The Lipid Library

CERAMIDE PHOSPHORYLINOSITOL, PHYTOGLYCOSPHINGOLIPIDS AND RELATED GLYCOPHOSPHOSPHINGOLIPIDS

OCCURRENCE, COMPOSITION and BIOCHEMISTRY

1.   Ceramide Phosphorylinositol

Ceramide phosphorylinositol or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, the sphingolipid analogue of phosphatidylinositol, is an important component of the sphingolipids in many eukaryotic species other than mammals.

Some bacteria and parasitic organisms, such as Leishmania sp. (in some stages of its growth), contain ceramide phosphorylinositol, and it is present in many species of filamentous fungi and mushrooms, usually together with glycosylated forms with mannose as the most common additional hexose. Ceramide phosphorylinositol has also been detected in some marine invertebrates (echinoderms), such as starfish, where it is the precursor of more complex ganglioside-like lipids.

In higher plants and other organisms, ceramide phosphorylinositol and glycosylated forms of this are components of the membranes. In the first step, ceramide phosphorylinositol synthase catalyses the transfer of inositol phosphate from phosphatidylinositol to ceramide before further hexose units are added. However, little is known of how the more complex phosphoinositides are built up.

The lipid components of the ceramide phosphorylinositol of the few plant species to have been studied are mainly saturated, with primarily phytosphingosine as the long-chain base and tetracosanoic acid (24:0) as the fatty acid component. Ceramide phosphorylinositol per se tends to contain a wider range of lipid constituents.

In fungi, it is intriguing that the glycosyl inositol phosphorylceramides contain sphinganine as the main long-chain base, not (4E,8E)-9-methylsphinga-4,8-dienine as in the glucosylceramides, suggesting that separate pools of ceramide are used in the biosynthesis of each of these lipids. The main long-chain base in ceramide phosphorylinositol in S. cerivisiae is phytosphingosine, and this is linked to a C₂₆ hydroxy fatty acid. Interestingly, there appears to be a parallel function with sphingomyelin in that ceramide phosphorylinositol occurs in specific membranes domains (rafts) together with the yeast sterol, ergosterol, where both interact with specific membrane proteins with signalling functions. This is certainly true in higher plants also.

2.   Ceramide Phosphorylinositol-Glycan Anchors for Proteins

Both phosphatidylinositol and ceramide phosphorylinositol are also the lipid components for oligosaccharide-linked proteins in an analogous way to the glycosylphosphatidylinositol(GPI)-anchors, which as in animals contain the highly conserved core unit –

Manα1–4 Manα1–4Manα1–4GlcNα1–6Ins–1–P–Cer/DAG

The proteins can remain tethered to the cell wall in this way or they can be released by action of a phospholipase. Gene studies suggest that over 200 different proteins occur in membranes in this form in Arabidopsis thaliana, though a relatively small proportion are based on ceramides. However, there is evidence that the complex ceramide-containing proteolipids (together with the phytoglycosphingolipids below) are the most abundant sphingolipids in plants, but as they are not easily extracted by conventional methodologies, they are rarely detected by analysts. A corollary is that the glycerophospholipids of plant membranes may be relatively less abundant than has been considered hitherto. Thus the plasma membrane in plants is usually considered to contain roughly 10% of glucosylceramide, 40% sterols and 50% phospholipids, and the glycosyl inositol phosphorylceramides (GPCI) are ignored. In contrast, when the last are taken into account, it now appears likely that sphingolipids make up 55% of the total lipids and phospholipids only 25% in this membrane.

Species of yeast other than S. cerivisiae also contain highly complex lipids of this type, most of which are based on a ceramide core, that serve to anchor proteins to cell surfaces. In this instance, addition of a glycosylphosphatidylinositol precursor to proteins occurs first, before the ceramide moiety is incorporated by an exchange reaction. Ceramide phosphorylinositol per se is not the precursor. A similar process probably occurs in higher plants, but this has still to be confirmed experimentally.

3.   Ceramide Phosphorylinositol Mannosides and Phytoglycosphingolipids

In yeasts, such as Saccharomyces cerivisiae, ceramide phosphorylinositol is accompanied by two further inositol-containing sphingophospholipids, mannosylinositolphosphorylceramide (Cer-P-Inos-Man) and mannosyldiinositolphosphorylceramide (Cer-P-Inos-Man-P-Inos). In this instance, there is a Manα1–2Inos core, but in other species there are series of related lipids with Manα1–6Inos or GlcNα1–2Inos linkages, often attached to further mannose or other hexose units.

Higher plants, yeasts and fungi contain a number of distinctive lipids with ceramide phosphorylinositol as the backbone with carbohydrate moieties linked to inositol, and they have been termed 'phytoglycosphingolipids'. More than twenty molecular forms have been identified, though only a few of these have been fully characterized, and research on these appeared to stop many years ago. Nothing is known of their biological functions. It is evident that the nature of the carbohydrate moiety is dependent on the type of organism and can be highly complex, including glucuronic acid, glucosamine (and its N-acetyl derivative) and many others. In such complex sphingolipids, the oligosaccharide chains are usually linked at position 2 and/or position 6 of the inositol moiety, as with the analogous glycerophospholipids, leading to both linear and branched chains of hexose units. The overall structures can be very variable.

In higher plants, the basic structural building block is – Glucosamine–Glucuronic acid–Ins–P–Cer

   – in yeasts and fungi (as discussed above) –   Man–Ins–P–Cer

   – in protozoa –   Man–GlcNH₂–Ins–P–Cer

One of the simplest lipids of this type in higher plants is N-acetylglucosamine-glucuronic-inositolphosphoceramide, which is now believed to be the most abundant sphingolipid in the membranes of leaves of tomato and soybean at roughly twice the concentration of glucosylceramide. In Arabidopsis, the N-acetyl moiety is replaced by a hydroxyl group. However, such lipids are not readily extracted from tissues by conventional means, because of their relatively high solubility in aqueous media, and they have usually been ignored by analysts. Modern techniques of electrospray-ionization and tandem mass spectrometry greatly simplify the problem of structural analysis now.

The composition of the long-chain bases tends to differ between species and between sphingolipid classes, but in general the more complex lipids tend to have a much higher proportion of trihydroxy bases than do the glucosylceramides.

Similarly, little is know of the catabolism lipids containing ceramide phosphorylinositol, although there is evidence that the complex phytoglycosphingolipids turn over much more rapidly, with generation of ceramides, than do the glucosylceramides, for example.

4.   Other Glycosphingophospholipids

Ceramide phosphorylmannose was recently identified and characterized for the first time in the lipids of the bacterium Sphingobacterium spiritivorum, where it occurred together with ceramide phosphorylethanolamine and ceramide phosphorylinositol. The ceramide unit contained 15-methylhexadecasphinganine and 13-methyltetradecanoic acid, primarily.

A second type of glycosphingophospholipid is known in which glycosphingolipids are apparently further phosphorylated, i.e. where the ceramide is linked directly to carbohydrate moieties not via phosphate. For example, cholinephosphoryl–6Galβ1–1Cer and cholinephosphoryl–6Galβ1–6Galβ1–1Cer were isolated and characterized from the earthworm, Pheretima hilgendorfi. In this instance, the main fatty acids were 22:0 and 24:0, and the sphingoid bases were octadeca- and nonadeca-4-sphingenine. Subsequently, related triglycosylsphingophospholipids with either a terminal mannose or galactose unit linked to phosphorylcholine were found in the same species, while a similar lipid to that illustrated was found in a clam worm, Marphysa sanguinea. Phosphocholine-containing glycosyl inositol phosphorylceramides have also been found in some filamentous fungi.

Suggested Reading

• Dickson, R.C. and Lester, R.L. Yeast sphingolipids. Biochim. Biophys. Acta, 1426, 347-357 (1999).
• Lynch, D.V. and Dunn, T.M. An introduction to plant sphingolipids and a review of recent advances in understanding their metabolism and function. New Phytologist, 161, 677-702 (2004).
• Markham, J.E., Li, J., Cahoon, E.B. and Jaworski, J.G. Separation and identification of major plant sphingolipid classes from leaves. J. Biol. Chem., 281, 22684-22694 (2006).
• Olsen, E. and Jantzen, E. Sphingolipids in bacterial and fungi. Anaerobe, 7, 103-112 (2001).
• Sperling, P. and Heinz, E. Plant sphingolipids: structural diversity, biosynthesis, first genes and functions. Biochim. Biophys. Acta, 1632, 1-15 (2003).
• Sperling, P., Warnecke, D. and Heinz, E. Plant sphingolipids. In: Lipid Metabolism and Membrane Biogenesis. pp. 337-381 (ed. G. Daum, Springer-Verlag, Heidelberg) (2004).

Updated: 4/8/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland	Right click on the icon to open in a new window