Import of proteins into mitochondria. Extramitochondrial pools and post-translational import of mitochondrial protein precursors in vivo.

Extramitochondrial pools of mitochondrial precursor polypeptides can be detected in yeast spheroplasts which had been pulse-labeled for 3 min in the absence of inhibitors or for 10 min in the presence of an uncoupler of oxidative phosphorylation. In either case, these precursors can be post-translationally converted to the mature forms during a subsequent chase. Post-translational maturation of the precursor to F1-ATPase beta-subunit is accompanied by uptake of the polypeptide into mitochondria. Pools of precursor polypeptides are also found in pulse-labeled intact yeast cells, the pool sizes depending on the metabolic status of the cells. For example, if the rate of protein synthesis is lowered by cycloheximide, the precursor pool size decreases. This indicates that living yeast cells can, and apparently do, import polypeptides into mitochondria post-translationally.