Catalyzes the synthesis of bis(monoacylglycero)phosphate (BMP) via transacylation of 2 molecules of lysophosphatidylglycerol (LPG). BMP also known as lysobisphosphatidic acid plays a key role in the formation of intraluminal vesicles and in maintaining intracellular cholesterol homeostasis. Can use only LPG as the exclusive lysophospholipid acyl donor for base exchange and displays BMP synthase activity towards various LPGs (LPG 14:0, LPG 16:0, LPG 18:0, LPG 18:1) with a higher preference for longer chain lengths. Plays a role in influencing the retrograde trafficking of lysosomal sorting receptors SORT1 and IGF2R from the endosomes to the trans-Golgi network by controlling the recruitment of retromer complex to the endosomal membrane. Regulates the localization and activation of RAB7A which is required to recruit the retromer complex to the endosomal membrane