Transmembrane transport of peptidoglycan precursors across model and bacterial membranes

Abstract

SummaryTranslocation of the peptidoglycan precursor Lipid II across the cytoplasmic membrane is a key step in bacterial cell wall synthesis, but hardly understood. Using NBD-labelled Lipid II, we showed by fluorescence and TLC assays that Lipid II transport does not occur spontaneously and is not induced by the presence of single spanning helical transmembrane peptides that facilitate transbilayer movement of membrane phospholipids. MurG catalysed synthesis of Lipid II from Lipid I in lipid vesicles also did not result in membrane translocation of Lipid II. These findings demonstrate that a specialized protein machinery is needed for transmembrane movement of Lipid II. In line with this, we could demonstrate Lipid II translocation in isolated Escherichia coli inner membrane vesicles and this transport could be uncoupled from the synthesis of Lipid II at low temperatures. The transport process appeared to be independent from an energy source (ATP or proton motive force). Additionally, our studies indicate that translocation of Lipid II is coupled to transglycosylation activity on the periplasmic side of the inner membrane.

Lipid II: A central component in bacterial cell wall synthesis and a target for antibiotics

“…MurG was found to catalyze the synthesis of Lipid II from its precursor Lipid I in model membranes, but the resulting Lipid II remained on the synthesis side of the bilayer, excluding the possibility that MurG itself is solely responsible for rapid translocation [21]. Consistent with this finding was the observation that Lipid II synthesis in the bacterial system is not obligatorily coupled to translocation, but must be mediated by a specialized protein machinery [21].…”

“…Consistent with this finding was the observation that Lipid II synthesis in the bacterial system is not obligatorily coupled to translocation, but must be mediated by a specialized protein machinery [21]. Transport was found to be ATP or pmf independent and appeared to be coupled to transglycosylation activities on the periplasmic side of the inner membrane [21].…”

“…Studies using model membranes demonstrated that translocation across a lipid bilayer is extremely slow and, in contrast to membrane phospholipids, is not stimulated by the presence of transmembrane helices [21], suggesting a more specific proteinmediated mechanism that is involved in phospholipid translocation.…”

Vancomycin: ligand recognition, dimerization and super‐complex formation

“…2) is comprised of the glycopeptide N-acetyl-muramyl-Nacetyl-glucosaminyl-L-Ala-c-D-Glu-L-Lys-D-Ala-D-Ala attached to a C55 hydrophobic tail via a pyrophosphate linker [7]. After synthesis, lipid II is translocated to the periplasmic side of the plasma membrane where the peptidoglycan group is cleaved from the lipid tail and incorporated into the nascent cell wall [8,9]. Vancomycin primarily interacts with the peptidic region of lipid II and the interaction between vancomycin and a range of lipid II peptidoglycan analogs has been studied using a variety of techniques.…”

The Expanding Role of Lipid II as a Target for Lantibiotics

“…At this stage the intact lipid II monomer is translocated across the plasma membrane and is delivered to the periplasmic (exterior) side of the bacterial cell for incorporation into the growing peptidoglycan network. While the mechanism by which this translocation occurs is unknown, recent evidence has shown that it is not a spontaneous process and may be coupled to transglycosylation on the periplasmic cell surface [8].…”