The enzyme converts monogalactosyldiacylglycerol to digalactosyldiacylglycerol, trigalactosyldiacylglycerol and tetragalactosyldiacylglycerol. All residues are connected by beta linkages. The activity is localized to chloroplast envelope membranes, but it does not contribute to net galactolipid synthesis in plants, which is performed by EC 126.96.36.199, monogalactosyldiacylglycerol synthase, and EC 188.8.131.52, digalactosyldiacylglycerol synthase. Note that the beta,beta-digalactosyldiacylglycerol formed by this enzyme is different from the more common alpha,beta-digalactosyldiacylglycerol formed by EC 184.108.40.206. The enzyme provides an important mechanism for the stabilization of the chloroplast membranes during freezing and drought stress.
under normal growth conditions, the in vivo activity of the galactolipid:galactolipid galactosyltransferase is not detectable in the wild type, but becomes apparent in the tgd mutant. The enzyme does not provide the bulk of glactoglycerolipids in the chloroplast, but is speculated to play a role during ozone-induced injury or possibly during senescence of leaves when chloroplast membranes are turned over
alpha-linolenic acid causes drastic increase in activity under limiting concentrations of MgCl2, without affecting its maximum activity at higher MgCl2 concentration, free alpha-linolenic acid alone does not affect the activity; EDTA inhibits stimulation; unsaturated 16- and 18-carbon fatty acids stimulate in presence of MgCl2
analysis of dgd1 and dgd2 double mutants show that galactolipid:galactolipid galactosyltransferase is different from the genuine digalactosyldiacylglycerol synthases DGD1 and DGD2, because chloroplasts of a double mutant still are capable of oligogalactolipid biosynthesis.; analysis of tgd mutants show that the galactosyl residues are all beta1->6-linked.
dgd2 null mutant, no effects on growth and lipid composition. Dgd1 dgd2 double null mutant, only trace amounts of digalactosyldiacylglycerol, but some monogalactosyldiacylglycerol and unusual oligogalactolipids like tri- and tetragalactosyldiacylglycerol