A PLANT ACYLTRANSFERASE INVOLVED IN TRIACYLGLYCEROL BIOSYNTHESIS COMPLEMENTS AN ESCHERICHIA-COLI SN-1-ACYLGLYCEROL-3-PHOSPHATE ACYLTRANSFERASE MUTANT

The second acylation reaction in glycerolipid biosynthesis is catalyze d by an sn-1-acylglycerol-3-phosphate acyltransferase. The enzyme of L imnanthes douglasii involved in triacylglycerol synthesis has an unusu al specificity for very long chain acyl groups in both of its substrat es, namely acyl-CoA and sn-1-acylglycerol-3-phosphate, and causes the enrichment of erucoyl groups in the sn-2 position of the seed oil of t his plant species. We have isolated a cDNA clone encoding this embryo- specific, microsomal acyltransferase via heterologous complementation of an Escherichia coli mutant deficient in sn-1-acylglycerol-3-phospha te acyltransferase activity. The open reading frame of the cDNA insert encodes a protein with a length of 281 amino acids, with three predic ted membrane-spanning domains and of about 31.7 kDa. The sequence exhi bits substantial sequence similarity to the sn-1-acylglycerol-3-phosph ate acyltransferase of E. coli. The corresponding transcript was detec table in developing embryos but not in leaves of L. douglasii, and exp ression of the open reading frame in E. coli caused sn-1-acylglycerol- 3-phosphate acyltransferase activity which showed properties different from those of the bacterial acyltransferase but typical of the L. dou glasii enzyme involved in triacylglycerol biosynthesis.