Yeast translational activator Cbs2p: Mitochondrial targeting and effect ofoverexpression

The yeast translational activator protein Cbs2p is imported into mitochondr ia without obvious proteolytic processing. To test the importance of amino- terminal amino acids for mitochondrial targeting we fused varying portions of the N-terminus with green fluorescent protein and examined the intracell ular distribution of the reporter protein. We show that the 25 N-terminal a mino acids are sufficient to direct the majority of the fusion protein into mitochondria. Cbs2p derivatives lacking 9 to 35 amino acids from the N-ter minus fail to complement the respiratory deficiency of a Delta cbs2 strain, but are still imported into mitochondria. Therefore Cbs2p contains at leas t one independent mitochondrial targeting information in addition to the N- terminal signal. We further analyzed the effect of over-expression of Cbs2p on mitochondrial function. Elevated concentrations of Cbs2p lead to slight ly impaired mitochondrial gene expression, probably as the result of the fo rmation of inactive Cbs2p aggregates.