Molecular structure of the 8.0 kDa subunit of cytochrome-c reductase from potato and its delta psi-dependent import into isolated mitochondria.

The cytochrome-c reductase (EC 1.10.2.2) of the mitochondrial respiratory chain couples electron transport from ubiquinol to cytochrome c with proton translocation across the inner mitochondrial membrane. The enzyme from potato was shown to be composed of 10 subunits. Isolation and characterization of cDNA clones for the second smallest subunit reveal an open reading frame of 216 bp encoding a protein of 8.0 kDa. The protein exhibits similarities to a 7.2/7.3 kDa subunit of cytochrome-c reductase from bovine and yeast, that is localized on the intermembrane space side of the enzyme complex. It also shows similarity to a previously unidentified 7.8 kDa protein of cytochrome-c reductase from Euglena. The potato 8.0 kDa protein has a segmental structure, as its sequence can be divided into four parts, each comprising a central Arg-(Xaa)5-Val motif. N-terminal sequencing of the mature 8.0 kDa proteins indicates the absence of a cleavable mitochondrial targeting sequence. Import of the in vitro synthesized 8.0 kDa protein into isolated potato mitochondria confirms the lack of a presequence and reveals a dependence of the transport on the membrane potential delta psi across the inner mitochondrial membrane. These features are unique among the intermembrane space proteins known so far.