(1/2197) Receptor-mediated targeting of fluorescent probes in living cells.
A strategy was developed to label specified sites in living cells with a wide selection of fluorescent or other probes and applied to study pH regulation in Golgi. cDNA transfection was used to target a single-chain antibody to a specified site such as an organelle lumen. The targeted antibody functioned as a high affinity receptor to trap cell-permeable hapten-fluorophore conjugates. Synthesized conjugates of a hapten (4-ethoxymethylene-2-phenyl-2-oxazolin-5-one, phOx) and fluorescent probes (Bodipy Fl, tetramethylrhodamine, fluorescein) were bound with high affinity (approximately 5 nM) and specific localization to the single-chain antibody expressed in the endoplasmic reticulum, Golgi, and plasma membrane of living Chinese hamster ovary cells. Using the pH-sensitive phOx-fluorescein conjugate and ratio imaging microscopy, pH was measured in the lumen of Golgi (pH 6.25 +/- 0.06). Measurements of pH-dependent vacuolar H+/ATPase pump activity and H+ leak in Golgi provided direct evidence that resting Golgi pH is determined by balanced leak-pump kinetics rather than the inability of the H+/ATPase to pump against an electrochemical gradient. Like expression of the green fluorescent protein, the receptor-mediated fluorophore targeting approach permits specific intracellular fluorescence labeling. A significant advantage of the new approach is the ability to target chemical probes with custom-designed spectral and indicator properties. (+info)
(2/2197) Interaction of purified human proteinase 3 (PR3) with reconstituted lipid bilayers.
Proteinase 3 (PR3), the major target autoantigen in Wegener's granulomatosis is a serine proteinase that is normally stored intracellularly in the primary granules of quiescent neutrophils and monocytes. Upon cell activation, a significant portion of this antigen is detected on the cell surface membrane. The nature of the association of PR3 with the membrane and its functional significance are unknown. We investigated the interaction of purified human PR3 with mixtures of zwitterionic (dimyristoyl-L-alpha-phosphatidylcholine, DMPC) and anionic (dimyristoyl-L-alpha-phosphatidylglycerol, DMPG) phospholipids in reconstituted lipid bilayers using differential scanning calorimetry and lipid photolabeling, and measured the affinity of this interaction using spectrophotometry. Two other primary granule constituents, human neutrophil elastase (HNE) and myeloperoxidase (MPO) were investigated for comparison. In calorimetric assays, using lipid vesicles of mixed DMPC/DMPG, increasing PR3 concentrations (protein/lipid molar ratio from 0 to 1 : 110) induced a significant decrease of the main chain transition enthalpy and a shift in chain melting temperatures which is indicative of partial insertion of PR3 into the hydrophobic region of the lipid membranes. This was confirmed by hydrophobic photolabeling using liposomes containing trace amounts of the photoactivable [125I]-labeled phosphatidylcholine analog TID-PC/16. The molar affinity of PR3, HNE, and MPO to lipid vesicles of different DMPC/DMPG ratios was then determined by spectrophotometry. At a DMPC/DMPG ratio of 1 : 1, molar affinities of PR3, Kd = 4.5 +/- 0.3 microm; HNE, 14.5 +/- 1.2 microm; and MPO, 50 +/- 5 microm (n = 3) were estimated. The lipid-associated PR3 exhibited two-fold lower Vmax and Km values, and its enzyme activity was slightly more inhibited (Ki) by the natural alpha1-proteinase inhibitor (alpha1-PI) or an autoantibody to PR3. (+info)
(3/2197) Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase.
To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase. (+info)
(4/2197) Identification, purification, and characterization of the rat liver golgi membrane ATP transporter.
Phosphorylation of secretory and integral membrane proteins and of proteoglycans also occurs in the lumen of the Golgi apparatus. ATP, the phosphate donor in these reactions, must first cross the Golgi membrane before it can serve as substrate. The existence of a specific ATP transporter in the Golgi membrane has been previously demonstrated in vitro using intact Golgi membrane vesicles from rat liver and mammary gland. We have now identified and purified the rat liver Golgi membrane ATP transporter. The transporter was purified to apparent homogeneity by a combination of conventional ion exchange, dye color, and affinity chromatography. An approximately 70,000-fold purification (2% yield) was achieved starting from crude rat liver Golgi membranes. A protein with an apparent molecular mass of 60 kDa was identified as the putative transporter by a combination of column chromatography, photoaffinity labeling with an analog of ATP, and native functional size determination on a glycerol gradient. The purified transporter appears to exist as a homodimer within the Golgi membrane, and when reconstituted into phosphatidylcholine liposomes, was active in ATP but not nucleotide sugar or adenosine 3'-phosphate 5'-phosphosulfate transport. The transport activity was saturable with an apparent Km very similar to that of intact Golgi vesicles. (+info)
(5/2197) Ectodomain cleavage and shedding of the type III transforming growth factor-beta receptor in lung membranes effect of temperature, ligand binding and membrane solubilization.
Previous studies from our laboratory [Philip, A. & O'Connor-McCourt, M. D. (1991) J. Biol. Chem. 266, 22290--22296] have shown that the lung exhibited the highest uptake of circulating [125I]-transforming growth factor-beta1 (TGF-beta1) on a per gram basis. This observation, together with the lack of information on TGF-beta receptor expression in the lung, prompted us to attempt to characterize TGF-beta receptors in this tissue. In the present report we show that the type III TGF-beta receptor is the most abundant TGF-beta binding protein in rat lung membranes and that it exhibits a 10-fold higher affinity for TGF-beta2 than for TGF-beta1. We observed that the majority of the type III receptor population in lung membranes is cleaved at a site in the central portion of the ectodomain, the resulting two fragments (95 kDa and 58 kDa) being held together by disulfide bonds. Furthermore, we demonstrate that a soluble form of the ectodomain of the type III receptor is shed from rat lung membranes in an efficient manner, with protease cleavage occurring at a site close to the transmembrane domain. This shedding is controllable by temperature, thus providing a system to study the mechanism of ectodomain release. Using this system, we show that the shedding is inhibited by prior ligand binding and by membrane solubilization. The identification of a membrane preparation which exhibits controllable and quantitative release of the type III receptor ectodomain provides a unique cell-free system for further studies of the mechanism of shedding of the type III TGF-beta receptor ectodomain. (+info)
(6/2197) Photoaffinity labeling and mass spectrometry identify ribosomal protein S3 as a potential target for hybrid polar cytodifferentiation agents.
The ability of a novel class of hybrid polar compounds (HPCs) to induce differentiation and consequent cessation of proliferation of transformed cells has led to their development as potential chemotherapeutic agents in the treatment of cancer. Suberoylanilide hydroxamic acid (SAHA) is a prototype of a family of hydroxamic acid based compounds (SAHA-like HPCs) that can, at micromolar concentrations, induce a variety of transformed cell lines to differentiate. The mechanism of action of the HPCs is not entirely understood. Searching for a cellular target of the SAHA-like HPCs, we synthesized a photoaffinity labeling reagent structurally based on SAHA, and probed for SAHA-binding proteins in murine erythroleukemia (MEL) cells. Photoaffinity labeling in cell free extracts identified a 32-kDa protein (p32) that was specifically labeled by the photoaffinity reagent. Cell fractionation assays localized p32 to the P100 fraction. p32 was partially purified and identified by mass spectrometry as the 40 S ribosomal protein S3. Expression of epitope-tagged S3 in bacterial lysates followed by photoaffinity labeling confirmed its specific labeling. Identification of a cytodifferentiation agent target may shed light on the mechanism by which the SAHA-like HPCs exert their antitumor effects. (+info)
(7/2197) Comparison of paclitaxel-, 5-fluoro-2'-deoxyuridine-, and epidermal growth factor (EGF)-induced apoptosis. Evidence for EGF-induced anoikis.
Epidermal growth factor (EGF), a hormone that stimulates proliferation of many cell types, induces apoptosis in some cell lines that overexpress the EGF receptor. To evaluate the mechanism of EGF-induced apoptosis, MDA-MB-468 breast cancer cells were examined by microscopy, flow cytometry, immunoblotting, enzyme assays, and affinity labeling after treatment with EGF, paclitaxel, or 5-fluoro-2'-deoxyuridine (5FUdR). Apoptosis induced by all three agents was accompanied by activation of caspases-3, -6, and -7, as indicated by disappearance of the corresponding zymogens from immunoblots, cleavage of substrate polypeptides in situ, and detection of active forms of these caspases in cytosol and nuclei using fluorogenic assays and affinity labeling. Further analysis indicated involvement of the cytochrome c/Apaf-1/caspase-9 pathway of caspase activation, but not the Fas/Fas ligand pathway. Interestingly, caspase activation was consistently lower after EGF treatment than after paclitaxel or 5FUdR treatment. Additional experiments revealed that the majority of cells detaching from the substratum after EGF (but not paclitaxel or 5FUdR) were morphologically normal and retained the capacity to readhere, suggesting that EGF-induced apoptosis involves cell detachment followed by anoikis. These observations not only indicate that EGF- and chemotherapy-induced apoptosis in this cell line involve the same downstream pathways but also suggest that detachment-induced apoptosis is responsible for the paradoxical antiproliferative effects of EGF. (+info)
(8/2197) Opening mechanism of a cyclic nucleotide-gated channel based on analysis of single channels locked in each liganded state.
Cyclic nucleotide-gated channels contain four subunits, each with a binding site for cGMP or cAMP in the cytoplasmic COOH-terminal domain. Previous studies of the kinetic mechanism of activation have been hampered by the complication that ligands are continuously binding and unbinding at each of these sites. Thus, even at the single channel level, it has been difficult to distinguish changes in behavior that arise from a channel with a fixed number of ligands bound from those that occur upon the binding and unbinding of ligands. For example, it is often assumed that complex behaviors like multiple conductance levels and bursting occur only as a consequence of changes in the number of bound ligands. We have overcome these ambiguities by covalently tethering one ligand at a time to single rod cyclic nucleotide-gated channels (Ruiz, ML., and J.W. Karpen. 1997. Nature. 389:389-392). We find that with a fixed number of ligands locked in place the channel freely moves between three conductance states and undergoes bursting behavior. Furthermore, a thorough kinetic analysis of channels locked in doubly, triply, and fully liganded states reveals more than one kinetically distinguishable state at each conductance level. Thus, even when the channel contains a fixed number of bound ligands, it can assume at least nine distinct states. Such complex behavior is inconsistent with simple concerted or sequential allosteric models. The data at each level of liganding can be successfully described by the same connected state model (with different rate constants), suggesting that the channel undergoes the same set of conformational changes regardless of the number of bound ligands. A general allosteric model, which postulates one conformational change per subunit in both the absence and presence of ligand, comes close to providing enough kinetically distinct states. We propose an extension of this model, in which more than one conformational change per subunit can occur during the process of channel activation. (+info)