RELATIONSHIP OF PROTEIN AND LIPID SYNTHESIS DURING BIOGENESIS OF MITOCHONDRIAL MEMBRANES

Rat liver mitochondria were fractionated into inner and outer membrane components at various times after the intravenous injection of14C-leucine or14C-glycerol. The time curves of protein and lecithin labeling were similar in the intact mitochondria, the outer membrane fraction, and the inner membrane fraction. In rat liver slices also, the kinetics of3H-phenylalanine incorporation into mitochondrial KCl-insoluble proteins was identical to that of14C-glycerol incorporation into mitochondrial lecithin. These results suggest a simultaneous assembly of protein and lecithin during membrane biogenesis The proteins and lecithin of the outer membrane were maximally labeled in vivo within 5 min after injection of the radioactive precursors, whereas the insoluble proteins and lecithin of the inner membrane reached a maximum specific acitivity 10 min after injection. Phospholipid incorporation into mitochondria of rat liver slices was not affected when protein synthesis was blocked by cycloheximide, puromycin, or actinomycin D. The injection of cycloheximide 3 to 30 min prior to14C-choline did not affect the in vivo incorporation of lecithin into the mitochondrial inner or outer membranes; however treatment with the drug for 60 min prior to14C-choline resulted in a decrease in lecithin labeling. These results suggest that phospholipid incorporation into membranes may be regulated by the amount of newly synthesized protein available. When mitochondria and microsomes containing labeled phospholipids were incubated with the opposite unlabeled fraction in vitro, a rapid exchange of phospholipid between the microsomes and the outer membrane occurred. A slight exchange with the inner membrane was observed. © 1969 Springer-Verlag New York Inc.