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FATTY ACYLATED PROTEINS

(PROTEOLIPIDS)



During the course of the study of brain sulfatides with Lees MB, Folch J described for the first time in 1951 the presence of special proteins in rat brain myelin which could be solubilized in organic solvents (chloroform / methanol / water mixtures) (Folch J et al., J Biol Chem 1951, 191, 807). These substances were named "proteolipides" and were considered as a novel lipoprotein but quite different from the other known lipoproteins.

The discovery of these special proteins was made after drying and extracting a brain lipid extract with organic solvents. All the lipids re-dissolved except for an insoluble residue which accounted for about 15% of the initial lipid extract. The residue was completely insoluble in any organic solvents and also in aqueous solutions. As it contained about 13% amino acid nitrogen and some phosphorus, the residue was considered an insoluble protein with some contaminant lipids. For an exhaustive history of proteolipids, see the review of Marjorie B Lees (Neurochem Res 1998, 23, 261). 

These proteolipids were shown to be present mainly in neural tissues but also in heart, kidney, liver, and muscles but absent from blood plasma.

During thirty years the definition of proteolipids was exclusively used to refer to a family of various proteins which are related by their solubility in mixtures of chloroform and methanol (Lees MB et al., Biochim Biophys Acta 1979, 559, 209). Thus, the archetypal proteolipid found initially in myelin is now known as "proteolipid protein" or PLP.

The presence of fatty acids covalently associated with hydrophobic proteins was first described in Gram-negative bacteria (Braun V et al., Eur J Biochem 1969, 10, 426 and 1972, 28, 51) but rapidly extended to myelin PLP (Sherman G et al., Biochem Biophys Res Comm 1971, 44, 157) and to the Ca++-dependent ATPase complex of sarcoplasmic reticulum (McLennan DH, Can J Biochem 1975, 53, 251). These discoveries led to the new definition for proteolipid : a protein that contains a lipid moiety as part of its primary structure (Schlesinger MJ, Ann Rev Biochem 1981, 50, 193).
Thus, fatty acylated proteins should now be characterized by more strict criteria than hydrophobicity, i.e. presence of lipids even after exhaustive extraction with organic solvents and boiling SDS and identification of covalent lipids after chemical cleavage of the protein-lipid linkage.

Since that restrictive definition, a wide variety of fatty acylated proteins were reported to be present in virus, bacteria and eukaryotic cells leading to the acceptance that acylation may be one of the most widespread modifications of proteins (Schultz AM et al., Ann Rev Cell Biol, 1988, 4, 611; Schmidt MFG, Biochim Biophys Acta 1989, 988, 411).


Curiously, only two types of acylated proteins have been identified :

 

- Myristoylated proteins

Myristic acid (C14:0) is bound to the amino-terminal glycine residue (stable amide linkage)

- Palmitoylated proteins

Palmitic acid (C16:0) is bound to side chains of cystein residues (labile thioester linkage). Other fatty acids can also be present (C16:1, C18:2, C20:0 ..)