Degrades mitochondrial transit peptides after their cleavage in the intermembrane space or in the matrix, and presequence peptides; clearance of these peptides is required to keep the presequence processing machinery running. Preferentially cleaves the N-terminal side of paired basic amino acid residues. Also degrades other unstructured peptides. May function as an ATP-dependent peptidase as opposed to a metalloendopeptidase 