Mitochondrial Localization of the Yeast Forkhead Factor Hcm1

Hcm1 is a member of the forkhead transcription factor family involved in segregation, spindle pole dynamics, and budding in Saccharomyces cerevisiae. Our group described the role of Hcm1 in mitochondrial biogenesis and stress resistance, and in the cellular adaptation to mitochondrial respiratory metabolism when nutrients decrease. Regulation of Hcm1 activity occurs at the protein level, subcellular localization, and transcriptional activity. Here we report that the amount of protein increased in the G1/S transition phase when the factor accumulated in the nucleus. In the G2/M phases, the Hcm1 amount decreased, and it was translocated outside the nucleus with a network-like localization. Preparation of highly purified mitochondria by a sucrose gradient density demonstrated that Hcm1 colocalized with mitochondrial markers, inducing expression of COX1, a mitochondrial encoded subunit of cytochrome oxidase, in the G2/M phases. Taken together, these results show a new localization of Hcm1 and suggest that it acts as a mitochondrial transcription factor regulating the metabolism of this organelle.