Lability of the products of mitochondrial protein synthesis in Saccharomyces cerevisiae yeasts

Estimation of the rate of degradation of the products of mitochondrial protein synthesis in S. cerevisiae cells is reported. The method developed for this purpose is based on pulse incorporation of a labeled amino acid in the presence of an inhibitor of cytoplasmic protein synthesis and allows one to monitor postincorporation of the label. The label incorporated is shown to be rapidly released from mitochondria. Its content is decreased 2-fold during 20-30 min at the beginning and 50-60 min at the end of the exponential phase of growth. The label is detected in cytosol proteins and the TCA-soluble fraction of mitochondria, which is indicative of possible proteolysis of mitochondrial membrane proteins. Since release of the label does not undergo inhibition by specific inhibitors of yeast cell proteinases (pepstatin and phenylmethylsulfonyl fluoride), it may be assumed that these proteinases are either not involved in the digestion of the products of mitochondrial protein synthesis or do not represent a rate-limiting step of the process.