TOPOGENESIS OF CYTOCHROME-OXIDASE SUBUNIT II - MECHANISMS OF PROTEIN EXPORT FROM THE MITOCHONDRIAL MATRIX

Cytochrome c oxidase subunit II (COXII) in yeast mitochondria is synth esized as a precursor (preCOXII) and is sorted across the inner membra ne, whereby both N and C termini become exposed to the intermembrane s pace. We describe here how this process can be experimentally dissecte d into a number of distinct stages. Our results demonstrate that the t ranslation of COXII is not obligatorily coupled to translocation, Inse rtion into the inner membrane and export of the N and C-terminal domai ns require an energized inner membrane. The export of COXII is indepen dent of both maturation by the Imp1p protease and assembly into the cy tochrome c oxidase complex. When linked to a mitochondrial matrix-targ eting sequence, the N terminal portion of preCOXII (fused to mouse dih ydrofolate reductase) can be imported into the mitochondrial matrix. F ollowing accumulation in the matrix, this chimeric protein can become exported across the inner membrane, delivering the N terminus into the intermembrane space where it undergoes processing by the Imp1p protea se. This export process displays a number of similarities to bacterial protein export and supports the view that the principles of sorting a re conserved from prokaryotes to eukaryotic organelles.