Intramitochondrial fatty acylation of a cytoplasmic imported protein in animal cells

Mitochondrial digitonin particles from mouse liver (and also from other tissues) incorporate [3H]myristic acid into a 52-kilodalton (kDa) protein in an energy-dependent manner. The 52-kDa N-myristylated protein is located inside the mitochondrial inner membrane since it is protected against proteolytic degradation in intact mitoplasts. Disruption of mitochondrial inner membrane by sonication results in severalfold higher labeling of the 52-kDa protein, further confirming that the enzyme system for protein fatty acylation as well as the 52-kDa target protein are compartmentalized inside the mitochondrial inner membrane matrix. The results of in vitro labeling of submitochondrial fractions suggest that both the 52-kDa target protein and the enzyme system for fatty acylation are in the matrix fraction, although the N-myristylated protein is found loosely associated with the inner membrane. Finally, immunoprecipitation of cytoplasmic free polysome translation products and in vitro transport of proteins into isolated mitochondria show that the 52-kDa protein is of cytoplasmic translation origin. These results demonstrate that the intramitochondrial N-myristylation of the 52-kDa protein is not translationally linked.