Osmoregulated Periplasmic Glucans

Abstract

Among all the systems developed by enterobacteria to face osmotic stress, only osmoregulated periplasmic glucans (OPGs) were found to be modulated during osmotic fluxes. First detected in 1973 by E.P. Kennedy's group in a study of phospholipid turnover in Escherichia coli, OPGs have been shown across alpha, beta, and gamma subdivisions of the proteobacteria. Discovery of OPG-like compounds in the epsilon subdivision strongly suggested that the presence of periplasmic glucans is essential for almost all proteobacteria. This article offers an overview of the different classes of OPGs. Then, the biosynthesis of OPGs and their regulation in E. coli and other species are discussed. Finally, the biological role of OPGs is developed. Beyond structural function, OPGs are involved in pathogenicity, in particular, by playing a role in signal transduction pathways. Recently, OPG synthesis proteins have been suggested to control cell division and growth rate.

Figure 1

Backbone structure of the four families of OPGs. Reprinted with permission from reference .

Figure 2

Working model of the OPG biosynthetic complex of E. coli. The variety of backbone structures and patterns of substitution are schematically represented (see text for details). Ptd-Gro, phosphatidylglycerol; Ptd-Etn, phosphatidylethanolamine; DG, diacylglycerol; Suc-CoA, succinyl-coenzyme A; CoA, coenzyme A; UDP, uridine diphosphate.

Figure 3

Occurrences of OPG genes in various representative genomes. Protein-protein BLAST program was used to search in the nonredundant database sequences highly similar to OpgG, OpgH, and OpgD from E. coli; OpgI from R. sphaeroides, NdvB, NdvC, and NdvD from B. japonicum, and ChvB and ChvA from A. tumefaciens, OpgB and OpgE from E. coli, OpgC from E. coli; and D. dadantii, and CgmB from S. meliloti.

Figure 4

Schematic of UDP-glucose production, utilization, and connection with trehalose and OPG biosynthesis in E. coli. The Kyoto Encyclopedia of Genes and Genomes (KEGG) was used to establish the metabolic pathway (125, 126). Glucose is transported into the cell via the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS). During uptake, glucose is activated and transformed into glucose-6-P. Glucose-6-P is used by Pgm and GalU enzymes to produce uridine-diphosphate (UDP)-glucose (in red). The UDP-glucose can be used either to produce OPGs via the OpgH/OpgG complex or trehalose-6-P via OtsA. Trehalose-6-P is transformed into trehalose by OtsB enzyme. Then, it can either be excreted via the stretch-activated proteins (SAP) into the periplasm or maintained inside the cell. In both cases, the trehalose will be degraded in 2 glucoses by TreF in the cytoplasm or TreA in the periplasm. P, Phosphate; OPG, osmoregulated periplasmic glucan.