The Presequence of a Precursor to the Delta-subunit of Sweet Potato Mitochondrial F1ATPase is Not Sufficient for the Transport of Beta-glucuronidase (GUS) into Mitochondria of Tobacco, Rice and Yeast Cells

A precursor to the delta-subunit of sweet potato mitochondrial F1ATPase (pre-F1 delta) has an amino-terminal (N-terminal) presequence of 45 amino acid residues and its N-terminal 18 residues may form an amphiphilic alpha-helix, which is typical of mitochondrial targeting signals [Kimura et al. (1990) J. Biol. Chem. 265: 6079]. Fusion genes consisting of sequences that encoded the 25 (DG25), 46 (DG46) and 73 (DG73) N-terminal amino acids from pre-F1 delta fused to the N-terminus of the coding sequence of bacterial beta-glucuronidase (GUS) were placed downstream of the 35S promoter of cauliflower mosaic virus and used to transform suspension-cultured tobacco cells, rice calli and tobacco plants. Fusion genes were also placed downstream of the yeast GAL10 promoter and introduced into Saccharomyces cerevisiae cells. In these transformed cells, only the DG73 GUS-fusion protein was transported into mitochondria and subjected to proteolytic cleavage of the presequence. Neither transport to mitochondria nor processing of the presequence of the DG46 GUS-fusion protein, which contained the entire presequence and the processing site, occurred in either plant or yeast cells. These results indicate that the presequence of pre-F1 delta is not sufficient for the transport of the GUS protein into mitochondria in tobacco, rice and yeast cells. The requirement for the longer polypeptide from pre-F1 delta in the transport of the GUS protein into mitochondria could be due either to the lack of sufficient information for mitochondrial targeting within the presequence or to the nature of the passenger protein, GUS, used in this study.