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The Sd(a)/Cad antigen reported on glycoconjugates of human tissues has an increasingly recognized wide impact on the physio-pathology of different biological systems. The last step of its biosynthesis relies on the enzymatic activity of the 1,4-N-acetylgalactosaminyltransferase-II (B4GALNT2), which shows the highest expression level in healthy colon. Previous studies reported the occurrence in human colonic cells of two B4GALNT2 protein isoforms that differ in the length of their cytoplasmic tail, the long isoform showing an extended 66-amino acid tail. We examined here, the subcellular distribution of the two B4GALNT2 protein isoforms in stably transfected colonic LS174T cells and in transiently transfected HeLa cells using fluorescence microscopy. While a similar subcellular distribution at the trans-Golgi cisternae level was observed for the two isoforms, our study pointed to an atypical subcellular localization of the long B4GALNT2 isoform into dynamic vesicles. We demonstrated a critical role of its extended cytoplasmic tail for its Golgi targeting and post-Golgi sorting and highlighted the existence of a newly described post-Golgi sorting signal as well as a previously undescribed fate of a Golgi glycosyltransferase.DatabaseThe proteins 1,4GalNAcT II, 1,4-GalT1, FucT I, FucT VI and ST3Gal IV are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4, whereas the corresponding human genes are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4 according to the HUGO nomenclature.

International audience; Significant efforts are currently deployed in order to find optimum GaN-on-silicon epitaxial structuresenabling outstanding DC performances beyond 1 kV while delivering low trapping effects. We report on thedevelopment of GaN-on-silicon heterostructures, designed and manufactured by the company EpiGaNtargeting 1200 V power applications. In particular, it is shown that the insertion of superlattices (SL) into thebuffer layers allows pushing the vertical breakdown voltage above 1200 V without generating additionaltrapping effects as compared to a more standard step-graded AlGaN-based epi-structure using similar totalbuffer thickness. DC characterizations by means of back-gating measurements of fabricated transistorsreflect both the enhancement of the breakdown voltage and the low trapping effects up to 1200 V. Theseresults show that a proper buffer optimization along with the insertion of SL pave the way to GaN-on-siliconlateral power transistors operating at 1200 V with low on-resistance and low trapping effects.Two buffer layers have been compared: one reference buffer without SL with a total thickness of5.5µm and one SL-based buffer with a total thickness of 5µm. Vertical and lateral breakdown voltages havebeen measured at various temperatures. A vertical breakdown of 1300 V at 0.01 A/cm² is reached for thestructure with SL compared to 1000 V for the more standard structure, even though the latter is slightlythicker. Furthermore, it can be noticed that a low vertical leakage current is observed up to a temperatureof 150°C for the SL heterostructure. The leakage current increase up to 150°C is significantly lower than thereference buffer without SL. Also, the lateral breakdown voltage (floating substrate) is well-above 2000V forboth structures when using large contact distances.The effects of buffer traps have been studied through substrate bias ramp measurements usingvarious sweep rates. The reference heterostructure without SL shows low trapping effect up to 800V with astrong trapping activation starting from 900 V. However, the optimized SL buffer uniformly delivers state-ofthe-art low trapping effects all the way to 1300 V.Electrical characterizations have been realized on 2x50 µm transistors with a gate length of 2 µm forseveral gate-drain distances (GD) for both structures. DC characteristics ID-VD and ID-VG reveal a low leakagecurrent and an excellent pinch-off behavior reflecting the absence of parasitic punch-through effect or gateleakage current. It can be noticed that low static on-state resistance (RON_STATIC) of about 5 mΩ/cm² with agate-drain distance of 5 µm has been obtained. For larger GD, RON_STATIC increases as expected but remains stillbelow 15 mΩ/cm² for a GD = 40 µm for instance. Trapping in the buffer (between the 2DEG and the substrate)can be studied by back-gating operation with high vertical drain-to-substrate potential. In this case, tuningthe transistor / substrate bias conditions allows in turn to distinguish the surface and bulk traps. In the presentstudy, this technique has been applied and fully confirms the low buffer trapping up to 1200 V despite residualsurface traps due to the processing that still needs to be optimized.

International audience; The aim of this work is to investigate the role of the epitaxial structure on the low trapping effects of GaN-on-silicon heterostructures use for power application. Structures with and without superlattices (SL) are analysed. In particular, it is shown that the insertion of SL into the buffer layers allows pushing the vertical breakdown voltage above 1200 V without generating additional trapping effects as compared to a more standard GaN-based epi-structure using similar total buffer thickness. A low trapping effect down to-1.2 kV has been observed with substrate ramp measurements. Indeed, we demonstrated that a structure with SL shows a reduction in the trapping effects with high vertical breakdown.

Sphingolipids are considered to play a key role in protein sorting and membrane trafficking. In melanocytic cells, sorting of lysosomal and melanosomal proteins requires the sphingolipid glucosylceramide (GlcCer). This sorting information is located in the lumenal domain of melanosomal proteins. We found that two processes dependent on lumenal pH, protein sialylation and lysosomal acid lipase (LAL) activity were aberrant in GM95 melanocyte cells, which do not produce glycosphingolipids. Using fluorescence lifetime imaging microscopy (FLIM), we found that the lumenal pH in the trans-Golgi network and lysosomes of wild-type melanocyte MEB4 cells are >1 pH unit lower than GM95 cells and fibroblasts. In addition to the lower pH found in vivo, the in vitro activity of the proton pump, the vacuolar-type H+-translocating ATPase (V-ATPase), was twofold higher in MEB4 compared to GM95 cells. The apparent Ki for inhibition of the V-ATPase by concanamycin A and archazolid A, which share a common binding site on the c-ring, was lower in glycosphingolipid-deficient GM95 cells. No difference between the MEB4 and GM95 cells was found for the V-ATPase inhibitors apicularen A and salicylihalimide. We conclude that hyperacidification in MEB4 cells requires glycosphingolipids and propose that low pH is necessary for protein sorting and melanosome biogenesis. Furthermore, we suggest that glycosphingolipids are indirectly involved in protein sorting and melanosome biogenesis by stimulating the proton pump, possibly through binding of GlcCer. These experiments establish, for the first time, a link between pH, glycosphingolipids and melanosome biogenesis in melanocytic MEB4 cells, to suggest a role for glycosphingolipids in hyperacidification in melanocytes.

International audience; Despite superior performance demonstrated by GaN-based power devices [1] as compared to conventional Si-and GaAs-based components, HEMT devices commercialization is still rather limited. This is because the mechanisms such as current collapse, self-heating, strain into the heterostructure, deep traps and surface states still plague the device reliability. Significant improvements have been achieved in GaN technology to efficiently reduce the current collapse, namely by improving the material quality and optimizing proper surface passivation like SiN. However, self-heating and strain relaxation effects strongly affect the device characteristics and result in device degradation [2]. Therefore, investigation and enhancement of the thermal stability of GaN devices are desirable for higher reliability. In this work, the thermal stability of strained AlGaN/GaN, AlN/GaN and unstrained InAlN/GaN heterostructures on low cost Si substrates are compared for the first time. Two-dimensional electron gas (2DEG) channel and Schottky contacts degradation have been studied by high temperature storage tests up to 900°C for various GaN-based configurations. In order to overcome the problem of gate degradation at high temperature, a novel MOSHEMT by means of a thermally grown oxide is proposed for both exponential gate leakage reduction and thermal stability enhancement.

Glycosphingolipids, a family of heterogeneous lipids with biophysical properties conserved from fungi to mammals, are key components of cellular membranes. Because of their tightly packed backbone, they have the ability to associate with other sphingolipids and cholesterol to form microdomains called lipid rafts, with which a variety of proteins associate. These microdomains are thought to originate in the Golgi apparatus, where most sphingolipids are synthesized, and are enriched at the plasma membrane. They are involved in an increasing number of processes, including sorting of proteins by allowing selectivity in intracellular membrane transport. Apart from being involved in recognition and signaling on the cell surface, glycosphingolipids may fulfill unexpected roles on the cytosolic surface of cellular membranes. © 2004 Elsevier Ltd. All rights reserved.

UDP-galactose reaches the Golgi lumen through the UDP-galactose transporter (UGT) and is used for the galactosylation of proteins and lipids. Ceramides and diglycerides are galactosylated within the endoplasmic reticulum by the UDP-galactose: ceramide galactosyltransferase. It is not known how UDP-galactose is transported from the cytosol into the endoplasmic reticulum. We transfected ceramide galactosyltransferase cDNA into CHOlec8 cells, which have a defective UGT and no endogenous ceramide galactosyltransferase. Cotransfection with the human UGT1 greatly stimulated synthesis of lactosylceramide in the Golgi and of galactosylceramide in the endoplasmic reticulum. UDP-galactose was directly imported into the endoplasmic reticulum because transfection with UGT significantly enhanced synthesis of galactosylceramide in endoplasmic reticulum membranes. Subcellular fractionation and double label immunofluorescence microscopy showed that a sizeable fraction of ectopically expressed UGT and ceramide galactosyltransferase resided in the endoplasmic reticulum of CHOlec8 cells. The same was observed when UGT was expressed in human intestinal cells that have an endogenous ceramide galactosyltransferase. In contrast, in CHOlec8 singly transfected with UGT 1, the transporter localized exclusively to the Golgi complex. UGT and ceramide galactosyltransferase were entirely detergent soluble and form a complex because they could be coimmunoprecipitated. We conclude that the ceramide galactosyltransferase ensures a supply of UDP-galactose in the endoplasmic reticulum lumen by retaining UGT in a molecular complex.