A UNIQUE MITOCHONDRIA-ASSOCIATED MEMBRANE-FRACTION FROM RAT-LIVER HASA HIGH-CAPACITY FOR LIPID-SYNTHESIS AND CONTAINS PRE-GOLGI SECRETORY PROTEINS INCLUDING NASCENT LIPOPROTEINS

An endoplasmic reticulum-like membrane fraction, termed the ''mitochon dria-associated membrane'' (MAM), that co-isolates with mitochondria f rom rat liver has been characterized. One potential function of the MA M is as a membrane bridge between the endoplasmic reticulum and mitoch ondria that may be involved in transfer of phospholipids between these two organelles (Vance, J. E. (1990) J. Biol. Chem. 265, 7248-7256). A polyclonal antibody directed against a peptide corresponding to the C terminus of phosphatidylethanolamine N-methyltransferase-2, a specifi c marker protein of the MAM (Cui, Z., Vance, J. E., Chen, M. H., Voelk er, D. R., and Vance, D. E. (1993) J. Biol. Chem. 268, 16655-16663), w as used in immunofluorescence and immunogold electron microscopy local ization studies in rat hepatocytes. Immunoreactive protein was cluster ed in regions of the cell that did not correspond to the bulk of endop lasmic reticulum. A second potential role for the MAM, as a component of the secretory pathway that supplies lipids for assembly into very l ow density lipoproteins, has been examined. The MAM contains enzymes o f similar, or higher, specific activities to those enzymes in the endo plasmic reticulum for the synthesis of phospholipids, triacylglycerols , cholesterol, and cholesteryl esters. Specific activities of diacylgl ycerol acyltransferase, acyl coenzyme A:cholesterol acyltransferase, a nd phosphatidylserine synthase (base exchange enzyme) are enriched 2.2 -3.4-fold in the MAM compared with endoplasmic reticulum. In addition, the microsomal triacylglycerol transfer protein, which is required fo r the assembly/secretion of apolipoprotein B containing lipoproteins, was present in the MAM. Nascent apolipoprotein B-containing lipoprotei ns were isolated from the lumen of the MAM. These lipoproteins had the same average density and composition as nascent apolipoprotein B-cont aining lipoproteins isolated from heavy and light endoplasmic reticulu m fractions, from the Golgi, and lipoproteins newly secreted by cultur ed rat hepatocytes. The MAM is a pre-Golgi compartment of the secretor y route, as shown by pulse-chase studies with apolipoprotein B and alb umin, as well as the sensitivity of luminal apolipoprotein B to endogl ycosidase H.