Catalyzes the transfer of a malonyl group from malonyl-CoA to the 6'''-hydroxyl group of the 5-glucosyl moiety of anthocyanins. Active towards bisdemalonylsalvianin (pelargonidin 3-O-(6-caffeoyl-beta-D-glucoside) 5-O-beta-D-glucoside) and shisonin, but not towards nodemalonylsalvianin, salvianin, pelargonidin 3,5-diglucoside and delphinidin 3,5-diglucoside. malonyl-CoA + pelargonidin 3-O-(6-O-[(E)-caffeoyl]-beta-D-glucoside) 5-O-beta-D-glucoside = 4'''-demalonylsalvianin + CoA Completely inhibited by 10 mM p-coumaric acid, this inhibition is rapid, reversible and non-competitive. Completely inhibited by 0.1 mM Cu(2+), 0.1 mM Hg(2+) and 10 mM caffeic acid. Partially inhibited by 5 mM N-ethylmaleimide, 1 mM diethylpyrocarbonate and 1 mM acetyl-CoA. Optimum pH is 8.0. Active over the pH range 6.0-11.0. Stable at pH 7.5 for 20 hours at 20 degrees Celsius. Stable below 30 degrees Celsius for 20 minutes at pH 7.0. Pigment biosynthesis; anthocyanin biosynthesis. Detected in petals and sepals, and at lower levels in bracts and red stems. Highest levels are present in the petals and sepals of young to opening flowers. Belongs to the plant acyltransferase family.