br Future directions The proposed
Future directions The proposed membrane topology model of DGAT1 has only three TMDs, with the N-terminus oriented toward the cytosol and the C-terminus in the ER lumen. The catalytic activity is exclusively on the luminal side of the ER membrane. The second and third TM domains are only about 10 residues, and may not span the membrane but be embedded within the lipid bilayer. A length of 18–20 residues is sufficient to span the usual width of a lipid bilayer . If the interpretation by McFie et al.  is correct, DGAT1 would only have one genuine TMD, which is unlikely as it is predicted to have between 8 and 10 TMDs. DGAT1 has been shown to express catalytic activity on both sides of the membrane . Therefore, further studies are necessary to confirm the number of TMDs and the orientation of the protein termini of DGAT1. Although the last two decades have seen the cloning of the two DGAT genes and characterization of their role in lipid metabolism, the next step in understanding DGAT function requires the purification of the BI 2536 for detailed structural analysis. McFie et al.  have proposed a DGAT2 topology based on experimental data that has the protein embedded in the ER membrane to facilitate the synthesis of TAG channelled into LDs. Further understanding of the three-dimensional structure of DGAT2 could provide the next step in understanding the specific role the enzyme plays in the process of LD formation and growth. Elucidation of the DGAT2 structure would also serve as a guide to understanding how other ER-resident ‘integral’ membrane proteins make their way to the LD. Additional proteomic studies focused on the LD would also help identify other proteins that interact with DGAT2.
Acknowledgments Bhumika Bhatt-Wessel's PhD was supported by a Victoria University of Wellington PhD scholarship and a research grant was provided by the Ministry of Science and Innovation (VLAC0801), New Zealand. Sonja Hummel helped with Fig. 2.
Introduction The African oil palm, Elaeis guineensis, is the most important global vegetable oil crop in terms of both yield efficiency (tonnes oil/hectare) and overall volume of production [, , ]. Oil palm fruits contain two types of storage oil located respectively in the fleshy mesocarp tissue of the fruit and the triploid endosperm tissue of the seed kernel. In contrast to many prominent temperate oilseed crops, such as soybean and rapeseed, where the storage oil accumulates in the embryo, oil storage in palm fruits occurs in non-embryo, maternally-derived tissues and may therefore be subject to different forms of genetic regulation. Moreover, in contrast to the storage role of the seed/kernel oil, the major role of the mesocarp oil is as an attractant to potential animal vectors that serve to disseminate ingested seeds. Again this distinctive role may result in different evolutionary constraints that could affect genetic regulation of mesocarp versus kernel oils. The mesocarp oil, commonly referred to as palm oil (PO), is mostly made up of triacylglycerols (TAGs) containing long-chain C16 and C18 fatty acids (about 44% palmitate, 39% oleate and 11% linoleate) . In contrast, the seed endosperm oil commonly referred to as palm kernel oil (PKO), is enriched in medium-chain C12 and C14 fatty acids (about 48% laurate and 16% myristate). Together these two palm-derived oils account for about 38% of global production of commercial vegetable oils . Further information about palm oil has been reviewed by Sambanthamurthi et al. and Murphy. Two key recent developments in oil palm research have been the publication of the genome sequence in 2013  and the compilation of an updated gene model dataset for the species in 2017 . Like the vast majority of plant storage oils, the two types of palm oil are overwhelmingly made up of TAGs that are synthesised in the ER and then accumulated as cytosolic lipid droplets (LDs) . The overall nature of the pathways involved in TAG biosynthesis in plants is well established [, , , ], although several new enzymes have recently been discovered  and the relative contributions of different reactions and the details of their regulation have yet to be fully resolved . Central to lipid assembly in oil crops is the Kennedy pathway  which converts glycerol 3-phosphate in four steps to TAG using acyl-CoAs as the source of fatty acyl residues. The final reaction in this sequence is catalysed by diacylglycerol acyltransferase (DGAT) (diacylglycerol:acyl-CoA acyltransferase, EC 126.96.36.199).