Understanding phospholipid function: Why are there so many lipids?

Abstract

In the 1970s, phospholipids were still considered mere building blocks of the membrane lipid bilayer, but the subsequent realization that phospholipids could also serve as second messengers brought new interest to the field. My own passion for the unique amphipathic properties of lipids led me to seek other, non-signaling functions for phospholipids, particularly in their interactions with membrane proteins. This seemed to be the last frontier in protein chemistry and enzymology to be conquered. I was fortunate to find my way to Eugene Kennedy's laboratory, where both membrane proteins and phospholipids were the foci of study, thus providing a jumping-off point for advancing our fundamental understanding of lipid synthesis, membrane protein biosynthesis, phospholipid and membrane protein trafficking, and the cellular roles of phospholipids. After purifying and characterizing enzymes of phospholipid biosynthesis in Escherichia coli and cloning of several of the genes encoding these enzymes in E. coli and Saccharomyces cerevisiae, I was in a position to alter phospholipid composition in a systematic manner during the cell cycle in these microorganisms. My group was able to establish, contrary to common assumption (derived from the fact that membrane proteins retain activity in detergent extracts) that phospholipid environment is a strong determining factor in the function of membrane proteins. We showed that molecular genetic alterations in membrane lipid composition result in many phenotypes, and uncovered direct lipid-protein interactions that govern dynamic structural and functional properties of membrane proteins. Here I present my personal "reflections" on how our understanding of phospholipid functions has evolved.

Keywords: lipid; membrane; membrane lipid; membrane transport; phospholipid.

Figure 1.

Gottfried Schatz (left), H. Ronald Kaback (middle), and William Dowhan (right). Taken outside Basel, Switzerland in 1984.

Figure 2.

2003 Gordon Research Conference. Left to right: Dennis Voelker, Anthony Fischl (first doctoral student with George Carman and last postdoctoral fellow with Eugene Kennedy), Eugene Kennedy, William Dowhan, George Carman, Edward Dennis, and Christian Raetz.

Figure 3.

Synthesis of native and foreign phospholipids in E. coli. Pathways native to E. coli are noted with solid arrows, and pathways resulting from foreign genes introduced into E. coli are noted with dashed arrows. The genes encoding the following enzymes and associated with each biosynthetic step are listed next to the arrows: 1) CDP-diacylglycerol synthase; 2) PS synthase; 3) PS decarboxylase; 4) PGP synthase; 5) PGP phosphatases; 6) CL synthases; 7) PG:MDO sn-glycerol-1-P transferase; 8) diacylglycerol kinase; 9) glucosyl diacylglycerol synthase (Acholeplasma laidlawii); 10) diglucosyl diacylglycerol synthase (A. laidlawii); 11) PC synthase (Legionella pneumophila); 12) PI synthase (S. cerevisiae); 13) O-lysyl PG synthase (Staphylococcus aureus). Figure originally published in Ref. (Dowhan, W., et al. (2017) Functional roles of individual membrane phospholipids in Escherichia coli and Saccharomyces cerevisiae. in: Biogenesis of Fatty Acids, Lipids and Membranes (Otto Geiger (ed)), DOI: 10.1007/978-3-319-43676-0_36-1). With kind permission from Springer International Publishing.

Figure 4.

Topological organization of LacY as a function of membrane lipid composition. Transmembrane domains (Roman numerals) and extramembrane domains (Arabic numerals) are sequentially numbered from the N terminus to C terminus. LacY orientation with respect to the periplasm and cytoplasm is noted. Net charge of extramembrane domains is shown, and approximate position of positively (red dots) and negatively (green dots) charged residues is shown. Topology of LacY is shown after initial assembly in PE-containing cells (+PE) or after initial assembly in PE-lacking cells (−PE). The interconversion of topological conformers after assembly in one orientation and the ratio of native to inverted conformer are reversible in both directions in vivo and in vitro depending on the dynamic level of PE in membranes. At intermediate levels of PE, LacY exists in a mixture of topological forms.

Figure 5.

Walter Shaw (left), president of Avanti Polar Lipids, presenting the ASBMB Avanti Award in Lipids to William Dowhan (center) in San Diego, CA, April 2005. Christian Raetz (right) nominated William Dowhan for the award.

Figure 6.

William Dowhan Symposium April 2014. A gathering in Houston of former lab members, collaborators and members of the Texas Medical Center community organized by former postdoctoral fellows George Carman and Weiming Xia.