Cholinephosphotransferase and Diacylglycerol Acyltransferase (Substrate Specificities at a Key Branch Point in Seed Lipid Metabolism)

Abstract

Many oilseed plants accumulate triacylglycerols that contain unusual fatty acyl structures rather than the common 16- and 18-carbon fatty acids found in membrane lipids of these plants. In vitro experiments demonstrate that triacylglycerols are synthesized via diacylglycerols in microsomal preparations and that this same sub-cellular fraction is the site for the synthesis of phosphatidylcholine, which in seeds is synthesized from diacylglycerol by CDP-choline: diacylglycerol cholinephosphotransferase. In microsomes from Cuphea lanceolata, a plant that accumulates fatty acids with 10 carbons and no double bonds (10:0) in its oil, the diacylglycerol acyltransferase exhibited 4-fold higher activity with 10:0/10:0 molecular species of diacylglycerol than with molecular species containing 18-carbon fatty acids. In castor bean (Ricinus communis), which accumulates oil containing ricinoleic acid, diricinoleoyldiacylglycerol was the favored substrate for triacylglycerol synthesis. In contrast to these modest specificities of the diacylglycerol acyltransferases, the cholinephosphotransferases from these plants and from safflower (Carthamus tinctorius) and rapeseed (Brassica napus) showed little or no specificity across a range of different diacylglycerol substrates. Consideration of these results and other data suggests that the targeting of unusual fatty acids to triacylglycerol synthesis and their exclusion from membrane lipids are not achieved on the basis of the diacylglycerol substrate specificities of the enzymes involved and may instead require the spatial separation of two different diacylglycerol pools.

Biosynthesis and function of polyacetylenes and allied natural products

“…This would, in turn, promote the channeling of these atypical fatty acids to TAG, particularly in plants, such as Crepis sp., and fungi, such as Cantharellus sp., that accumulate acetylenic TAGs. While CDP-CPT is an appealing branch point for acyl lipids, in vitro studies for VLCFA, MCFA, and hydroxy fatty acid-producing plants indicate that CDP-CPT is of minor importance as it has little acyl group selectivity [128].…”

Isolation of a Gene Encoding a 1,2-Diacylglycerol-sn-acetyl-CoA Acetyltransferase from Developing Seeds of Euonymus alatus

“…Triacylglycerol synthesis has been described in developing seed extracts from various plant species using medium-chain acyl-CoAs and/or medium-chain diacylglycerols in DGAT as-says (17)(18)(19)(20). Frequently, the substrate specificity and selectivity of these DGATs are fairly broad.…”

Seed-Specific Over-Expression of an Arabidopsis cDNA Encoding a Diacylglycerol Acyltransferase Enhances Seed Oil Content and Seed Weight

“…The biochemical properties of microsomal DGAT have been examined in a number of plant systems (Frentzen, 1993), including developing seeds (Cao and Huang, 1987;Bernerth and Frentzen, 1990;Vogel and Browse, 1996) and embryo cultures (Taylor et al, 1991(Taylor et al, , 1992Weselake et al, 1991;Little et al, 1994) of Brassica napus. In general, studies with developing seeds indicate that DGAT activity increases rapidly during the active phase of oil accumulation and then decreases markedly as seed lipid content reaches a plateau (Tzen et al, 1993;Weselake et al, 1993).…”