The peptidoglycan layers surrounding bacterial membranes are essential for bacterial cell survival and provide an important target for antibiotics. Many antibiotics have mechanisms of action that involve binding to Lipid II, the prenyl chain-linked donor of the peptidoglycan building blocks. One of these antibiotics, the pore-forming peptide nisin uses Lipid II as a receptor molecule to increase its antimicrobial efficacy dramatically. Nisin is the first example of a targeted membranepermeabilizing peptide antibiotic. However, it was not known whether Lipid II functions only as a receptor to recruit nisin to bacterial membranes, thus increasing its specificity for bacterial cells, or whether it also plays a role in pore formation. We have developed a new method to produce large amounts of Lipid II and variants thereof so that we can address the role of the lipidlinked disaccharide in the activity of nisin. We show here that Lipid II is not only the receptor for nisin but an intrinsic component of the pore formed by nisin, and we present a new model for the pore complex that includes Lipid II.The cell wall is an essential structure of a bacterium, providing its shape and protecting it from bursting because of the high osmotic pressures of the cytoplasm. This wall is a threedimensional network built of identical subunits consisting of two amino sugars, N-acetylglucosamine (GlcNAc) and N-acetylmu-is attached to the carboxyl group of MurNAc. These subunits are assembled in the cytosol of bacteria using UDP-activated precursors on a special lipid carrier, undecaprenyl phosphate (for a review see Ref. 1). The integral membrane protein MraY and the peripherally membraneassociated MurG that synthesize the precursors Lipid I and II, respectively, are the key enzymes in the last two cytoplasmic steps in the formation of the subunits (Fig. 1). Subsequently, Lipid II is transported across the plasma membrane via an as of yet unknown mechanism. Thereafter, the subunits are polymerized and inserted into the pre-existing cell wall by means of the penicillin-binding proteins (for review see Ref.2). Numerous antibiotics target the cell wall synthesis, including a diverse group of antibiotics that bind to Lipid II. Perhaps the best known of these antibiotics is vancomycin, the antibiotic of last resort to treat MRSA infections (3). However, there are many others, including the polypeptide nisin, that kill cells by permeabilizing bacterial membranes. Efficient membrane permeabilization by nisin requires an interaction with Lipid II (4, 5). This designates nisin as the first example of a targeted poreforming peptide antibiotic.Two recent studies have shed light on the structural requirements within nisin for the interaction with Lipid II. A genetic approach indicated that the N terminus of nisin is involved in the interaction with Lipid II (6), and more recently we could map the binding interface toward specific residues in the N terminus using 15 N-labeled nisin (7). It is not clear yet what events lead to pore formation after the...

SUMMARY N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the Congenital Disorders of Glycosylation (CDG). We describe a new type of CDG caused by mutations in the steroid 5α-reductase type 3 (SRD5A3) gene. Patients have mental retardation, ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.

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The alarming rise of antimicrobial resistance requires antibiotics with unexploited mechanisms. Ideal templates could be antibiotics that target the peptidoglycan precursor lipid II, known as the bacterial Achilles heel, at an irreplaceable pyrophosphate group. Such antibiotics would kill multidrug-resistant pathogens at nanomolecular concentrations without causing antimicrobial resistance. However, due to the challenge of studying small membrane-embedded drug–receptor complexes in native conditions, the structural correlates of the pharmaceutically relevant binding modes are unknown. Here, using advanced highly sensitive solid-state NMR setups, we present a high-resolution approach to study lipid II-binding antibiotics directly in cell membranes. On the example of nisin, the preeminent lantibiotic, we show that the native antibiotic-binding mode strongly differs from previously published structures, and we demonstrate that functional hotspots correspond to plastic drug domains that are critical for the cellular adaptability of nisin. Thereby, our approach provides a foundation for an improved understanding of powerful antibiotics.

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