| Popis: |
Proteins which are able to transfer phospholipids between membranes in vitro,have been the subject of investigation for many years (1,2). Much information about their biochemical properties is available. Since a few years the actual cellular function of these proteins is subject of intensive investigations. Brain tissue has been the major source of one of the most interesting of these proteins: the phosphatidylinositol transfer protein (PI-TP). This protein is present in all mammalian tissues examined sofar as well as in many lower species and micro-organisms (reviewed in 1, 2). The protein functions as a carrier and, in addition to PI, can also bind a PC molecule. The affinity for PC is about 16-fold lower than for PI. Because of this dual specificity, it has been proposed that in ivoPI-TP is able to cause net transfer of PI between membranes in exchange for PC (3). As a consequence, PI-TP could be involved in restoring the phosphatidylinositol-4,5- bisphosphate (PIP2) levels in the plasma membrane after stimulation of PLC-mediated PIP2 degradation. This could be accomplished by direct transfer of PI from the endoplas- mic reticulum (ER) where the phospholipids are synthesized, to the site of phosphoryla- tion in the plasma membrane. On the other hand, PI-TP could also increase PI transfer to the plasma membrane by manipulation of the PI/PC content of secretory vesicles budding from the trans-Golgi membranes. These possible involvements of PI-TP in PI transfer from the ER/Golgi membranes to the plasma membrane are shown in Figure 1. |
