PtdCho and PtdEtn, the two most abundant phospholipids are largely synthe-sised by the integral membrane aminoalcoholphosphotransferases (AAPTs) from sn-1,2 DAG and either CDP-choline or CDP-ethanolamine, respectively (Fig. 2, step 6). In these reactions CMP is displaced from CDP-choline or CDP-ethanolamine by the sn-3 hydroxyl of sn-1,2 DAG. Biochemical evidence that the same enzyme is responsible for catalysing both reactions (Lord 1975; Sparace et al. 1981) was confirmed once genes encoding AAPTs from soybean (Dewey et al. 1994), Arabidopsis (Goode and Dewey 1999) and Brassica napus (Qi et al. 2003) were cloned and the proteins expressed and characterised. The Arabidopsis AAPT proteins share 85% sequence identity and both contain ER membrane retention signals. Although all AAPTs characterised were shown to use both CDP-choline and CDP-ethanolamine as substrates, the AAPT2 from Arabidopsis and AAPT1 from Brassica preferentially use CDP-choline. Interestingly, the AAPT from Chlamydomonas reinhardtii, an organism which does not synthesise PtdCho but only PtdEtn, is capable of synthesising both PtdCho and PtdEtn if the substrates are provided (Yang et al. 2004). The AAPTs are activated by Mg2+ or Mn2+ and inhibited by Ca2+ and CMP (Goode and Dewey 1999; Qi et al. 2003). Strong expression of the soybean AAPT in tobacco leaves did not cause any change in the fatty-acid composition of phospholipids (Goode and Dewey 1999). On the other hand expression of the AAPT from Brassica napus in Arabidopsis caused a small but significant increase in the polyunsaturated fatty-acid content (Qi et al. 2003). The AAPTs in microsomal membranes isolated from developing seeds from safflower and oilseed rape showed no specificity for the acyl chain composition of the sn-1,2-DAG, suggesting that this enzyme draws on DAG molecules in the pool surrounding the enzyme in the membrane (Vogel and Browse 1996). However, the AAPTs from pea leaves and germinating soybeans were all shown to prefer 1-palmitoyl-2-linoleoyl DAG as lipid substrate (Justin et al. 1987). A detailed study of the substrate selectivity of purified heterol-ogously expressed AAPT enzymes in mixed detergent/substrate micelles has yet to be made.
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