Equilibrium binding assays reveal the elevated stoichiometry and salt dependence of the interaction between full-length human sex-determining region on the Y chromosome (SRY) and DNA

In an effort to better define the molecular mechanism of the functional specificity of human sex-determining region on the Y chromosome (SRY), we have carried out equilibrium binding assays to study the interaction of the full-length bacterial-expressed protein with a DNA response element derived from the CD3epsilon gene enhancer. These assays are based on the observation of the fluorescence anisotropy of a fluorescein moiety covalently bound to the target oligonucleotide. The low anisotropy value due to the fast tumbling of the free oligonucleotide in solution increases substantially upon binding the protein to the labeled target DNA. Our results indicate that the full-length human wild-type SRY (SRY(WT)) forms a complex of high stoichiometry with its target DNA. Moreover, we have demonstrated a strong salt dependence of both the affinity and specificity of the interaction. We have also addressed the DNA bending properties of full-length human SRY(WT) in solution by fluorescence resonance energy transfer and revealed that maximal bending is achieved with a protein to DNA ratio significantly higher than the classical 1:1. Oligomerization thus appears, at least in vitro, to be tightly coupled to SRY-DNA interactions. Alteration of protein-protein interactions observed for the mutant protein SRY(Y129N), identified in a patient presenting with 46,XY sex reversal, suggests that oligomerization may play an important role in vivo as well.     

Protein kinase C stimulates the acid-sensing ion channel ASIC2a via the PDZ domain-containing protein PICK1

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular protons. They are expressed in central and sensory neurons where they are involved in neuromodulation and in pain perception. Recently, the PDZ domain-containing protein PICK1 (protein interacting with C-kinase) has been shown to interact with ASIC1a and ASIC2a, raising the possibility that protein kinase C (PKC) could regulate ASICs. We now show that the amplitude of the ASIC2a current, which was only modestly increased ( approximately +30%) by the PKC activator 1-oleyl-2-acetyl-sn-glycerol (OAG, 50 microm) in the absence of PICK1, was strongly potentiated ( approximately +300%) in the presence of PICK1. This PICK1-dependent regulatory effect was inhibited in the presence of a PKC inhibitory peptide and required the PDZ domain of PICK1 as well as the PDZ-binding domain of ASIC2a. We have also shown the direct PICK1-dependent phosphorylation of ASIC2a by [(32)P]phosphate labeling and immunoprecipitation and identified a major phosphorylation site, (39)TIR, on the N terminus part of ASIC2a. The OAG-induced increase in ASIC2a current amplitude did not involve any change in the unitary conductance of the ASIC2a channel, whether co-expressed with PICK1 or not. These data provide the first demonstration of a regulation of ASICs by protein kinase phosphorylation and its potentiation by the partner protein PICK1.     

Stimulation of human flap endonuclease 1 by human immunodeficiency virus type 1 integrase: possible role for flap endonuclease 1 in 5'-end processing of human immunodeficiency virus type 1 integration intermediates

Human immunodeficiency virus type 1 (HIV-1) DNA integration intermediates consist of viral and host DNA segments separated by a 5-nucleotide gap adjacent to a 5'-AC unpaired dinucleotide. These short-flap (pre-repair) integration intermediates are structurally similar to DNA loci undergoing long-patch base excision repair in mammalian cells. The cellular proteins flap endonuclease 1 (FEN-1), proliferating cell nuclear antigen, replication factor C, DNA ligase I and DNA polymerase delta are required for the repair of this type of DNA lesion. The role of FEN-1 in the base excision repair pathway is to cleave 5'-unpaired flaps in forked structures so that DNA ligase can seal the single-stranded breaks that remain following gap repair. The rate of excision by FEN-1 of 5'-flaps from short- and long-flap oligonucleotide substrates that mimic pre- and post-repair HIV-1 integration intermediates, respectively, and the effect of HIV-1 integrase on these reactions were examined in the present study. Cleavage of 5'-flaps by FEN-1 in pre-repair HIV-1 integration intermediates was relatively inefficient and was further decreased 3-fold by HIV-1 integrase. The rate of removal of 5'-flaps by FEN-1 from post-repair HIV-1 integration intermediates containing relatively long (7-nucleotide) unpaired 5'-tails and short (1-nucleotide) gaps was increased 3-fold relative to that seen with pre-repair substrates and was further stimulated 5- to 10-fold by HIV-1 integrase. Overall, post-repair structures were cleaved 18 times more effectively in the presence of HIV-1 integrase than pre-repair structures. The site of cleavage was 1 or 2 nucleotides 3' of the branch point and was unaffected by HIV-1 integrase. Integrase alone had no detectable activity in removing 5'-flaps from either pre- or post-repair substrates.     

A novel 2'-(N-methylpyridinium acetate) prodrug of paclitaxel induces superior antitumor responses in preclinical cancer models

The development of novel strategies for the treatment of malignancies by successful intervention in advanced stage disease is a major challenge in oncology. We tested the hypothesis that this can be achieved by the rational design of taxoid onium salts modified at C-7 and C-2' positions. The characterization of these molecules revealed a dramatically improved water solubility and prodrug behavior in plasma. Specifically, all compounds released parental paclitaxel with half-lives ranging from 0.9 to 180 min. In the absence of plasma, only the 2'-(N-methylpyridinium acetate) derivative of paclitaxel (2'-MPA-paclitaxel) revealed a complete abrogation of paclitaxel specific microtubule assembly disassembly dynamics and a 3 log reduction in cellular binding, indicating that reversible blockage of the C-2' position by methylpyridinium acetate yields a true paclitaxel prodrug. Structure/activity profiles of all compounds in tissue culture revealed cytotoxicity effective at picomolar concentrations with a panel of 16 cancer cell lines in contrast to 4 nonmalignant cell lines. Importantly, the decisive cytotoxic potential observed in vitro for all compounds correlated only with in vivo findings for 2'-MPA-paclitaxel. Specifically, the 2'-MPA-paclitaxel prodrug induced regression of primary tumors in three xenograft models of nonsmall cell lung carcinoma, ovarian carcinoma and prostate cancer, in contrast to ineffective C-7 derivatives and parental paclitaxel. At the same time, a reduced systemic toxicity of 2'-MPA-paclitaxel was observed in contrast to a far more toxic parental paclitaxel. Taken together, these findings demonstrate that the 2'-MPA-paclitaxel prodrug is a promising new candidate for cancer therapy.     

Multiple active states and oligomerization of CCR5 revealed by functional properties of monoclonal antibodies

CC-chemokine receptor 5 (CCR5) is the principal coreceptor for macrophage-tropic strains of human immunodeficiency virus type 1 (HIV-1). We have generated a set of anti-CCR5 monoclonal antibodies and characterized them in terms of epitope recognition, competition with chemokine binding, receptor activation and trafficking, and coreceptor activity. MC-4, MC-5, and MC-7 mapped to the amino-terminal domain, MC-1 to the second extracellular loop, and MC-6 to a conformational epitope covering multiple extracellular domains. MC-1 and MC-6 inhibited regulated on activation normal T cell expressed and secreted (RANTES), macrophage inflammatory polypeptide-1beta, and Env binding, whereas MC-5 inhibited macrophage inflammatory polypeptide-1beta and Env but not RANTES binding. MC-6 induced signaling in different functional assays, suggesting that this monoclonal antibody stabilizes an active conformation of CCR5. Flow cytometry and real-time confocal microscopy showed that MC-1 promoted strong CCR5 endocytosis. MC-1 but not its monovalent isoforms induced an increase in the transfer of energy between CCR5 molecules. Also, its monovalent isoforms bound efficiently, but did not internalize the receptor. In contrast, MC-4 did not prevent RANTES binding or subsequent signaling, but inhibited its ability to promote CCR5 internalization. These results suggest the existence of multiple active conformations of CCR5 and indicate that CCR5 oligomers are involved in an internalization process that is distinct from that induced by the receptor's agonists.     

Computer-aided design of a PDZ domain to recognize new target sequences

PDZ domains are small globular domains that recognize the last 4-7 amino acids at the C-terminus of target proteins. The specificity of the PDZ-ligand recognition is due to side chain-side chain interactions, as well as the positioning of an alpha-helix involved in ligand binding. We have used computer-aided protein design to produce mutant versions of a Class I PDZ domain that bind to novel Class I and Class II target sequences both in vitro and in vivo, thus providing an alternative to primary antibodies in western blotting, affinity chromatography and pull-down experiments. Our results suggest that by combining different backbone templates with computer-aided protein design, PDZ domains could be engineered to specifically recognize a large number of proteins.     

Solution structure of two xenoantigens: alpha Gal-LacNAc and alpha Gal-Lewis X

Organ hyperacute rejection, a phenomenon occurring during discordant xenotransplantation, is due to the recognition of an oligosaccharide epitope by human xenoreactive natural antibodies. In addition to the alpha Gal(1-3)beta Gal(1-4)GlcNAc trisaccharide, a fucosylated structure, alpha Gal-Lewis X, has been shown to be recognized by the antibodies. Both the trisaccharide and the tetrasaccharide have been synthesized by chemical methods. A complete nuclear magnetic resonance characterization of the two compounds has been performed, including the measurements of two-dimensional nuclear Overhauser effect spectroscopy data. Molecular dynamics simulations were run for several ns in the presence of explicit water molecules. The combination of experimental and theoretical approaches revealed the effect of an additional fucose residue on the conformational behavior of the xenoantigen. This branched fucose strongly rigidifies the N-acetyllactosamine. The effect on the alpha Gal(1-3)Gal fragment is less marked. In the presence of fucose, the terminal alpha Gal residue can still adopt two different conformations, but the equilibrium populations are modified.     

Adaptation of aequorin functional assay to high throughput screening

AequoScreen, a cellular aequorin-based functional assay, has been optimized for luminescent high-throughput screening (HTS) of G protein-coupled receptor (GPCRs). AequoScreen is a homogeneous assay in which the cells are loaded with the apoaequorin cofactor coelenterazine, diluted in assay buffer, and injected into plates containing the samples to be tested. A flash of light is emitted following the calcium increase resulting from the activation of the GPCR by the sample. Here we have validated a new plate reader, the Hamamatsu Photonics FDSS6000, for HTS in 96- and 384-well plates with CHO-K1 cells stably coexpressing mitochondrial apoaequorin and different GPCRs (AequoScreen cell lines). The acquisition time, plate type, and cell number per well have been optimized to obtain concentration-response curves with 4000 cells/well in 384-well plates and a high signal:background ratio. The FDSS6000 and AequoScreen cell lines allow reading of twenty 96- or 384-well plates in 1 h with Z' values of 0.71 and 0.78, respectively. These results bring new insights to functional assays, and therefore reinforce the interest in aequorin-based assays in a HTS environment.