Novel method to study neutron capture of 235U and 238U simultaneously at keV energies.

The neutron capture cross sections of the main uranium isotopes, (235)U and (238)U, were measured simultaneously for keV energies, for the first time by combining activation technique and atom counting of the reaction products using accelerator mass spectrometry. New data, with a precision of 3%-5%, were obtained from mg-sized natural uranium samples for neutron energies with an equivalent Maxwell-Boltzmann distribution of kT ∼ 25 keV and for a broad energy distribution peaking at 426 keV. The cross-section ratio of (235)U(n,γ)/(238)U(n,γ) can be deduced in accelerator mass spectrometry directly from the atom ratio of the reaction products (236)U/(239)U, independent of any fluence normalization. Our results confirm the values at the lower band of existing data. They serve as important anchor points to resolve present discrepancies in nuclear data libraries as well as for the normalization of cross-section data used in the nuclear astrophysics community for s-process studies.

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis.

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from kT=5-100 keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Measurement of the 60Fe(n, gamma)61Fe Cross Section at Stellar Temperatures.

Observations of galactic gamma-ray activity have challenged the current understanding of nucleosynthesis in massive stars. Recent measurements of (60)Fe abundances relative to ;{26}Al;{g} have underscored the need for accurate nuclear information concerning the stellar production of (60)Fe. In light of this motivation, a first measurement of the stellar (60)Fe(n, gamma)(61)Fe cross section, the predominant destruction mechanism of (60)Fe, has been performed by activation at the Karlsruhe Van de Graaff accelerator. Results show a Maxwellian averaged cross section at kT = 25 keV of 9.9 +/-_{1.4(stat)};{2.8(syst)}mbarn, a significant reduction in uncertainty with respect to existing theoretical discrepancies. This result will serve to significantly constrain models of (60)Fe nucleosynthesis in massive stars.

Stellar (n,gamma) cross section of 62Ni.

The 62Ni(n,gamma)63Ni(t(1/2)=100+/-2 yr) reaction plays an important role in the control of the flow path of the slow neutron-capture (s) nucleosynthesis process. We have measured for the first time the total cross section of this reaction for a quasi-Maxwellian (kT=25 keV) neutron flux. The measurement was performed by fast-neutron activation, combined with accelerator mass spectrometry to detect directly the 63Ni product nuclei. The experimental value of 28.4+/-2.8 mb, fairly consistent with a recent calculation, affects the calculated net yield of 62Ni itself and the whole distribution of nuclei with 62

Neutron capture cross section measurement of 151Sm at the CERN neutron time of flight facility (n_TOF).

The151Sm(n,gamma)152Sm cross section has been measured at the spallation neutron facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The new facility combines excellent resolution in neutron time-of-flight, low repetition rates, and an unsurpassed instantaneous luminosity, resulting in rather favorable signal/background ratios. The 151Sm cross section is of importance for characterizing neutron capture nucleosynthesis in asymptotic giant branch stars. At a thermal energy of kT=30 keV the Maxwellian averaged cross section of this unstable isotope (t(1/2)=93 yr) was determined to be 3100+/-160 mb, significantly larger than theoretical predictions.

Evidence for M1 scissors resonances built on the levels in the quasicontinuum of 163Dy.

Spectra of two-step gamma cascades following the thermal 162Dy(n,gamma)163Dy reaction have been measured. Distinct peaklike structures observed at the midpoints of these spectra are interpreted as a manifestation of the low-energy isovector M1 vibrational mode of excited 163Dy nuclei.

Elastic alpha-12C scattering and the 12C(alpha,gamma)16O E2 S factor.

Angular distributions of 12C(alpha,alpha)12C have been measured for E(alpha) = 2.6-8.2 MeV, at angles from 24 to 166, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of gamma12 = 0.47 +/- 0.06 MeV(1/2) for the Ex = 6.917 MeV (2+) state in 16O(a = 5.5 fm). The dependence of the chi2 surface on the interaction radius a has been investigated and a deep minimum is found at a = 5.42(+0.16)(-0.27) fm. Using this value of gamma12, radiative alpha capture and 16N beta-delayed alpha-decay data, the S factor is calculated at E(c.m.) = 300 keV to be S(E2)(300) = 53(+13)(-18) keV b for destructive interference between the subthreshold resonance tail and the ground state E2 direct capture.

Neutron capture on (180)Ta(m): clue for an s-process origin of nature's rarest isotope.

The neutron capture cross section of (180)Ta(m) has been measured in the keV range, yielding a stellar average of 1465+/-100 mb at kT = 30 keV. Though the sample contained only 6.7 mg (180)Ta(m) (at an enrichment of 5.5%), the few capture events could be separated from much larger backgrounds by a unique combination of high efficiency, good energy resolution, and high granularity of the Karlsruhe 4 pi BaF(2) detector. A detailed s-process analysis based on this first experimental value indicates that (180)Ta(m) is predominantly of s-process origin.

ERGIC-53 and traffic in the secretory pathway.

The ER-Golgi intermediate compartment (ERGIC) marker ERGIC-53 is a mannose-specific membrane lectin operating as a cargo receptor for the transport of glycoproteins from the ER to the ERGIC. Lack of functional ERGIC-53 leads to a selective defect in secretion of glycoproteins in cultured cells and to hemophilia in humans. Beyond its interest as a transport receptor, ERGIC-53 is an attractive probe for studying numerous aspects of protein trafficking in the secretory pathway, including traffic routes, mechanisms of anterograde and retrograde traffic, retention of proteins in the ER, and the function of the ERGIC. Understanding these fundamental processes of cell biology will be crucial for the elucidation and treatment of many inherited and acquired diseases, such as cystic fibrosis, Alzheimer's disease and viral infections.

The lectin ERGIC-53 is a cargo transport receptor for glycoproteins.

Soluble secretory proteins are transported from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC) in vesicles coated with COP-II coat proteins. The sorting of secretory cargo into these vesicles is thought to involve transmembrane cargo-receptor proteins. Here we show that a cathepsin-Z-related glycoprotein binds to the recycling, mannose-specific membrane lectin ERGIC-53. Binding occurs in the ER, is carbohydrate- and calcium-ion-dependent and is affected by untrimmed glucose residues. Binding does not, however, require oligomerization of ERGIC-53, although oligomerization is required for exit of ERGIC-53 from the ER. Dissociation of ERGIC-53 occurs in the ERGIC and is delayed if ERGIC-53 is mislocalized to the ER. These results strongly indicate that ERGIC-53 may function as a receptor facilitating ER-to-ERGIC transport of soluble glycoprotein cargo.

Protein targeting to endoplasmic reticulum by dilysine signals involves direct retention in addition to retrieval.

Dilysine signals confer localization of type I membrane proteins to the endoplasmic reticulum (ER). According to the prevailing model these signals target proteins to the ER by COP I-mediated retrieval from post-ER compartments, whereas the actual retention mechanism in the ER is unknown. We expressed chimeric membrane proteins with a C-terminal -Lys-Lys-Ala-Ala (KKAA) or -Lys-Lys-Phe-Phe (KKFF) dilysine signal in Lec-1 cells. Unlike KKFF constructs, which had access to post-ER compartments, the KKAA chimeras were localized to the ER by confocal microscopy and were neither processed by cis-Golgi-specific enzymes in vivo nor included into ER-derived transport vesicles in an in vitro budding assay, suggesting that KKAA-bearing proteins are permanently retained in the ER. The ER localization was nonsaturable and exclusively mediated by the dilysine signal because mutating the lysines to alanines led to cell surface expression of the chimeras. Although the KKAA signal avidly binds COP I in vitro, the ER retention by this signal does not depend on intact COP I in vivo because it was not affected in an epsilon-COP-deficient cell line. We propose that dilysine ER targeting signals can mediate ER retention in addition to retrieval.

A novel direct interaction of endoplasmic reticulum with microtubules.

The positioning and dynamics of organelles in eukaryotic cells critically depend on membrane-cytoskeleton interactions. Motor proteins play an important role in the directed movement of organelle membranes along microtubules, but the basic mechanism by which membranes stably interact with the microtubule cytoskeleton is largely unknown. Here we report that p63, an integral membrane protein of the reticular subdomain of the rough endoplasmic reticulum (ER), binds microtubules in vivo and in vitro. Overexpression of p63 in cell culture led to a striking rearrangement of the ER and to concomitant bundling of microtubules along the altered ER. Mutational analysis of the cytoplasmic domain of p63 revealed two determinants responsible for these changes: an ER rearrangement determinant near the N-terminus and a central microtubule-binding region. The two determinants function independently of one another as indicated by deletion experiments. A peptide corresponding to the cytoplasmic tail of p63 promoted microtubule polymerization in vitro. p63 is the first identified integral membrane protein that can link a membrane organelle directly to microtubules. By doing so, it may contribute to the positioning of the ER along microtubules.

Mistargeting of the lectin ERGIC-53 to the endoplasmic reticulum of HeLa cells impairs the secretion of a lysosomal enzyme.

ERGIC-53, a homo-oligomeric recycling protein associated with the ER-Golgi intermediate compartment (ERGIC), has properties of a mannose-selective lectin in vitro, suggesting that it may function as a transport receptor for glycoproteins in the early secretory pathway. To investigate if ERGIC-53 is involved in glycoprotein secretion, a mutant form of this protein was generated that is incapable of leaving the ER. If expressed in HeLa cells in a tetracycline-inducible manner, this mutant accumulated in the ER and retained the endogenous ERGIC-53 in this compartment, thus preventing its recycling. Mistargeting of ERGIC-53 to the ER did not alter the gross morphology of the early secretory pathway, including the distribution of beta'-COP. However, it impaired the secretion of one major glycoprotein, identified as the precursor of the lysosomal enzyme cathepsin C, while overexpression of wild-type ERGIC-53 had no effect on glycoprotein secretion. Transport of two other lysosomal enzymes and three post-Golgi membrane glycoproteins was unaffected by inactivating the recycling of ERGIC-53. The results suggest that the recycling of ERGIC-53 is required for efficient intracellular transport of a small subset of glycoproteins, but it does not appear to be essential for the majority of glycoproteins.

Carbohydrate-mediated Golgi to cell surface transport and apical targeting of membrane proteins.

Polarized expression of most epithelial plasma membrane proteins is achieved by selective transport from the Golgi apparatus or from endosomes to a specific cell surface domain. In Madin-Darby canine kidney (MDCK) cells, basolateral sorting generally depends on distinct cytoplasmic targeting determinants. Inactivation of these signals often resulted in apical expression, suggesting that apical transport of transmembrane proteins occurs either by default or is mediated by widely distributed characteristics of membrane glycoproteins. We tested the hypothesis of N-linked carbohydrates acting as apical targeting signals using three different membrane proteins. The first two are normally not glycosylated and the third one is a glycoprotein. In all three cases, N-linked carbohydrates were clearly able to mediate apical targeting and transport. Cell surface transport of proteins containing cytoplasmic basolateral targeting determinants was not significantly affected by N-linked sugars. In the absence of glycosylation and a basolateral sorting signal, the reporter proteins accumulated in the Golgi complex of MDCK as well as CHO cells, indicating that efficient transport from the Golgi apparatus to the cell surface is signal-mediated in polarized and non-polarized cells.

The recycling of ERGIC-53 in the early secretory pathway. ERGIC-53 carries a cytosolic endoplasmic reticulum-exit determinant interacting with COPII.

Further investigation of the targeting of the intracellular membrane lectin endoplasmic reticulum (ER)-Golgi intermediate compartment-53 (ERGIC-53) by site-directed mutagenesis revealed that its lumenal and transmembrane domains together confer ER retention. In addition we show that the cytoplasmic domain is required for exit from the ER indicating that ERGIC-53 carries an ER-exit determinant. Two phenylalanines at the C terminus are essential for ER-exit. Thus, ERGIC-53 contains determinants for ER retention as well as anterograde transport which, in conjunction with a dilysine ER retrieval signal, control the continuous recycling of ERGIC-53 in the early secretory pathway. In vitro binding studies revealed a specific phenylalanine-dependent interaction between an ERGIC-53 cytosolic tail peptide and the COPII coat component Sec23p. These results suggest that the ER-exit of ERGIC-53 is mediated by direct interaction of its cytosolic tail with the Sec23p.Sec24p complex of COPII and that protein sorting at the level of the ER occurs by a mechanism similar to receptor-mediated endocytosis or Golgi to ER retrograde transport.