Arabidopsis RNA-binding protein UBA2a relocalizes into nuclear speckles in response to abscisic acid

The Arabidopsis thaliana RNA binding protein UBA2a is the closest homologue of the Vicia faba AKIP1 (56% identity). Like AKIP1, UBA2a is a constitutively-expressed nuclear protein and in response to ABA it is also reorganized within the nucleus in "speckles" suggesting a possible role of this protein in the regulation of mRNA metabolism during ABA signaling. AKIP1 interacts with, and is phosphorylated by, the upstream ABA-activated protein kinase AAPK. We have investigated if a pathway similar to that described in Vicia faba also exists in Arabidopsis. Our results showed that despite the resemblance between the corresponding Vicia and Arabidopsis proteins, it appears that the function of UBA2a is independent of OST1 phosphorylation.     

Glycanogenomics: a qPCR-approach to investigate biological glycan function

As an indirect approach towards glycan structures, qRT-PCR analyses using the ΔΔC_T method were performed to investigate changes in expression levels of heparan sulfate-synthesising enzymes of stimulated and unstimulated HMVECs. We chose NDSTs as early enzymes initiating sulfation and 3OSTs which act late generating specific binding sites. Major changes in expression patterns were found for the NDST3 and 3OST1 isoforms. Both enzymes were down-regulated 7- and 6-fold, respectively, following TNF-α stimulation, and 3.5- and 7.6-fold following LPS-stimulation suggesting a common restructuring process of HS in inflammation leading to a less diverse sulfation pattern. Immunostaining of TNF-α-stimulated cells using a phage display-derived antibody specific for 3-O-sulfation and unsulfated regions of HS resulted in significant fluorescence changes between unstimulated and stimulated.     

Natural products as modulators of the nuclear receptors and metabolic sensors LXR,FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets.This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.     

Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.     

Screening for OST deficiencies in unsolved CDG-I patients

Congenital Disorders of Glycosylation (CDG) are a group of inherited disorders caused by deficiencies in glycosylation. Since 1980, 14 CDG type I (CDG-I) defects have been identified in the endoplasmic reticulum, all affecting the assembly of the oligosaccharide precursor. However, the number of unsolved CDG-I (CDG-Ix) patients displaying protein hypoglycosylation in combination with an apparently normal assembly of the oligosaccharide precursor is currently expanding.We hypothesized that the hypoglycosylation observed in some of these patients could be caused by a deficiency in the transfer of the oligosaccharide precursor onto protein, a reaction catalyzed by the oligosaccharyltransferase (OST) complex. For this purpose, the different subunits of the OST complex were screened in 27 CDG-Ix patients for whom structural analysis of the lipid-linked oligosaccharides revealed a normal level and intact structure of the oligosaccharide precursor. Among these 27 patients, one was identified with a homozygous missense mutation (c.1121G > A; p.G374D) in the ribophorin 2 (RPN2) subunit of the OST complex. The pathogenic nature of this mutation remains unproven due to the complexity of tackling a possible OST defect.     

The structure of Arabidopsis thaliana OST1 provides insights into the kinase regulation mechanism in response to osmotic stress

SnRK [SNF1 (sucrose non-fermenting-1)-related protein kinase] 2.6 [open stomata 1 (OST1)] is well characterized at molecular and physiological levels to control stomata closure in response to water-deficit stress. OST1 is a member of a family of 10 protein kinases from Arabidopsis thaliana (SnRK2) that integrates abscisic acid (ABA)-dependent and ABA-independent signals to coordinate the cell response to osmotic stress. A subgroup of protein phosphatases type 2C binds OST1 and keeps the kinase dephosphorylated and inactive. Activation of OST1 relies on the ABA-dependent inhibition of the protein phosphatases type 2C and the subsequent self-phosphorylation of the kinase. The OST1 ABA-independent activation depends on a short sequence motif that is conserved among all the members of the SnRK2 family. However, little is known about the molecular mechanism underlying this regulation. The crystallographic structure of OST1 shows that ABA-independent regulation motif stabilizes the conformation of the kinase catalytically essential α C helix, and it provides the basis of the ABA-independent regulation mechanism for the SnRK2 family of protein kinases.     

Membrane insertion and topology of the translocating chain-associating membrane protein (TRAM)

The translocating chain-associating membrane protein (TRAM) is a glycoprotein involved in the translocation of secreted proteins into the endoplasmic reticulum (ER) lumen and in the insertion of integral membrane proteins into the lipid bilayer. As a major step toward elucidating the structure of the functional ER translocation/insertion machinery, we have characterized the membrane integration mechanism and the transmembrane topology of TRAM using two approaches: photocross-linking and truncated C-terminal reporter tag fusions. Our data indicate that TRAM is recognized by the signal recognition particle and translocon components, and suggest a membrane topology with eight transmembrane segments, including several poorly hydrophobic segments. Furthermore, we studied the membrane insertion capacity of these poorly hydrophobic segments into the ER membrane by themselves. Finally, we confirmed the main features of the proposed membrane topology in mammalian cells expressing full-length TRAM.     

Protein O-Mannosyltransferases Associate with the Translocon to Modify Translocating Polypeptide Chains

O-Mannosylation and N-glycosylation are essential protein modifications that are initiated in the endoplasmic reticulum (ER). Protein translocation across the ER membrane and N-glycosylation are highly coordinated processes that take place at the translocon-oligosaccharyltransferase (OST) complex. In analogy, it was assumed that protein O-mannosyltransferases (PMTs) also act at the translocon, however, in recent years it turned out that prolonged ER residence allows O-mannosylation of un-/misfolded proteins or slow folding intermediates by Pmt1-Pmt2 complexes. Here, we reinvestigate protein O-mannosylation in the context of protein translocation. We demonstrate the association of Pmt1-Pmt2 with the OST, the trimeric Sec61, and the tetrameric Sec63 complex in vivo by co-immunoprecipitation. The coordinated interplay between PMTs and OST in vivo is further shown by a comprehensive mass spectrometry-based analysis of N-glycosylation site occupancy in pmtΔ mutants. In addition, we established a microsomal translation/translocation/O-mannosylation system. Using the serine/threonine-rich cell wall protein Ccw5 as a model, we show that PMTs efficiently mannosylate proteins during their translocation into microsomes. This in vitro system will help to unravel mechanistic differences between co- and post-translocational O-mannosylation.