Phospholipid-transfer proteins (PL-TP) have been isolated from a number of tissues, including heart, liver, and brain (1 ). These proteins catalyze the one-for-one exchange in vitro of phospholipid molecules between membranes, lipoproteins, or liposomes. The functions of PL-TP in vivo have not been established; however, it is likely that they are involved in the transfer of phospholipid between organelle membranes during membrane biogenesis and membrane remodeling. Since PL-TP only exchange the phospholipid of the outer leaflet of membrane bilayers and do not appear to alter the structure of membranes, they provide a unique tool for investigating the transverse distribution of membrane phospholipids.
