Alg14 Recruits Alg13 to the Cytoplasmic Face of the Endoplasmic Reticulum to Form a Novel Bipartite UDP-N-acetylglucosamine Transferase Required for the Second Step of N-Linked Glycosylation

N-linked glycosylation requires the synthesis of an evolutionarily conserved lipid-linked oligosaccharide (LLO) precursor that is essential for glycoprotein folding and stability. Despite intense research, several of the enzymes required for LLO synthesis have not yet been identified. Here we show that two poorly characterized yeast proteins known to be required for the synthesis of the LLO precursor, GlcNAc2-PP-dolichol, interact to form an unusual hetero-oligomeric UDP-GlcNAc transferase. Alg13 contains a predicted catalytic domain, but lacks any membrane-spanning domains. Alg14 spans the membrane but lacks any sequences predicted to play a direct role in sugar catalysis. We show that Alg14 functions as a membrane anchor that recruits Alg13 to the cytosolic face of the ER, where catalysis of GlcNAc2-PP-dol occurs. Alg13 and Alg14 physically interact and under normal conditions, are associated with the ER membrane. Overexpression of Alg13 leads to its cytosolic partitioning, as does reduction of Alg14 levels. Concomitant Alg14 overproduction suppresses this cytosolic partitioning of Alg13, demonstrating that Alg14 is both necessary and sufficient for the ER localization of Alg13. Further evidence for the functional relevance of this interaction comes from our demonstration that the human ALG13 and ALG14 orthologues fail to pair with their yeast partners, but when co-expressed in yeast can functionally complement the loss of either ALG13 or ALG14. These results demonstrate that this novel UDP-GlcNAc transferase is a unique eukaryotic ER glycosyltransferase that is comprised of at least two functional polypeptides, one that functions in catalysis and the other as a membrane anchor.

Biosynthesis of lipid-linked oligosaccharides in Saccharomyces cerevisiae: Alg13p and Alg14p form a complex required for the formation of GlcNAc(2)-PP-dolichol

N-Glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to man. Here we identify and characterize two essential yeast proteins having homology to bacterial glycosyltransferases, designated Alg13p and Alg14p, as being required for the formation of GlcNAc(2)-PP-dolichol (Dol), the second step in the biosynthesis of the unique lipid-linked core oligosaccharide. Down-regulation of each gene led to a defect in protein N-glycosylation and an accumulation of GlcNAc(1)-PP-Dol in vivo as revealed by metabolic labeling with [(3)H]glucosamine. Microsomal membranes from cells repressed for ALG13 or ALG14, as well as detergent-solubilized extracts thereof, were unable to catalyze the transfer of N-acetylglucosamine from UDP-GlcNAc to [(14)C]GlcNAc(1)-PP-Dol, but did not impair the formation of GlcNAc(1)-PP-Dol or GlcNAc-GPI. Immunoprecipitating Alg13p from solubilized extracts resulted in the formation of GlcNAc(2)-PP-Dol but required Alg14p for activity, because an Alg13p immunoprecipitate obtained from cells in which ALG14 was down-regulated lacked this activity. In Western blot analysis it was demonstrated that Alg13p, for which no well defined transmembrane segment has been predicted, localizes both to the membrane and cytosol; the latter form, however, is enzymatically inactive. In contrast, Alg14p is exclusively membrane-bound. Repression of the ALG14 gene causes a depletion of Alg13p from the membrane. By affinity chromatography on IgG-Sepharose using Alg14-ZZ as bait, we demonstrate that Alg13-myc co-fractionates with Alg14-ZZ. The data suggest that Alg13p associates with Alg14p to a complex forming the active transferase catalyzing the biosynthesis of GlcNAc(2)-PP-Dol.

Mapping the targeted membrane pore formation mechanism by solution NMR: the nisin Z and lipid II interaction in SDS micelles

Nisin is an example of type-A lantibiotics that contain cyclic lanthionine rings and unusual dehydrated amino acids. Among the numerous pore-forming antimicrobial peptides, type-A lantibiotics form an unique family of post-translationally modified peptides. Via the recognition of cell wall precursor lipid II, nisin has the capacity to form pores against Gram-positive bacteria with an extremely high activity in the nanomolar (nM) range. Here we report a high-resolution NMR spectroscopy study of nisin/lipid II interactions in SDS micelles as a model membrane system in order to elucidate the mechanism of molecular recognition at residue level. The binding to lipid II was studied through (15)N-(1)H HSQC titration, backbone amide proton temperature coefficient analysis, and heteronuclear (15)N[(1)H]-NOE relaxation dynamics experiments. Upon the addition of lipid II, significant changes were monitored in the N-terminal part of nisin. An extremely low amide proton temperature coefficient (Delta delta/Delta T) was found for the amide proton of Ala3 (> -0.1 ppb/K) in the complex form. This suggests tight hydrogen bonding and/or isolation from the bulk solvent for this residue. Large chemical shift perturbations were also observed in the first two rings. In contrast, the C-terminal part of nisin was almost unaffected. This part of the molecule remains flexible and solvent-exposed. On the basis of our results, a multistep pore-forming mechanism is proposed. The N-terminal part of nisin first binds to lipid II, and a subsequent structural rearrangement takes place. The C-terminal part of nisin is possibly responsible for the activation of the pore formation. In light of the emerging antibiotic resistance problems, an understanding of the specific recognition mechanism of nisin with lipid II at the residue specific level may therefore aid in the development of novel antibiotics.

Phytanyl-pyrophosphate-linked substrate for a bacterial alpha-mannosyltransferase

The biochemical characterization of bacterial glycosyltransferases involved in the assembly of cell-wall-associated polysaccharides is often hindered by the lack of the appropriate undecaprenyl-pyrophosphate-linked acceptor substrate. In order to find a suitable synthetic substrate for the alpha1,3-mannosyltransferase AceA from Acetobacter xylinum, phytanyl-pyrophosphate-linked cellobiose was prepared. In the presence of GDP-[14C]mannose and recombinant AceA, the phytanyl-pyrophosphate-linked cellobiose afforded a 14C-labeled trisaccharide that was sensitive to alpha-mannosidase degradation in a fashion analogous to the natural undecaprenyl-pyrophosphate-linked cellobiose substrate. These results suggest that phytanyl-pyrophosphate-linked oligosaccharides may be useful substrates for other important bacterial glycosyltransferases.

The lantibiotic nisin, a special case or not?

Nisin is a 34-residue-long peptide belonging to the group A lantibiotics with antimicrobial activity against Gram-positive bacteria. The presence of dehydrated residues and lanthionine rings (thioether bonds) in nisin, imposing structural restrains on the peptide, make it an interesting case for studying the mode of action. In addition, the relatively high activity (nM range) of nisin against Gram-positive bacteria indicates that nisin may be a special case in the large family of pore-forming peptides antibiotics. In this review, we attempted to dissect the mode of action of nisin concentrating on studies that used model membranes or biological membranes. The picture that emerges suggests that in model membrane systems, composed of only phospholipids, nisin behaves similar to the antimicrobial peptide magainin, albeit with an activity that is much lower as compared to its activity towards biological membranes. This difference can be contributed to a missing factor which nisin needs for its high activity. Novel results have identified the factor as Lipid II, a precursor in the bacterial cell wall synthesis. The special high affinity interaction of nisin with Lipid II resulting in high activity and the active role of Lipid II in the pore-formation process make nisin a special case.

Regulation of the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol, feedback and product inhibition

The assembly of the core oligosaccharide region of asparagine-linked glycoproteins proceeds by means of the dolichol pathway. The first step of this pathway, the reaction of dolichol phosphate with UDP-GlcNAc to form N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-dolichol), is under investigation as a possible site of metabolic regulation. This report describes feedback inhibition of this reaction by the second intermediate of the pathway, N-acetylglucosaminyl-N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-GlcNAc-P-P-dolichol), and product inhibition by GlcNAc-P-P-dolichol itself. These influences were revealed when the reactions were carried out in the presence of showdomycin, a nucleoside antibiotic, present at concentrations that block the de novo formation of GlcNAc-GlcNAc-P-P-dolichol but not that of GlcNAc-P-P-dolichol. The apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol under basal conditions were 4.4 and 2.8 microM, respectively. Inhibition was also observed under conditions where mannosyl-P-dolichol (Man-P-dol) stimulated the biosynthesis of GlcNAc-P-P-dolichol; the apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol were 2.2 and 11 microM, respectively. Kinetic analysis of the types of inhibition indicated competitive inhibition by GlcNAc-P-P-dolichol toward the substrate UDP-GlcNAc and non-competitive inhibition toward dolichol phosphate. Inhibition by GlcNAc-GlcNAc-P-P-dolichol was uncompetitive toward UDP-GlcNAc and competitive toward dolichol phosphate. A model is presented for the kinetic mechanism of the synthesis of GlcNAc-P-P-dolichol. GlcNAc-P-P-dolichol also exerts a stimulatory effect on the biosynthesis of Man-P-dol, i.e. a reciprocal relationship to that previously observed between these two intermediates of the dolichol pathway. This network of inhibitory and stimulatory influences may be aspects of metabolic control of the pathway and thus of glycoprotein biosynthesis in general.

Two related proteolipids and dolichol-linked oligosaccharides accumulate in motor neuron degeneration mice (mnd/mnd), a model for neuronal ceroid lipofuscinosis

In this study, we show that two biochemical markers of neuronal ceroid lipofuscinoses (NCLs) are present in a mutant mouse (mnd/mnd) that exhibits symptoms of the disease. Subunit c of the mitochondrial F1F0-ATP synthase, a proteolipid that accumulates in storage bodies of most forms of NCL and several animal models, is dramatically increased in mnd/mnd mouse brain, kidney, liver, heart, and pancreas. Interestingly, another related proteolipid, subunit c of the vacuolar H(+)-ATPase, also accumulates in several mnd/mnd tissues. The molar ratio of the vacuolar subunit c to the F1F0 subunit c is approximately one to two in enriched storage bodies from brain. The relative accumulation of the vacuolar subunit c correlates with its abundance in normal tissues. It appears in decreasing amounts in brain, kidney, and liver and is not detected in heart or pancreas. Aged mice and two mutant mouse lines, juvenile bare (jb) and mucopolysaccharidosis, type VII (gusmps), did not accumulate either of these proteolipids. Dolichol-linked oligosaccharides also accumulate in NCLs and are increased 17-fold in mnd/mnd mouse brain. Thus, mnd/mnd mice seem to be an excellent model for NCLs since they not only share clinical signs and histopathology, but also two biochemical markers. The accumulation of the vacuolar subunit c in this model may prove to be a marker for distinguishing different forms of NCLs.

Stored dolichyl pyrophosphoryl oligosaccharides in Batten disease

Each of the 3 childhood forms of Batten disease, juvenile (JB), late-infantile (LIB), and infantile (IB), have abnormally high brain concentrations of dolichyl pyrophosphoryl oligosaccharides (Dol-PP-OS). In this study, the carbohydrate portions of Dol-PP-OS were analysed: in JB and LIB, they range in size from Man2GlcNAc2 to Glc3Man9GlcNAc2, predominant components being Man5-7GlcNAc2 and Glc3Man7GlcNAc2. In IB, they range from Man6-9GlcNAc2, no glucose containing oligosaccharides being identified. In Batten disease, the main subcellular location of Dol-PP-OS is within storage material, where it represents up to 7% of the dry weight. [3H]-Mannose incorporation experiments with cultured fibroblasts show that synthesis of Dol-PP-OS in JB is normal. We infer that the glycosylation intermediate Glc3Man9GlcNAc2-PP-dolichol is synthesised normally within the endoplasmic reticulum in Batten disease, but that catabolic derivatives accumulate within the lysosomes. It is unclear whether this process is central to the pathogenesis of the disease, though in IB a defect in the release of mannose residues from Dol-PP-OS is a distinct possibility.