Effect of enzymatic methylation of yeast iso-1-cytochrome c on its isoelectric point

Abstract

Yeast iso-1- unmethylated and methylated apocytochrome c were synthesized in vitro by translating yeast cytochrome c mRNA, and by subsequently methylating the protein product. Unmethylated and methylated iso-1-holocytochrome c were extracted from Saccharomyces cerevisiae. By employing a column isoelectrofocusing technique, the p I values of these proteins were determined. The p I values of unmethylated and methylated apocytochrome c were found to be 9.60 and 8.70, respectively, with a difference of 0.90 p I unit. On the other hand, the p I values of unmethylated and methylated holocytochrome c were 9.72 and 9.68, respectively, with a difference of 0.04 unit. Therefore, although the p I values of both apo- and holocytochrome c decreased by methylation, methylation of apocytochrome c had a more profound effect on the p I of the protein. The result also indicated that conjugation of heme to apocytochrome c increased its p I value, resulting in the more “compact” and basic structure of the protein. The observed magnitude of the p I change subsequent to the methylation of apocytochrome c (decrease of 0.90 unit) seemed to be contradictory to the predicted increase in the value, since the positive charge is fixed on the quaternary amino group of trimethyllysine and there is no proton to titrate. Trimethylation of ϵ-NH 2 group of Res-72 lysine of apocytochrome c could disrupt any possible hydrogen bond formed by the nitrogen atom of Res-72 lysine residues, as visualized by a space-filling model. The model and observed shift in the “effective charge” of the protein strongly suggest that conformational change in the apoprotein takes place upon methylation. This presumably altered conformation along with the decrease in p I caused by methylation may play a role in enhancement of apocytochrome c import into mitochondria.