Physiological function of membrane lipids and cell surface structure in bacteria

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Bacteria need to respond quickly to environmental stresses for survival because of their unicellular nature. All cells are partitioned by cell membranes, and in the stress response process an environmental stress is initially sensed at the cell surface as a signal by membrane proteins. Cells then respond to the stress by altering gene expression. A lipid bilayer structure for these membranes was proposed by Singer and Nicolson in 1972. In their model, membrane proteins are floating in lipids and can move freely and work in the membrane. In the past several years, the cell membrane has been increasingly considered as important not only for compartmentalization of cells from their environment but also for stress responses as well as cell surface structure. To date, there have been many reports on so-called lipid rafts in the eukaryotes and lipid microdomains in bacteria, and such domains, involving specific lipids and proteins, are believed to be required for the cell’s physiological functions. Thus, many studies suggest the importance of particular lipid molecules in the membrane. Moreover, the lipid content of cell membranes differs in various organisms, suggesting that lipid content is related to biological function in each organism. In fact, the loss of particular lipids by disruption of their genes may lead to defects not only of membrane proteins but also of cytoplasmic proteins, and finally to loss of cell function. However, the full extent of physiological function for each lipid molecule is still unclear because the loss of lipid molecules affects many aspects in the cell. To reveal the extent of lipid function, a variety of studies on membrane lipids are currently being conducted.

In this special issue, three reviews on lipid function and stress responses in bacteria are presented. In the first, I introduce the biosynthetic pathway of membrane lipids and cell surface structure in Bacillus subtilis, a model organism in the Gram-positive bacteria, and outline the biological function of glucolipids, which are synthesized processively by UgtP. A B. subtilis ugtP disruptant shows abnormal morphology and the activation of several extracytoplasmic function (ECF) sigma factors, which regulate functions related to the cell surface response to extracytoplasmic stimuli. Biological functions of glucolipids in B. subtilis are discussed in the context of the results of molecular genetic analysis of the ugtP disruptant and expression of heterologous glucolipid synthases in glucolipid-lacking cells.

Next, Kei Asai outlines the mechanism of the stress response mediated by these ECF sigma factors in B. subtilis. Bacillus subtilis has seven genes that encode ECF sigma factors, of which six are apparently regulated by individual anti-sigma factors as the genes encoding ECF sigma factors and their cognate anti-sigma factors constitute an operon. He focuses on regulatory aspects of anti-sigma factor activity, especially based on cell membrane and cell wall dynamics, mainly in B. subtilis, compared to the representative model proposed in other bacteria.

Finally, Egi T. Apdila and Koichiro Awai outline the physiological roles of glycolipids in photosynthesis by heterologous gene expression of glycolipid synthases in cyanobacteria. Glycolipids are major constituents in the photosynthetic cyanobacteria: three glycolipids comprise around 90% of thylakoid membranes. As well as functioning as constituents of the membrane system, these glycolipids also integrate into photosynthetic protein complexes and are essential for photosynthetic electron transport reactions. Apdila and Awai introduce the structure and biosynthetic pathway of galactolipids in the thylakoid membrane, and describe the physiological significance of galactolipids and attempts to complement the functions of these lipids by introducing glycolipid (galactolipid and glucolipid) synthetic genes into cyanobacteria.

I hope these reviews will help readers to appreciate the importance of lipid molecules and cell surface structures in bacterial cells, and stimulate research to further elucidate membrane lipids and cell surface functions in bacteria.