FIBROBLAST MEMBRANE STEROL KINETIC DOMAINS - MODULATION BY STEROL CARRIER PROTEIN-2 AND LIVER FATTY-ACID-BINDING PROTEIN

The mechanism(s) of intracellular sterol trafficking among subcellular organelle membranes is not well understood. Relative contributions of vesicular, sterol carrier protein, and membrane sterol domain pathway s are not resolved. A sterol kinetic assay was used to resolve multipl e sterol domains in microsome (MICRO), mitochondria (MITO), and plasma (Phl) membrane: exchangeable, 20-40% of total; non-exchangeable, 60-8 0% of total. Spontaneous sterol transfer between dissimilar donor and acceptor membranes was vectorial and depended both on acceptor and don or membrane properties. For example, sterol transfer from Phl to MICRO or to MITO, or from MICRO to MITO was 3- to 5-fold slower as compared to sterol movement in the opposite direction. Sterol carrier protein- 2 (SCP-2) stimulated sterol transfer in most donor/acceptor membrane c ombinations by decreasing exchange half-time but not domain size. SCP- 2 enhanced sterol transfer selectively: PM-MICRO (12-fold); MITO-MITO, MICRO-MICRO, MICRO-PM (3-fold); PM-PM (1.4-fold); PM-MITO, MICRO-MITO (no effect), Thus, SCP-2-mediated sterol movement was vectorial and n ot necessarily down a membrane sterol concentration gradient. In contr ast, liver fatty acid binding protein (L-FABP) revealed a modest (2-fo ld) stimulatory effect on sterol transfer only between PM-MITO and MIC RO-MICRO. jlr In conclusion, in vitro studies of sterol transfer among isolated subcellular membranes provided kinetic evidence for sterol d omains in microsomes and mitochondria as weil as plasma membranes. Fur thermore, both spontaneous and protein-mediated sterol transfer appear ed vectorial and selective in nature.