Redox-sensitive loops D and E regulate NADP(H) binding in domain III and domain I-domain III interactions in proton-translocating Escherichia coli transhydrogenase.

Membrane-bound transhydrogenases are conformationally driven proton-pumps which couple an inward proton translocation to the reversible reduction of NADP+ by NADH (forward reaction). This reaction is stimulated by an electrochemical proton gradient, Delta p, presumably through an increased release of NADPH. The enzymes have three domains: domain II spans the membrane, while domain I and III are hydrophilic and contain the binding sites for NAD(H) and NADP(H), respectively. Separately expressed domain I and III together catalyze a very slow forward reaction due to tightly bound NADP(H) in domain III. With the aim of examining the mechanistic role(s) of loop D and E in domain III and intact cysteine-free Escherichia coli transhydrogenase by cysteine mutagenesis, the conserved residues beta A398, beta S404, beta I406, beta G408, beta M409 and beta V411 in loop D, and residue beta Y431 in loop E were selected. In addition, the previously made mutants betaD392C and betaT393C in loop D, and beta G430C and beta A432C in loop E, were included. All loop D and E mutants, especially beta I406C and beta G430C, showed increased ratios between the rates of the forward and reverse reactions, thus approaching that of the wild-type enzyme. Determination of values indicated that the former increase was due to a strongly increased dissociation of NADPH caused by an altered conformation of loops D and E. In contrast, the cysteine-free G430C mutant of the intact enzyme showed the same inhibition of both forward and reverse rates. Most domain III mutants also showed a decreased affinity for domain I. The results support an important and regulatory role of loops D and E in the binding of NADP(H) as well as in the interaction between domain I and domain III.     

Notch function in the vasculature: insights from zebrafish, mouse and man

Vascular development entails multiple cell-fate decisions to specify a diverse array of vascular structures. Notch proteins are signaling receptors that regulate cell-fate determination in a variety of cell types. The finding that Notch genes are robustly expressed in the vasculature suggests roles for Notch in guiding endothelial and associated mural cells through the myriad of cell-fate decisions needed to form the vasculature. In fact, mice with defects in genes encoding Notch, Notch ligands, and components of the Notch signaling cascade invariably display vascular defects. Human Notch genes are linked to Alagille's Syndrome, a developmental disorder with vascular defects, and CADASIL, a cerebral arteriopathy. Studies in zebrafish, mice and humans indicate that Notch works in conjunction with other angiogenic pathways to pattern and stabilize the vasculature. Here, we will focus on established functions for Notch in vascular remodeling and arterial/venous specification and more speculative roles in vascular homeostasis and organ-specific angiogenesis.     

Rap2B-dependent stimulation of phospholipase C-epsilon by epidermal growth factor receptor mediated by c-Src phosphorylation of RasGRP3

Receptor tyrosine kinase regulation of phospholipase C-epsilon (PLC-epsilon), which is under the control of Ras-like and Rho GTPases, was studied with HEK-293 cells endogenously expressing PLC-coupled epidermal growth factor (EGF) receptors. PLC and Ca(2+) signaling by the EGF receptor, which activated both PLC-gamma1 and PLC-epsilon, was specifically suppressed by inactivation of Ras-related GTPases with clostridial toxins and expression of dominant-negative Rap2B. EGF induced rapid and sustained GTP loading of Rap2B, binding of Rap2B to PLC-epsilon, and Rap2B-dependent translocation of PLC-epsilon to the plasma membrane. GTP loading of Rap2B by EGF was inhibited by chelation of intracellular Ca(2+) and expression of lipase-inactive PLC-gamma1 but not of PLC-epsilon. Expression of RasGRP3, a Ca(2+)/diacylglycerol-regulated guanine nucleotide exchange factor for Ras-like GTPases, but not expression of various other exchange factors enhanced GTP loading of Rap2B and PLC/Ca(2+) signaling by the EGF receptor. EGF induced tyrosine phosphorylation of RasGRP3, but not RasGRP1, apparently caused by c-Src; inhibition of c-Src interfered with EGF-induced Rap2B activation and PLC stimulation. Collectively, these data suggest that the EGF receptor triggers activation of Rap2B via PLC-gamma1 activation and tyrosine phosphorylation of RasGRP3 by c-Src, finally resulting in stimulation of PLC-epsilon.     

Purification and partial characterization of feline alpha1-proteinase inhibitor (falpha1-PI) and the development and validation of a radioimmunoassay for the measurement of falpha1-PI in serum

Alpha(1)-proteinase inhibitor (alpha(1)-PI) of the domestic cat (Felis catus) was purified from serum and a radioimmunoassay (RIA) for the measurement of feline alpha(1)-PI concentration in serum was developed and validated. Feline alpha(1)-PI (falpha(1)-PI) was isolated using ammonium sulfate precipitation, anion-exchange, size-exclusion, ceramic hydroxyapatite, and hydrophobic interaction chromatography. The molecular weight of falpha(1)-PI was estimated at 57,000 and the relative molecular mass (M(r)) was determined to be approximately 54.5 kDa. Isoelectric focusing revealed four bands with isoelectric points (pI) between 4.3 and 4.5. The N-terminal amino acid sequence of the first 19 residues was Glu-Gly-Leu-Gln-Gly-Ala-Ala-Val-Gln-Glu-Thr-Val-Ala-Ser-Gln-His-Asp-Gln-Glu. Antiserum against feline alpha(1)-PI was raised in rabbits. Tracer was produced by iodination ((125)I) of feline alpha(1)-PI using the chloramine T method. A radioimmunoassay was established and validated by determination of sensitivity, dilutional parallelism, spiking recovery, intra-assay variability, and inter-assay variability. A control range for serum feline alpha(1)-PI concentration was established from 50 healthy cats using the central 95th percentile. The sensitivity of the assay was 0.042 mg/ml. Observed to expected ratios for serial dilutions ranged from 105% to 141.18% for four different serum samples at dilutions of 1 in 35,000, 1 in 70,000, 1 in 140,000 and 1 in 280,000. Observed to expected ratios for spiking recovery ranged from 88.14% to 152.17% for four different serum samples and five different spiking concentrations. Coefficients of variation for four different serum samples were 4.57%, 6.45%, 8.52%, and 4.27% for intra-assay variability and 6.88%, 9.57%, 7.44%, and 9.94% for inter-assay variability. The reference range was established as 0.25-0.6 mg/ml. In summary, feline alpha(1)-PI was successfully purified from serum using a rapid and efficient method. The radioimmunoassay described here is sensitive, linear, accurate, precise, and reproducible and will facilitate further studies of the physiological or potential pathological role of alpha(1)-PI in cats.     

High salt solution structure of a left-handed RNA double helix

Right-handed RNA duplexes of (CG)_n sequence undergo salt-induced helicity reversal, forming left-handed RNA double helices (Z-RNA). In contrast to the thoroughly studied Z-DNA, no Z-RNA structure of natural origin is known. Here we report the NMR structure of a half-turn, left-handed RNA helix (CGCGCG)₂ determined in 6 M NaClO₄. This is the first nucleic acid motif determined at such high salt. Sequential assignments of non-exchangeable proton resonances of the Z-form were based on the hitherto unreported NOE connectivity path [H6_(n)-H5′/H5″_(n)-H8_(n+1)-H1′_(n+1)-H6_(n+2)] found for left-handed helices. Z-RNA structure shows several conformational features significantly different from Z-DNA. Intra-strand but no inter-strand base stacking was observed for both CpG and GpC steps. Helical twist angles for CpG steps have small positive values (4–7°), whereas GpC steps have large negative values (−61°). In the full-turn model of Z-RNA (12.4 bp per turn), base pairs are much closer to the helix axis than in Z-DNA, thus both the very deep, narrow minor groove with buried cytidine 2′-OH groups, and the major groove are well defined. The 2′-OH group of cytidines plays a crucial role in the Z-RNA structure and its formation; 2′-O-methylation of cytidine, but not of guanosine residues prohibits A to Z helicity reversal.     

Sequential dimerization of human zipcode-binding protein IMP1 on RNA: a cooperative mechanism providing RNP stability

Active cytoplasmic RNA localization depends on the attachment of RNA-binding proteins that dictate the destination of the RNA molecule. In this study, we used an electrophoretic mobility-shift assay in combination with equilibrium and kinetic analyses to characterize the assembly of the human zipcode-binding protein IMP1 on targets in the 3′-UTR from Igf-II mRNA and in H19 RNA. In both cases, two molecules of IMP1 bound to RNA by a sequential, cooperative mechanism, characterized by an initial fast step, followed by a slow second step. The first step created an obligatory assembly intermediate of low stability, whereas the second step was the discriminatory event that converted a putative RNA target into a ‘locked’ stable RNP. The ability to dimerize was also observed between members of the IMP family of zipcode-binding proteins, providing a multitude of further interaction possibilities within RNP granules and with the localization apparatus.