Biosynthesis of four rat liver mitochondrial acyl-CoA dehydrogenases: In vitro synthesis, import into mitochondria, and processing of their precursors in a cell-free system and in cultured cells

Abstract

The synthesis, translocation, processing, and assembly of rat liver short chain acyl-CoA, medium chain acyl-CoA, long chain acyl-CoA, and isovaleryl-CoA dehydrogenases were studied. These four acyl-CoA dehydrogenases are homotetrameric flavoproteins which are located in the mitochondrial matrix. They were synthesized in a cell-free rabbit reticulocyte lysate system, programmed by rat liver polysomal RNA, as precursor polypeptides which are 2–4 kDa larger than their corresponding mature subunits ( M r 41,000–45,000). When the radiolabeled precursors were incubated with intact rat liver mitochondria, they appeared to bind tightly to the mitochondrial outer membrane. At this stage they were completely susceptible to the action of exogenous trypsin. The precursors bound to mitochondria at 0 °C were translocated into the mitochondria and processed when the temperature was raised to 30 °C. No reaction occurred when the temperature was kept at 0 °C, however, suggesting that the binding of the precursors is temperature independent while the subsequent steps of the pathway are energy dependent. Indeed, the translocation reaction was inhibited by compounds such as dinitrophenol and rhodamine 6G which inhibit mitochondrial energy metabolism. The newly imported (mature) enzymes were inaccessible to the proteolytic action of added trypsin. The processing of the precursors to mature subunits was proteolytically carried out in the mitochondrial matrix, and the processed mature subunits mostly assembled to their respective tetrameric forms. Newly synthesized larger precursors of each of the four acyl-CoA dehydrogenases were recovered from intact, cultured Buffalo rat liver cells in the presence of dinitrophenol. When dinitrophenol was removed in a pulse-chase protocol, the accumulated precursors were rapidly ( t 1 2 3–5 min ) converted to their corresponding mature subunits. On the other hand, when the chase was performed in the presence of the inhibitor, the labeled precursors disappeared with t 1 2 of >4 h for long chain acyl-CoA dehydrogenase and 1–2 h for the other three enzyme precursors.