Effect of methylation on the stability of cytochrome c of Saccharomyces cerevisiae in vivo.

The in vivo stability of methylated and unmethylated cytochrome c in Saccharomyces cerevisiae was studied by pulse-labeling the hemoproteins with [methyl-3H]-methionine and/or [2-14C]methionine and following the fate of these proteins under anaerobiosis and in the presence of cycloleucine. These two conditions will respectively block further cytochrome c synthesis and inhibit methylation by lowering the cellular S-adenosyl-L-methionine pool and, thus, permit an unambiguous interpretation of the data. The results showed that the rate of degradation of unmethylated cytochrome c was constant throughout the chase period, while methylated cytochrome c degradation was seen only in the later part of cold chase. At the end of the chase period (40 h), the extent of degradation of the unmethylated species was three times higher than the methylated species. This indicated that the methylation of cytochrome c has a protective effects against the intracellular proteolytic enzyme attack on itself. Furthermore, this protective effect was considerably reduced in the petite mutant, which lacks high affinity cytochrome c binding sites, functional cytochrome c reductase, and oxidase, and possesses a less integrated and organized mitochondrial membrane. These results led us to the conclusion that the mechanism of methylated cytochrome c stabilization is best explained by a higher efficacy of binding to the mitochondria.