Is phosphatidylglycerol essential for terrestrial life?
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REVIEW
Is phosphatidylglycerol essential for terrestrial life? Samuel Furse 1
Received: 25 July 2016 / Accepted: 29 August 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract Lipids are of increasing importance in understanding biological systems. Lipids carrying an anionic charge are noted in particular for their electrostatic interactions with both proteins and divalent cations. However, the biological, analytical, chemical and biophysical data of such species are rarely considered together, limiting our ability to assess the true role of such lipids in vivo. In this review, evidence from a range of studies about the lipid phosphatidylglycerol is considered. This evidence supports the conclusions that this lipid is ubiquitous in living systems and generally of low abundance but probably fundamental for terrestrial life. Possible reasons for this are discussed and further questions posed. Keywords Lipid . Phosphatidylglycerol . Membrane . Anionic . Signal
Introduction The structure of phosphatidylglycerol (PG) was formally determined from lipid isolates of the single-cell photosynthetic organism Scenedesmus in the late 1950s [1]. Within a decade of its discovery, PG was found in higher plants [2, 3], Gram negative bacteria [4, 5] and mammals [6, 7]. The principal steps of the biosynthesis were elucidated in the Kennedy laboratory [8], and a laboratory synthesis was completed [9] at around the same time. Lipid derivatives of PG produced in vivo were discovered about ten years after structural determination of * Samuel Furse [email protected]; [email protected] 1
Molekylærbiologisk institutt, Unversitetet i Bergen, Thormøhlens gate 55, 5006 Bergen, Norway
the original [10, 11]. The discovery of PG in the archaeon Haloferax volcanii in the 1970s indicated that it is a constituent of all three domains of terrestrial life, a fact that hints that it may be ubiquitous and perform one or more fundamental biological functions. Recent lipid profiling has shown that PG has a relatively low abundance with respect to phosphatidylethanolamine (PE) in prokaryotes and either PE or phosphatidylcholine (PC) in eukaryotes [12, 13, 14]. This suggests that its structural contribution is minimal or controlled tightly. Its molecular structure (Fig. 1) is analogous to phosphatidylinositol (PI, which comprises an inosityl moiety instead of the glyceryl one in PG), which may be the reason that the two sometimes have similar activity [15] and transport [16, 17], and why they are sometimes bracketed together [18–20]. PG’s head group comprises a second glyceryl moiety on the phosphate (Fig. 1), which attracts hydration and thus has a larger effective head group diameter than phosphatidic acid (PA). This means that PG is both a cylindrical (type 0) lipid under model and physiological conditions [21] and possesses an anionic charge. The latter directs its relationship with cations [21–25]. PA, the progenitor of lipids in several species [26] is regarded as a cone-shaped (type II) lipid [27–29]. The molecular structure of PG also
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