Two Distinct and Independent Mitochondrial Targeting Signals Function in the Sorting of an Inner Membrane Protein, Cytochromec 1

Proteins of the mitochondrial inner membrane display a wide variety of orientations, many spanning the membrane more than once. Some of these proteins are synthesized with NH 2 -terminal cleavable targeting sequences (presequences) whereas others are targeted to mitochondria via internal signals. Here we report that two distinct mitochondrial targeting signals can be present in precursors of inner membrane proteins, an NH 2 -terminal one and a second, internal one. Using cytochrome c 1 as a model protein, we demonstrate that these two mitochondrial targeting signals operate independently of each other. The internal targeting signal, consisting of a transmembrane segment and a stretch of positively charged amino acid residues directly following it, initially directs the translocation of the preprotein into the intermembrane space. It then inserts into the inner membrane from the intermembrane space side in a Δψ-dependent manner and thereby determines the orientation the protein attains in the inner membrane. Analysis of a number of other presequence-containing protein of the inner membrane suggest that they too contain such internal targeting signals.