Additional Pathways Leading to Triacylglycerol Synthesis in the ER

Metabolic labelling studies using microsomal preparations from safflower and castor bean showed that extensive acyl-CoA independent transacylation between membrane lipids, DAG and TAG is possible (Mancha and Stymne 1997; Stobart et al. 1997). In one reaction, fatty acids are rapidly transferred from the sn-2 position of PtdCho to DAG, generating TAG and lysoPtdCho (Fig. 1, step 6). A candidate yeast gene encoding this activity was identified by homology search using the logic that a lecithin:cholesterol acyltransferase-related enzyme could be responsible for the transfer of fatty acids from PtdCho to TAG, (Dahlqvist et al. 2000). The encoded protein partially complemented the corresponding yeast knockout mutant and the activity was described as phospholipid:diacylglycerol acyltransferase (PDAT). In yeast, this enzyme plays an important role in triacylglycerol synthesis, however, after characterisation of the Arabidopsis PDAT enzyme, the authors now question the significance of PDAT in controlling lipid composition or redistribution of acyl groups between lipids (Stahl et al. 2004). Thus, the physiological role of PDAT in Arabidopsis is unclear. However, since it is involved in the breakdown of major membrane lipids to form lysophospholipids, PDAT may have a role in signalling or in membrane lipid repair (Stahl et al. 2004). The characterisation of an Arabidopsis PDAT T-DNA insertion mutant confirmed the existence of a diacylglycerol:diacylglycerol transacylation (DDAT) activity (Fig. 1, step 8), which contributes to triacylglycerol synthesis or remodelling but is normally masked by PDAT activity (Stahl et al. 2004). DDAT activity was first reported from microsomal preparations of safflower seeds (Stobart et al. 1997), and a similar activity was observed in sunflower seed microsomes (Frazer et al. 2000). These membrane preparations possess activities that catalyse the interconversion of mono-, di-, and triacylglycerols.

In addition to producing TAG, the DDAT reaction generates monoacylglyc-erol (MAG) but the mechanism for reincorporation of this lipid into Kennedy pathway intermediates was not clear (Stobart et al. 1997). Recently, the gene encoding MAG acyltransferase (MGAT) that catalyses the synthesis of the DAG has been cloned from mammals where the enzyme plays an important role in the resynthesis of TAGs in the intestine (Yen et al. 2002). The MGAT shares sequence homology with members of the DGAT2 gene family (Lardiz-abal et al. 2001). Although the existence of a peanut MGAT enzyme has been confirmed, the gene has not yet been cloned (Tumaney et al. 2001). The significance of MGAT and DDAT activities for storage lipid synthesis in plants may be related to their ability to enrich TAGs with polyunsaturated or unusual fatty acids and as well as to enhance the accumulation of TAG content in seeds.

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