Plants possess extracellular waxy polymer layers which protect them from dehydration and pathogens. This waxy layer is called suberin in the root and cutin on aerial tissues.  Mutant analyses and over-expression studies indicate that glycerol-3-phosphate acyltransferases (GPATs) 4,5,6, and 8 synthesize some of the building blocks these waxy extracellular polymers.  Yang, et al. have clarified the enzymatic mechanism of these GPATs (1).  The canonical GPAT reaction uses glycerol-3-phosphate and a coenyzyme A (CoA)-activated fatty acid as its substrates.  The reaction proceeds by bonding the fatty acid to the sn-1 position of the glycerol-3-phosphate, resulting in sn-1-lysophosphatidic acid (LPA).  This reaction is the rate-limiting step in the synthesis of membrane phospholipids and storage triacylglycerols.  The GPATs involved in cutin and suberin biosynthesis have a new trick.  Arabidopsis Gpats 4,5, and 6 expressed in yeast can use ω-oxidized acyl-CoAs as substrates for synthesis of LPA, however the ω-oxidized fatty acid is added at the sn-2 position of the glycerol-3-phosphate.  Yang, et al. noticed that GPATs 4 or 6 synthesized a second major product .  Mass spectrometry analysis identified this product as sn-2-monoacylglycerol (MAG), which is the dephosphorylated form of sn-2-LPA.  The authors go on to show that Arabidopsis GPATs 4 and 6 have two enzymatic activities - a glycerol-3-phosphate acyltransferase activity and an sn-2-LPA phosphatase activity.

This study is a major step toward understanding the synthesis of these waxy layers that are so important to plant growth and development, and it greatly expands the functional repertoire of GPATs as a whole.  A 2003 study reported that Arabidopsis GPATs 1, 4-7 are all able to synthesize LPA in vitro using “un-oxidized” acyl-CoAs for substrates (2).  The fact that these GPATs can use both ω-oxidized acyl-CoAs as well as “un-oxidized” acyl-CoAs as substrates suggests flexibility in the enzymes’ preference for its acyl-donor.  It would be interesting to determine if any of the mammalian GPATs can esterify oxidized acyl-CoAs, particularly as some oxidized fatty acids are bioactive.

1. Yang W, Pollard M, Li-Beisson Y, Beisson F, Feig M, Ohlrogge J. A distinct type of glycerol-3-phosphate acyltransferase with sn-2 preference and phosphatase activity producing 2-monoacylglycerol. Proceeding of the National Academy of Sciences USA. 2010 Jun 29;107(26):12040-5.
2.  Zheng Z, Xia Q, Dauk M, Shen W, Selvaraj G, Zou J. Arabidopsis AtGPAT1, a member of the membrane-bound glycerol-3-phosphate acyltransferase gene family, is essential for tapetum differentiation and male fertility. Plant Cell. 2003 Aug;15(8):1872-87.

Reviewed by: Matthew Keogh,UNC-Chapel Hill
Posted on: 21 July 2010