Removal of an N-terminal peptide from mitochondrial aspartate aminotransferase abolishes its interactions with mitochondria in vitro.

Treatment of mitochondrial aspartate aminotransferase from rat liver with trypsin leads to specific cleavage of the bonds between residues 26 and 27, and residues 31 and 32. The proteolysed enzyme has only a small residual catalytic activity, but retains a conformation similar to that of the native form as judged by accessibility and reactivity of cysteine residues. Proteolysis abolishes the ability of the enzyme either to bind to mitochondria or to be imported into the organelles. This suggests that the N-terminal segment of the native enzyme is essential for both of these functions, at least in the model system used to study the import process.