The relationship of protein and lipid synthesis during the biogenesis of mitochondrial membranes

Abstract

Rat liver mitochondria were fractionated into inner and outer membrane components at various times after the intravenous injection of(14)C-leucine or(14)C-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 of(3)H-phenylalanine incorporation into mitochondrial KCl-insoluble proteins was identical to that of(14)C-glycerol incorporation into mitochondrial lecithin. These results suggest a simultaneous assembly of protein and lecithin during membrane biogenesisThe proteins and lecithin of the outer membrane were maximally labeledin 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 to(14)C-choline did not affect thein vivo incorporation of lecithin into the mitochondrial inner or outer membranes; however treatment with the drug for 60 min prior to(14)C-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 fractionin vitro, a rapid exchange of phospholipid between the microsomes and the outer membrane occurred. A slight exchange with the inner membrane was observed.

Hepatic Subcellular Fractions Lipids in Rats Fed Millet (Sorghum vulgarie) at Different Protein Levels

“…Studies on labelling of mitochondrial and microsomal PC and PE have shown that an exchange mechanism is involved with the distribution of 32P-Iabelled PE and PC between microsomes and mitochondria. [32][33][34][35][36][37][38] This exchange Occurs in the presence of high speed supernatant.39) In the present investigation similar interpretation can be drawn, since the specific activities of mitochondrial PC and PE were always lower than the microsomal PC and PE (Table VI). This does not appear to be affected by the amount or quality of the protein in the diet.…”

STUDIES OF MEMBRANE FORMATION IN TETRAHYMENA PYRIFORMIS

“…(14) . Significant differences in incorporation rates can be noted even between the inner and outer membranes of mitochondria (2) .…”

“…New findings have seemed to complicate further this little understood process. Until recently, most popular models for membrane biosynthesis have assumed that lipids and proteins are inserted into expanding membranes simultaneously, perhaps in the form of a lipoprotein subunit, However, current observations (1,2,3,4) indicate that under certain conditions these two major membrane constituents may be added to a membrane independently of one another . Other evidence (5,6,7,8) suggests that at least some structural elements of membranes rapidly exchange with a cytoplasmic pool of membrane precursors .…”

Transfer of phospholipids between membrane

“…It was demonstrated by four groups of workers [73][74][75][76] that on incubation of mitochondria with 32p-labelled microsomes, both the outer and inner mitochondrial membrane phospholipids became labelled. This would explain the very extensive exchange of mitochondrial phospholipids observed.…”