Interaction between glycolipid MPIase and proteinaceous factors during protein integration into the cytoplasmic membrane of E. coli

Structures, functions, and syntheses of glycero-glycophospholipids

Biological membranes consist of integral and peripheral protein-associated lipid bilayers. Although constituent lipids vary among cells, membrane lipids are mainly classified as phospholipids, glycolipids, and sterols. Phospholipids are further divided into glycerophospholipids and sphingophospholipids, whereas glycolipids are further classified as glyceroglycolipids and sphingoglycolipids. Both glycerophospholipids and glyceroglycolipids contain diacylglycerol as the common backbone, but their head groups differ. Most glycerolipids have polar head groups containing phosphate esters or sugar moieties. However, trace components termed glycero-glycophospholipids, each possessing both a phosphate ester and a sugar moiety, exist in membranes. Recently, the unique biological activities of glycero-glycophospholipids have attracted considerable attention. In this review, we describe the structure, distribution, function, biosynthesis, and chemical synthetic approaches of representative glycero-glycophospholipids-phosphatidylglucoside (PtdGlc) and enterobacterial common antigen (ECA). In addition, we introduce our recent studies on the rare glycero-glyco"pyrophospho"lipid, membrane protein integrase (MPIase), which is involved in protein translocation across biomembranes.

Phosphatidylglycerol Is the Lipid Donor for Synthesis of Phospholipid-Linked Enterobacterial Common Antigen

The Gram-negative outer membrane (OM) is an asymmetric bilayer with phospholipids in its inner leaflet and mainly lipopolysaccharide (LPS) in its outer leaflet and is largely impermeable to many antibiotics. In Enterobacterales (e.g., Escherichia, Salmonella, Klebsiella, and Yersinia), the outer leaflet of the OM also contains phosphoglyceride-linked enterobacterial common antigen (ECA_PG). This molecule consists of the conserved ECA carbohydrate linked to diacylglycerol-phosphate (DAG-P) through a phosphodiester bond. ECA_PG contributes to the OM permeability barrier and modeling suggests that it may alter the packing of LPS molecules in the OM. Here, we investigate, in Escherichia coli K-12, the reaction synthesizing ECA_PG from ECA precursor linked to an isoprenoid carrier to identify the lipid donor that provides the DAG-P moiety to ECA_PG. Through overexpression of phospholipid biosynthesis genes, we observed alterations expected to increase levels of phosphatidylglycerol (PG) increased the synthesis of ECA_PG, whereas alterations expected to decrease levels of PG decreased the synthesis of ECA_PG. We discovered depletion of PG levels in strains that could synthesize ECA_PG, but not other forms of ECA, causes additional growth defects, likely due to the buildup of ECA precursor on the isoprenoid carrier inhibiting peptidoglycan biosynthesis. Our results demonstrate ECA_PG can be synthesized in the absence of the other major phospholipids (phosphatidylethanolamine and cardiolipin). Overall, these results conclusively demonstrate PG is the lipid donor for the synthesis of ECA_PG and provide a key insight into the reaction producing ECA_PG. In addition, these results provide an interesting parallel to lipoprotein acylation, which also uses PG as its DAG donor. IMPORTANCE The Gram-negative outer membrane is a permeability barrier preventing cellular entry of antibiotics. However, outer membrane biogenesis pathways are targets for small molecule development. Here, we investigate the synthesis of a form of enterobacterial common antigen (ECA), ECA_PG, found in the outer membrane of Enterobacterales (e.g., Escherichia, Salmonella, and Klebsiella). ECA_PG consists of the conserved ECA carbohydrate unit linked to diacylglycerol-phosphate-ECA is a phospholipid headgroup. The details of the reaction forming this molecule from polymerized ECA precursor are unknown. We determined the lipid donor providing the phospholipid moiety is phosphatidylglycerol. Understanding the synthesis of outer membrane constituents such as ECA_PG provides the opportunity for development of molecules to increase outer membrane permeability, expanding the antibiotics available to treat Gram-negative infections.

Coordinated upregulation of two CDP-diacylglycerol synthases, YnbB and CdsA, is essential for cell growth and membrane protein export in the cold

YnbB is a paralogue of CdsA, a CDP-diacylglycerol synthase. While the cdsA gene is essential, the ynbB gene is dispensable. So far, no phenotype of ynbB knockout has been observed. We found that a ynbB knockout strain acquired cold-sensitivity on growth under CdsA-limited conditions. We found that MPIase, a glycolipid involved in protein export, is cold-upregulated to facilitate protein export in the cold, by increasing the mRNA levels of not only CdsA but also that of YnbB. Under non-permissive conditions, phospholipid biosynthesis proceeded normally, however, MPIase upregulation was inhibited with accumulation of precursors of membrane and secretory proteins such as M13 procoat and proOmpA, indicating that YnbB is dedicated to MPIase biosynthesis, complementing the CdsA function.