Imported mitochondrial proteins cytochrome b2 and cytochrome c1 are processed in two steps.

Abstract

Cytochrome b2 of yeast [Saccharomyces cerevisiae] is located in the space between the inner and outer mitochondrial membranes whereas cytochrome c1 is bound to the outer face of the mitochondrial inner membrane. Both proteins are made outside the mitochondria as larger precursors that are processed to their mature forms in 2 steps. In the 1st step, at least a segment of the precursor polypeptide chain penetrates into the mitochondrial matrix and is cleaved to an intermediate form by a matrix-localized soluble protease. This step requires an electrochemical gradient across the mitochondrial inner membrane. In the 2nd step, the intermediate form is converted to the mature form. For cytochrome c1, this 2nd step requires heme. Import of these 2 cytochromes thus involves a detour across the inner membrane.