TARGETING AND ASSEMBLY OF THE OXOGLUTARATE CARRIER - GENERAL-PRINCIPLES FOR BIOGENESIS OF CARRIER PROTEINS OF THE MITOCHONDRIAL INNER MEMBRANE

We have studied the targeting and assembly of the 2-oxoglutarate carri er (OGC), an integral inner-membrane protein of mitochondria, The prec ursor of OGC, synthesized without a cleavable presequence, is transpor ted into mitochondria in an ATP- and membrane potential-dependent mann er. Import of the mammalian OGC occurs efficiently into both mammalian and yeast mitochondria. Targeting of OGC reveals a clear dependence o n the mitochondrial surface receptor Tom70 (the 70 kDa subunit of the translocase of the outer mitochondrial membrane), whereas a cleavable preprotein depends on Tom20 (the 20 kDa subunit)? supporting a model o f specificity differences of the receptors and the existence of distin ct targeting pathways to mitochondria. The assembly of minute amounts of OGC imported in vitro to the dimeric form can be monitored by blue native electrophoresis of digitonin-lysed mitochondria. The assembly o f mammalian OGC and fungal ADP/ATP carrier occurs with high efficiency in both mammalian and yeast mitochondria. These findings indicate a d ynamic behaviour of the carrier dimers in the mitochondrial inner memb rane and suggest a high conservation of the assembly reactions from ma mmals to fungi.