SPONTANEOUS AND PROTEIN-MEDIATED STEROL TRANSFER BETWEEN INTRACELLULAR MEMBRANES

Relatively little is known regarding intracellular cholesterol traffic king pathways. To resolve some of these potential pathways, spontaneou s and protein-mediated sterol transfer was examined between different donor acceptor membrane pairs in vitro using L-cell fibroblast plasma membrane (PM) and microsomal (MICRO) and mitochondrial (MITO) membrane s, Several new exciting insights were provided, First, the initial rat e of spontaneous molecular sterol transfer was more dependent on the t ype of acceptor than donor membrane, i.e. spontaneous intracellular st erol trafficking was vectorial. Therefore, the rate of sterol desorpti on from the donor membrane was not necessarily the rate-limiting step in molecular sterol transfer. Second, the rate of molecular sterol tra nsfer was not obligatorily correlated with the direction of the choles terol gradient. For example, although PM had a 3.2-fold higher cholest erol/phospholipid ratio than MITO, spontaneous sterol transfer was 4-5 fold faster up (MITO to PM) rather than down (PM to MITO) the concent ration gradient. Third, sterol carrier protein-2 differentially stimul ated the initial rate of sterol transfer for all donor-acceptor combin ations, being most effective with PM donors: PM-MICRO, 27-fold; and PM -MITO, Ig-fold, Sterol carrier protein-2 was less effective in enhanci ng sterol transfer in the reverse direction, i.e. MICRO-PM and MITO-PM (5- and 4-fold, respectively), Fourth, liver fatty acid-binding prote in was limited in stimulating the initial rate of sterol transfer from PM to PM (1.5-fold), from PM to MITO (3-fold), and from MICRO to MITO (3-fold). In summary, these observations present important insights i nto potential sterol trafficking pathways between the major membrane c omponents of the cell.