The precursor to ornithine carbamyl transferase is transported to mitochondria as a 5S complex containing an import factor.

The precursor to ornithine carbamyl transferase (Mr = 40,000) was synthesized in a rabbit reticulocyte lysate system and purified by immunoaffinity chromatography. Import of purified precursor by isolated mitochondria depended upon the presence of import factor(s) in fresh reticulocyte lysate. Velocity sedimentation analyses indicated that import factor binds to precursor to form a 5S complex (approximately 90 kDa); in this form, precursor was efficiently imported by isolated mitochondria. The ability of the 5S complex to deliver precursor into mitochondria was not affected by pretreatment with high concentrations of RNase. Import factor did not bind to mitochondria in the absence of precursor; upon binding of precursor to mitochondria in the presence of import factor, subsequent transmembrane uptake of precursor did not require the continued presence of additional lysate components.