Incomplete arrest in the outer membrane sorts NADH-cytochrome b5 reductase to two different submitochondrial compartments.

The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. The primary translation product has an amino-terminal matrix-targeting signal, followed by a stretch of 21 uncharged amino acids. This precursor protein is inserted into the outer membrane, but only about one-third of the molecules become firmly anchored to the outer face of that membrane. The remaining molecules pass through the outer membrane into the inner membrane, are cleaved by inner membrane protease 1, and are released into the intermembrane space. Incomplete translocation arrest in the outer membrane is a novel mechanism by which the product of a single gene is sorted into different compartments of the same organelle.