Mitochondrial import of rat pre-ornithine transcarbamylase: accurate processing of the precursor form is not required for uptake into mitochondria, nor assembly into catalytically active enzyme

Mitochondrial uptake of the cytoplasmically synthesized precursor of the mammalian enzyme ornithine transcarbamylase is mediated by an N-terminal leader sequence of 32 amino acids. In the mitochondrial matrix, the precursor form is processed to the mature subunit by proteolytic removal of this pre-sequence and in the enzyme from rat liver it has been suggested that this occurs in a two-step process which involves an intermediate cleavage at residue 24. We show that deletion of residues 20-26 spanning this intermediate cleavage site prevents correct processing to the mature subunit but it does not prevent mitochondrial targeting and internalization or assembly of the incorrectly processed product into a catalytically active enzyme. The incorrectly processed enzyme, which is larger than the normal mature enzyme, is nevertheless more susceptible to proteolytic degradation in permanently transfected human cells than the correctly processed enzyme.