Abstract
Main conclusion Plants have lysophosphatidylcholine transacylase (LPCT) and acyl-CoA:glycerophosphocholine acyltransferase (GPCAT) activities. The combined action of LPCT and GPCAT provides a novel route of PC re-synthesis after its deacylation. Phosphatidylcholine (PC) is the major lipid in eukaryotic membranes and has a central role in overall plant lipid metabolism. It is also the site of production of polyunsaturated fatty acids in plants. The recently discovered acyl-CoA:glycerophosphocholine acyltransferase (GPCAT) activity in yeast provides a novel route of re-synthesising PC via lysophosphatidylcholine (LPC) after its deacylation. This route does not require the degradation of the glycerophosphocholine (GPC) into free choline, the activation of choline to CDP-choline, nor the utilization of CDP-choline by the CDP-choline:diacylglycerol cholinephosphotransferase. We show here that GPCAT activities also are present in membrane preparations from developing oil seeds of safflower and other species as well as in membrane preparations of roots and leaves of Arabidopsis, indicating that GPCAT activity plays a ubiquitous role in plant lipid metabolism. The last step in formation of GPC, the substrate for GPCAT, is the deacylation of LPC. Microsomal membranes of developing safflower seeds utilized LPC in LPC:LPC transacylation reactions (LPCT activities) creating PC and GPC. The results demonstrate that safflower membranes have LPCT and GPCAT activities that represent novel reactions for PC acyl editing. The physiological relevance of these reactions probably has to await identification of the enzymes catalysing these reactions.
Highlights
Phosphatidylcholine (PC) is a central molecule in overall lipid metabolism outside the plastid in plant cells
The acyl groups can be removed from the glycerol backbone of PC for channelling to TAG by three different types of enzymes that all produce lysophosphatidylcholine (LPC): (1) The acyl groups from PC can be removed by phospholipases (Bafor et al 1991; Bates et al 2013), (2) by the reverse reaction of the acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT) (Lager et al 2013; Stymne and Stobart 1984) or (3) by the phospholipid:diacylglycerol acyltransferase (PDAT) (Banas et al 2013)
To make sure that all acyl-CoAs were removed from the membranes in the glycerophosphocholine:acylCoA acyltransferase (GPCAT) assays, they were re-suspended in large volumes of phosphate buffer containing 0.1 % BSA and repelleted by ultracentrifugation before used in GPCAT activity assays
Summary
Phosphatidylcholine (PC) is a central molecule in overall lipid metabolism outside the plastid in plant cells. The acyl groups in PC in these seed cells undergo rapid turnover whereby oleate is channelled into PC for desaturation and the resulting linoleate is channelled from PC into TAG. The acyl groups can be removed from the glycerol backbone of PC for channelling to TAG by three different types of enzymes that all produce lysophosphatidylcholine (LPC): (1) The acyl groups from PC can be removed by phospholipases (Bafor et al 1991; Bates et al 2013), (2) by the reverse reaction of the acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT) (Lager et al 2013; Stymne and Stobart 1984) or (3) by the phospholipid:diacylglycerol acyltransferase (PDAT) (Banas et al 2013). Microsomal fractions of developing safflower seeds have very high LPCAT activity in its forward reaction in presence of acyl-CoA and added LPC (Stymne and Stobart 1984)
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