Structural basis for the inhibition of the biosynthesis of biotin by the antibiotic amiclenomycin

The antibiotic amiclenomycin blocks the biosynthesis of biotin by inhibiting the pyridoxal-phosphate-dependent enzyme diaminopelargonic acid synthase. Inactivation of the enzyme is stereoselective, i.e. the cis isomer of amiclenomycin is a potent inhibitor, whereas the trans isomer is much less reactive. The crystal structure of the complex of the holoenzyme and amiclenomycin at 1.8 A resolution reveals that the internal aldimine linkage between the cofactor and the side chain of the catalytic residue Lys-274 is broken. Instead, a covalent bond is formed between the 4-amino nitrogen of amiclenomycin and the C4' carbon atom of pyridoxal-phosphate. The electron density for the bound inhibitor suggests that aromatization of the cyclohexadiene ring has occurred upon formation of the covalent adduct. This process could be initiated by proton abstraction at the C4 carbon atom of the cyclohexadiene ring, possibly by the proximal side chain of Lys-274, leading to the tautomer Schiff base followed by the removal of the second allylic hydrogen. The carboxyl tail of the amiclenomycin moiety forms a salt link to the conserved residue Arg-391 in the substrate-binding site. Modeling suggests steric hindrance at the active site as the determinant of the weak inhibiting potency of the trans isomer.     

GD3 recruits reactive oxygen species to induce cell proliferation and apoptosis in human aortic smooth muscle cells

Sialic acid containing glycosphingolipids (gangliosides) are expressed on the surface of all mammalian cells and have been implicated in regulating various biological phenomena; however, the detailed signaling mechanisms involved in this process are not known. We report here a novel aspect of disialoganglioside, GD3-mediated regulation of cell proliferation and cell death via the recruitment of reactive oxygen species (ROS). A low concentration (2.5-10 microm) of GD3, incubated with human aortic smooth muscle cells for a short period of time (10-30 min), stimulates superoxide generation via the activation of both NADPH oxidase and NADH oxidase activity. This leads to downstream signaling leading to cell proliferation and apoptosis. However, [(3)H]GD3 incubated with the cells under such conditions was found in a trypsin-sensitive fraction that was separable from endogenous GD3. The exact mechanism causing ROS generation and downstream signaling remains to be elucidated. The uptake of GD3 was accompanied by a 2.5-fold stimulation in the activity of mitogen-activated protein (MAP) kinase and 5-fold stimulation in cell proliferation. Preincubation of cells with membrane-permeable antioxidants, pyrrolidine dithiocarbamate, and N-acetylcysteine abrogated the superoxide generation and cell proliferation. In contrast, at higher concentrations (50-200 microm) GD3 inhibited the generation of superoxides but markedly stimulated the generation of nitric oxide (NO) (10-fold compared with control). This in turn stimulated mitochondrial cytochrome c release and intrachromosomal DNA fragmentation, which lead to apoptosis. In sum, at a low concentration, GD3 recruits superoxides to activate p44 MAPK and stimulates cell proliferation. In contrast, at high concentrations GD3 recruits nitric oxide to scavenge superoxide radicals that triggered signaling events that led to apoptosis. These observations might have relevance in regard to the potential role of GD3 in aortic smooth muscle cell proliferation and apoptosis that may contribute to plaque rupture in atherosclerosis.     

PIP(2)-PDZ domain binding controls the association of syntenin with the plasma membrane

PDZ proteins organize multiprotein signaling complexes. According to current views, PDZ domains engage in protein-protein interactions. Here we show that the PDZ domains of several proteins bind phosphatidylinositol 4,5-bisphosphate (PIP(2)). High-affinity binding of syntenin to PIP(2)-containing lipid layers requires both PDZ domains of this protein. Competition and mutagenesis experiments reveal that the protein and the PIP(2) binding sites in the PDZ domains overlap. Overlay assays indicate that the two PDZ domains of syntenin cooperate in binding to cognate peptides and PIP(2). Experiments on living cells demonstrate PIP(2)-dependent and peptide-dependent modes of plasma membrane association of the PDZ domains of syntenin. These observations suggest that local changes in phosphoinositide concentration control the association of PDZ proteins with their target receptors at the plasma membrane.     

SMN-mediated assembly of RNPs: a complex story

Although many RNA-protein complexes or ribonucleoproteins (RNPs) assemble spontaneously in vitro, little is known about how they form in the environment of a living cell. Insight into RNP assembly has come unexpectedly from functional analyses of the survival motor neuron (SMN) protein, a gene product that is affected in the neuromuscular disease spinal muscular atrophy. These studies show that the assembly of spliceosomal U-rich small nuclear RNPs in vivo depends on the activity of two large protein complexes, one of which contains the SMN protein. These complexes might also facilitate the assembly of other cellular RNPs.     

Stability of interleukin 6, soluble interleukin 6 receptor,interleukin 10 and CC16 in human serum

The stability of interleukin 6 (IL-6), its soluble receptor (sIL-6R), IL-10 and CC16 or uteroglobin (an endogenous cytokine inhibitor) in human serum was examined using an accelerated stability testing protocol according to the Arrhenius equation. Further, the effect of time delay between blood sampling and sample processing, clotting temperature and repeated freeze-thaw cycles on serum levels of these proteins were determined. Paired serum samples were stored at 4 degrees C, 20 degrees C, 30 degrees C and 40 degrees C for 1 to 21 days. We found that IL-6 and CC16 concentrations did not change at 4 degrees C, 20 degrees C and 30 degrees C. Interleukin-6 concentrations significantly declined after 11 days at 40 degrees C. The concentrations of sIL-6R and IL-10 did not change at 4 degrees C but significantly decreased at 20 degrees C (after 21 and 14 days respectively), 30 degrees C and 40 degrees C (after 1 day at both temperatures for both cytokines). Arrhenius-plots indicated that sIL-6R and IL-10 are stable for at least several years at -20 degrees C and -70 degrees C, respectively. Since their relative stability, no Arrhenius-plot could be calculated for IL-6 and CC16. The concentrations of the proteins examined were not significantly altered by repeated freeze-thaw cycles, nor by extended clotting times at 4 degrees C or 20 degrees C. We conclude that serum samples for the determination of IL-6, sIL-6R and CC16 can be stored at -20 degrees C for several years, but for IL-10 determinations, storage at -70 degrees C is recommended.     

Substrate-based design of reversible Pin1 inhibitors

Human Pin1, a peptidyl-prolyl cis/trans isomerase with high specificity to -Ser/Thr(PO(3)H(2))-Pro- motifs, is required for cell cycle progression. In an effort to design reversible Pin1 inhibitors by using a substrate structure based approach, a panel of peptides were applied to systematically analyze the minimal structural requirements for Pin1 substrate recognition. Pin1 catalysis (k(cat)/K(m) < 5 mM(-1) s(-1)) for Ala-Pro, Ser-Pro, and Ser(PO(3)H(2))-Pro was detected using direct UV-visible spectrophotometric detection of prolyl isomerization, while weak competitive inhibition of Pin1 by these dipeptides was observed (K(i) > 1 mM). Substrates with chain lengths extending from either the P2 to P1' or the P1 to P2' subsite gave k(cat)/K(m) values of 100 mM(-1) s(-1) for Ala-Ser(PO(3)H(2))-Pro and 38 mM(-1) s(-1) for Ser(PO(3)H(2))-Pro-Arg. For both Pin1 and its yeast homologue Ess1, the optimal subsite recognition elements comprise five amino acid residues with the essential Ser(PO(3)H(2)) in the middle position. The resulting substrate Ac-Ala-Ala-Ser(PO(3)H(2))-Pro-Arg-NH-4-nitroanilide possesses a very low cis/trans interconversion barrier in the presence of either Pin1 or Ess1, with k(cat)/K(m) = 9300 mM(-1) s(-1) and 12000 mM(-1) s(-1), respectively. The D-Ser(PO(3)H(2)) residue preceding proline could serve as a substrate-deactivating determinant without compromising ground state affinity. Similarly, substitution of the amide bond preceding proline with a thioxo amide bond produces a potent inhibitor. Pin1 is reversibly inhibited by such substrate analogue inhibitors with IC(50) values in the low micromolar range. The D-amino acid containing inhibitor also exhibits remarkable stability against phosphatase activity in cell lysate.