The Lipid Library

CERAMIDE PHOSPHORYLINOSITOL, PHYTOGLYCOSPHINGOLIPIDS AND RELATED GLYCOPHOSPHOSPHINGOLIPIDS

OCCURRENCE, COMPOSITION and BIOCHEMISTRY

1.  Ceramide Phosphorylinositol and Related Mannosyl Lipids

Ceramide phosphorylinositol or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, the sphingolipid analogue of phosphatidylinositol, is a major component of sphingolipids in yeasts, such as Saccharomyces cerivisiae, where it is accompanied by two further inositol-containing sphingophospholipids, mannosylinositolphosphorylceramide and mannosyldiinositolphosphorylceramide.

Ceramide phosphorylinositol and glycosylated forms of this are components of the membranes of higher plants, where they are also the basic anchor units in membranes for oligosaccharide-linked proteins. The latter can remain tethered to the cell wall in this way or they can be released by action of a phospholipase. Gene studies suggest that 248 different proteins occur in membranes in this form in Arabidopsis thaliana. There is evidence that these lipids (together with the phytoglycosphingolipids below) are by far 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. The lipid components of the GIPC 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.

Species of yeast other than S. cerivisiae can contain further highly complex lipids of this type, also including some that function in an analogous way to the glycosylphosphatidylinositols (GPI), which also occur in yeasts and serve to anchor specific proteins to cell surfaces. Indeed in this instance, the ceramide moiety is incorporated by an exchange reaction that occurs after the addition of the GPI precursor to proteins. 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. This is certainly true in higher plants also. 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.

Some bacteria and parasitic organisms 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.

2.  Phytoglycosphingolipids

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 –

Man–Ins–P–Cer

– in protozoa –

Man–GlcNH₂–Ins–P–Cer

– and in those lipids that serve as glycosyl inositol phosphorylceramide anchors for proteins (see above), it is the highly conserved –

Manα1–4Manα1–4Manα1–4GlcNα1–6Ins–1–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 missed 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.

Biosynthesis of ceramide phosphorylinositol in plants occurs by a mechanism analogous to that for the biosynthesis of sphingomyelin in animals, i.e. by transfer of inositol phosphate from phosphatidylinositol to ceramide with diacylglycerols as the other product of the reaction. However, little is known of how the more complex phosphoinositides are built up. Similarly, little is know of their catabolism, although there is evidence that the complex phytoglycosphingolipids turn over much more rapidly, with generation of ceramides, than do the glucosylceramides.

3.  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: 13/3/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland