Acyl-acyl-carrier protein: lysomonogalactosyldiacylglycerol acyltransferase from the cyanobacterium Anabaena variabilis.

Membranes isolated from the cyanobacterium, Anabaena variabilis, and washed free of soluble endogenous constituents, were capable of catalyzing the direct transfer of the acyl group from acyl-acyl-carrier protein to an endogenous lysomonogalactosyldiacylglycerol to form monogalactosyldiacylglycerol. Other glycolipids including monoglucosyldiacylglycerol and digalactosyldiacylglycerol were not products of this reaction. The transfer was not dependent on any added cofactors. Palmitoyl-, stearoyl- and oleoyl-acyl-carrier protein were approximately equally active as substrates. Transfer was exclusively to the C-1 of the glycerol, as demonstrated by hydrolysis of all incorporated acyl groups by the lipase from Rhizopus arrhizus delamar. In addition to the single galactolipid, a second minor reaction product was free fatty acid, presumably due to hydrolysis of the acyl-acyl-carrier protein. Using a double-labelled [14C]acyl-[14C]acyl-carrier protein, the reaction was demonstrated to be a transfer reaction, rather than a simple exchange of acyl groups with endogenous monogalactosyldiacylglycerol. The transfer reaction mechanism was also confirmed by increasing activity with the addition of liposomes of lysomonogalactosyldiacylglycerol.