CHO-K1 cells are intact in strain 58 cells. The heterologously expressed protein complemented the strain 58 defects in both temperature-sensitive growth and phosphatidylcholine biosynthesis, consistent with a single lesion in the structural gene for cytidylyltransferase being responsible for both phenomena. Overexpression of cytidylyltransferase activity at levels up to eightfold higher than those in CHO-K1 cells did not appreciably affect phosphatidylcholine metabolism. A putative casein kinase II phosphorylation site was altered by site-directed mutagenesis and expressed in the strain 58 cells. Alteration of this site did not affect expression and regulation of cytidylyltransferase activity.

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Proline-specific dipeptidyl peptidases (DPPs) are emerging targets for drug development. DPP4 inhibitors are approved in many countries, and other dipeptidyl peptidases are often referred to as DPP4 activity- and/or structure-homologues (DASH). Members of the DASH family have overlapping substrate specificities, and, even though they share low sequence identity, therapeutic or clinical cross-reactivity is a concern. Here, we report the structure of human DPP7 and its complex with a selective inhibitor Dab-Pip (L-2,4-diaminobutyryl-piperidinamide) and compare it with that of DPP4. Both enzymes share a common catalytic domain (α/β-hydrolase). The catalytic pocket is located in the interior of DPP7, deep inside the cleft between the two domains. Substrates might access the active site via a narrow tunnel. The DPP7 catalytic triad is completely conserved and comprises Ser162, Asp418 and His443 (corresponding to Ser630, Asp708 and His740 in DPP4), while other residues lining the catalytic pockets differ considerably. The “specificity domains” are structurally also completely different exhibiting a β-propeller fold in DPP4 compared to a rare, completely helical fold in DPP7. Comparing the structures of DPP7 and DPP4 allows the design of specific inhibitors and thus the development of less cross-reactive drugs. Furthermore, the reported DPP7 structures shed some light onto the evolutionary relationship of prolyl-specific peptidases through the analysis of the architectural organization of their domains.