Determinants of cytochrome c pro-apoptotic activity - The role of lysine 72 trimethylation

Cytochrome c released from vertebrate mitochondria engages apoptosis by tri ggering caspase activation. We previously reported that, whereas cytochrome s c from higher eukaryotes can activate caspases in Xenopus egg and mammali an cytosols, iso-l and iso-2 cytochromes c from the yeast Saccharomyces cer evisiae cannot. Here we examine whether the inactivity of the yeast isoform s is related to a post-translational modification of lysine 72, N-epsilon-t rimethylation. This modification was found to abrogate pro-apoptotic activi ty of metazoan cytochrome c expressed in yeast. However, iso-l cytochrome c lacking the trimethylation modification also was devoid of pro-apoptotic a ctivity. Thus, both lysine 72 trimethylation and other features of the iso- l sequence preclude pro-apoptotic activity. Competition studies suggest tha t the lack of pro-apoptotic activity was associated with a low affinity for Apaf-1. As cytochromes c that lack apoptotic function still support respir ation, different mechanisms appear to be involved in the two activities.