Triacylglycerol synthesis in crustacean hepatopancreas requires a mitochondrial glycerol-3-phosphate acyltransferase”

PELLON-MAISON M; GARCIA CF.; CATTANEO ER; COLEMAN RA; GONZALEZ-BARÓ M. R

Waterville Valley, NH, USA

Congreso; Gordon Research Conference “Molecular and Cellular Biology; 2007

Title: Triacylglycerol synthesis in Crustacean hepatopancreas requires a mitochondrial glycerol-3-phosphate acyltransferase        In mammals, four isoforms of glycerol-3-phosphate acyltransferase (GPAT, EC 2.3.1.15) have been described: GPAT1 and 2 are located on the outer mitochondrial membrane and GPAT 3 and 4 are in the endoplasmic reticulum (ER).  Although ER-GPATs account for the highest specific activity in mammalian tissues, it appears that GPAT1 may be the first-appearing acyltransferase in evolution.  In E. coli and other bacteria, the only protein that expresses GPAT activity is plsB, a protein with a molecular mass similar to that of rat GPAT1 and 30% amino acid identity. We report that triacylglycerol (TAG) synthesis in crustacean (Macrobrachium borellii) hepatopancreas depends solely on a mitochondrial GPAT.      Hepatopancreas is an organ that has a high-TAG biosynthetic activity, and exerts functions analogous to both mammalian liver and adipose tissue with TAG stores of 80% of total lipids. In hepatopancreas mitochondria, we identified both GPAT activity and protein similar to mammalian GPAT1.  The activity was resistant to inactivation by SH-reactive substances, it was activated by polymixin-B, and its substrate preference was for palmitoyl-CoA.  We also visualized a 67-kDa protein band reactive to anti-rat liver GPAT1 antibody. Surprisingly, we did not detect GPAT activity in hepatopancreas microsomes, even though these microsomes can synthesize TAG from  [14C]palmitate.  When we used [14C]glycerol-3-phosphate as substrate, no [14C]TAG was produced in microsomes, and in mitochondria the biosynthetic product was [14C]phosphatidate.  We conclude that this crustacean model is unique in that the first step in the de novo biosynthetic pathways for glycerolipids is carried out exclusively in mitochondria and that ER completes the synthesis.      When we tested TAG synthesis from [14C]glycerol-3-phosphate in membranes combined from both mitochondria and microsomes, only [14C]phosphatidate was formed. These results suggest that either the transfer of phosphatidate from mitochondria to ER requires the presence of a cytosolic transfer protein or that hydrolysis of the phosphatidic acid requires a soluble phosphohydrolase like Lipin.