Structure determination of murine mitochondrial carbonic anhydrase V at 2.45-A resolution: implications for catalytic proton transfer and inhibitor design.

The three-dimensional structure of murine mitochondrial carbonic anhydrase V has been determined and refined at 2.45-A resolution (crystallographic R factor = 0.187). Significant structural differences unique to the active site of carbonic anhydrase V are responsible for differences in the mechanism of catalytic proton transfer as compared with other carbonic anhydrase isozymes. In the prototypical isozyme, carbonic anhydrase II, catalytic proton transfer occurs via the shuttle group His-64; carbonic anhydrase V has Tyr-64, which is not an efficient proton shuttle due in part to the bulky adjacent side chain of Phe-65. Based on analysis of the structure of carbonic anhydrase V, we speculate that Tyr-131 may participate in proton transfer due to its proximity to zinc-bound solvent, its solvent accessibility, and its electrostatic environment in the protein structure. Finally, the design of isozyme-specific inhibitors is discussed in view of the complex between carbonic anhydrase V and acetazolamide, a transition-state analogue. Such inhibitors may be physiologically important in the regulation of blood glucose levels.

The angiotensin II type 2 (AT2) receptor antagonizes the growth effects of the AT1 receptor: gain-of-function study using gene transfer.

The type 1 angiotensin II (AT1) receptor is well characterized but the type 2 (AT2) receptor remains an enigma. We tested the hypothesis that the AT2 receptor can modulate the growth of vascular smooth muscle cells by transfecting an AT2 receptor expression vector into the balloon-injured rat carotid artery and observed that overexpression of the AT2 receptor attenuated neointimal formation. In cultured smooth muscle cells, AT2 receptor transfection reduced proliferation and inhibited mitogen-activated protein kinase activity. Furthermore, we demonstrated that the AT2 receptor mediated the developmentally regulated decrease in aortic DNA synthesis at the latter stages of gestation. These results suggest that the AT2 receptor exerts an antiproliferative effect, counteracting the growth action of AT1 receptor.

Protective immune responses induced by secretion of a chimeric soluble protein from a recombinant Mycobacterium bovis bacillus Calmette-Guérin vector candidate vaccine for human immunodeficiency virus type 1 in small animals.

A recombinant Mycobacterium bovis bacillus Calmette-Guérin (BCG) vector-based vaccine that secretes the V3 principal neutralizing epitope of human immunodeficiency virus (HIV) could induce immune response to the epitope and prevent the viral infection. By using the Japanese consensus sequence of HIV-1, we successfully constructed chimeric protein secretion vectors by selecting an appropriate insertion site of a carrier protein and established the principal neutralizing determinant (PND)-peptide secretion system in BCG. The recombinant BCG (rBCG)-inoculated guinea pigs were initially screened by delayed-type hypersensitivity (DTH) skin reactions to the PND peptide, followed by passive transfer of the DTH by the systemic route. Further, immunization of mice with the rBCG resulted in induction of cytotoxic T lymphocytes. The guinea pig immune antisera showed elevated titers to the PND peptide and neutralized HIVMN, and administration of serum IgG from the vaccinated guinea pigs was effective in completely blocking the HIV infection in thymus/liver transplanted severe combined immunodeficiency (SCID)/hu or SCID/PBL mice. In addition, the immune serum IgG was shown to neutralize primary field isolates of HIV that match the neutralizing sequence motif by a peripheral blood mononuclear cell-based virus neutralization assay. The data support the idea that the antigen-secreting rBCG system can be used as a tool for development of HIV vaccines.

Inflammation-induced recombinant protein expression in vivo using promoters from acute-phase protein genes.

We report that promoters for two murine acute-phase protein (APP) genes, complement factor 3 (C3) and serum amyloid A3 (SAA3), can increase recombinant protein expression in response to inflammatory stimuli in vivo. To deliver APP promoter-luciferase reporter gene constructs to the liver, where most endogenous APP synthesis occurs, we introduced them into a nonreplicating adenovirus vector and injected the purified viruses intravenously into mice. When compared with the low levels of basal luciferase expression observed prior to inflammatory challenge, markedly increased expression from the C3 promoter was detected in liver in response to both lipopolysaccharide (LPS) and turpentine, and lower-level inducible expression was also found in lung. In contrast, expression from the SAA3 promoter was found only in liver and was much more responsive to LPS than to turpentine. After LPS challenge, hepatic luciferase expression increased rapidly and in proportion to the LPS dose. Use of cytokine-inducible promoters in gene transfer vectors may make it possible to produce antiinflammatory proteins in vivo in direct relationship to the intensity and duration of an individual's inflammatory response. By providing endogenously controlled production of recombinant antiinflammatory proteins, this approach might limit the severity of the inflammatory response without interfering with the beneficial components of host defense and immunity.

Low concentrations of diacylglycerol promote the binding of apolipophorin III to a phospholipid bilayer: a surface plasmon resonance spectroscopy study.

The binding of the exchangeable apolipoprotein apolipophorin III (apoLp-III) to an egg phosphatidylcholine bilayer as a function of the concentration of diacylglycerol (DG) in the bilayer was studied by surface plasmon resonance spectroscopy. At a DG concentration of 2 mol % in the bilayer, the binding of apoLp-III reached saturation. Under saturating conditions, apoLp-III forms a closely packed monolayer approximately 55 A thick, in which each molecule of protein occupies approximately 500 A2 at the membrane surface. These dimensions are consistent with the molecular size of the apoLp-III molecule determined by x-ray crystallography, if apoLp-III binds to the bilayer with the long axis of the apoLp-III normal to the membrane surface. In the absence of protein, the overall structure of the lipid bilayer was not significantly changed up to 2.5 mol% DG. However, at 4 and 6 mol % DG, the presence of nonbilayer structures was observed. The addition of apoLp-III to a membrane containing 6 mol % DG promoted the formation of large lipid-protein complexes. These data support a two-step sequential binding mechanism for binding of apoLp-III to a lipid surface. The first step is a recognition process, consisting of the adsorption of apoLp-III to a nascent hydrophobic defect in the phospholipid bilayer caused by the presence of DG. This recognition process might depend on the presence of a hydrophobic sensor located at one of the ends of the long axis of the apoLp-III molecule but would be consolidated through H-bond and electrostatic interactions. Once primary binding is achieved, subsequent enlargement of the hydrophobic defect in the lipid surface would trigger the unfolding of the apolipoprotein and binding via the amphipathic alpha-helices. This two-step sequential binding mechanism could be a general mechanism for all exchangeable apolipoproteins. A possible physiological role of the ability of apoLp-III to bind to lipid structures in two orientations is also proposed.

CT-2576, an inhibitor of phospholipid signaling, suppresses constitutive and induced expression of human immunodeficiency virus.

Viruses such as human immunodeficiency virus (HIV) require cellular activation for expression. Cellular activation in lymphoid cells is associated with augmented accumulation of certain phosphatidic acid (PA) species derived from the hydrolysis of glycan phosphatidylinositol (GPI). This suggests that activation of a phospholipid pathway may play a role in initiation of viral replication. To test this hypothesis, we examined the effect of tat gene expression on the production of cellular PA species, as the Tat protein is essential for HIV expression and has been implicated in activating the expression of multiple host cellular genes. Expression of tat increased the expression of PA. We then tested whether synthetic inhibitors of PA metabolism would inhibit activation of the HIV long terminal repeat by Tat and tumor necrosis factor alpha (TNF-alpha). CT-2576 suppressed both PA generation induced by Tat and HIV long terminal repeat-directed gene expression in response to Tat or TNF-alpha at a posttranscriptional step. CT-2576 also inhibited constitutive as well as TNF-alpha- and interleukin 6-induced expression of HIV p24 antigen in chronically infected U1 cells and in peripheral blood lymphocytes acutely infected with a clinical isolate of HIV. Pharmacological inhibition of synthesis of selected PA species may therefore provide a therapeutic approach to suppression of HIV replication.

The protein deficient in Lowe syndrome is a phosphatidylinositol-4,5-bisphosphate 5-phosphatase.

Lowe syndrome, also known as oculocerebrorenal syndrome, is caused by mutations in the X chromosome-encoded OCRL gene. The OCRL protein is 51% identical to inositol polyphosphate 5-phosphatase II (5-phosphatase II) from human platelets over a span of 744 aa, suggesting that OCRL may be a similar enzyme. We engineered a construct of the OCRL cDNA that encodes amino acids homologous to the platelet 5-phosphatase for expression in baculovirus-infected Sf9 insect cells. This cDNA encodes aa 264-968 of the OCRL protein. The recombinant protein was found to catalyze the reactions also carried out by platelet 5-phosphatase II. Thus OCRL converts inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, and it converts inositol 1,3,4,5-tetrakisphosphate to inositol 1,3,4-trisphosphate. Most important, the enzyme converts phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 4-phosphate. The relative ability of OCRL to catalyze the three reactions is different from that of 5-phosphatase II and from that of another 5-phosphatase isoenzyme from platelets, 5-phosphatase I. The recombinant OCRL protein hydrolyzes the phospholipid substrate 10- to 30-fold better than 5-phosphatase II, and 5-phosphatase I does not cleave the lipid at all. We also show that OCRL functions as a phosphatidylinositol 4,5-bisphosphate 5-phosphatase in OCRL-expressing Sf9 cells. These results suggest that OCRL is mainly a lipid phosphatase that may control cellular levels of a critical metabolite, phosphatidylinositol 4,5-bisphosphate. Deficiency of this enzyme apparently causes the protean manifestations of Lowe syndrome.

Experimental evidence for hydrogen-bonded network proton transfer in bacteriorhodopsin shown by Fourier-transform infrared spectroscopy using azide as catalyst.

Experimental evidence for proton transfer via a hydrogen-bonded network in a membrane protein is presented. Bacteriorhodopsin's proton transfer mechanism on the proton uptake pathway between Asp-96 and the Schiff base in the M-to-N transition was determined. The slowdown of this transfer by removal of the proton donor in the Asp-96-->Asn mutant can be accelerated again by addition of small weak acid anions such as azide. Fourier-transform infrared experiments show in the Asp-96-->Asn mutant a transient protonation of azide bound to the protein in the M-to-N transition and, due to the addition of azide, restoration of the IR continuum band changes as seen in wild-type bR during proton pumping. The continuum band changes indicate fast proton transfer on the uptake pathway in a hydrogen-bonded network for wild-type bR and the Asp-96-->Asn mutant with azide. Since azide is able to catalyze proton transfer steps also in several kinetically defective bR mutants and in other membrane proteins, our finding might point to a general element of proton transfer mechanisms in proteins.

A mechanism for posttranslational modifications of proteins by yeast protein farnesyltransferase.

Protein farnesyltransferase catalyzes the alkylation of cysteine in C-terminal CaaX sequences of a variety of proteins, including Ras, nuclear lamins, large G proteins, and phosphodiesterases, by farnesyl diphosphate (FPP). These modifications enhance the ability of the proteins to associate with membranes and are essential for their respective functions. The enzyme-catalyzed reaction was studied by using a series of substrate analogs for FPP to distinguish between electrophilic and nucleophilic mechanisms for prenyl transfer. FPP analogs containing hydrogen, fluoromethyl, and trifluoromethyl substituents in place of the methyl at carbon 3 were evaluated as alternative substrates for alkylation of the sulfhydryl moiety in the peptide dansyl-GCVIA. The analogs were alternative substrates for the prenylation reaction and were competitive inhibitors against FPP. A comparison of kcat for FPP and the analogs with ksolv, the rate constants for solvolysis of related p-methoxybenzenesulfonate derivatives, indicated that protein prenylation occurred by an electrophilic mechanism.

Electrogenic light reactions in photosystem I: resolution of electron-transfer rates between the iron-sulfur centers.

Flash-induced voltage changes (electrogenic events) in photosystem I particles from spinach, oriented in a phospholipid layer, have been studied at room temperature on a time scale ranging from 1 micros to several seconds. A phospholipid layer containing photosystem I particles was adsorbed to a Teflon film separating two aqueous compartments. Voltage changes were measured across electrodes immersed in the compartments. In the absence of added electron donors and acceptors, a multiphasic voltage increase, associated with charge separation, was followed by a decrease, associated with charge recombination. Several kinetic phases were resolved: a rapid (<1 micros) increase, ascribed to electron transfer from the primary electron donor P700 to the iron-sulfur electron acceptor FB, was followed by a slower, biphasic increase with time constants of 30 and 200 micros. The 30-micros phase is assigned to electron transfer from FB to the iron-sulfur center FA. The voltage decrease had a time constant of 90 ms, ascribed to charge recombination from FA to P700. Upon chemical prereduction of FA and FB the 30- and 200-micros phases disappeared and the decay time constant was accelerated to 330 micros, assigned to charge recombination from the phylloquinone electron acceptor (A1) or the iron-sulfur center FX to P700.