Functional analysis of a recently originating, atypical presequence: mitochondrial import and processing of GUS fusion proteins in transgenic tobacco and yeast.

A gene family of at least five members encodes the tobacco mitochondrial Rieske Fe-S protein (RISP). To determine whether all five RISPs are translocated to mitochondria, fusion proteins containing the putative presequences of tobacco RISPs and Escherichia coli beta-glucuronidase (GUS) were expressed in transgenic tobacco, and the resultant GUS proteins were localized by cell fractionation. The amino-terminal 75 and 71 residues of RISP2 and RISP3, respectively, directed GUS import into mitochondria, where fusion protein processing occurred. The amino-terminal sequence of RISP4, which contains an atypical mitochondrial presequence, can translocate the GUS protein specifically into tobacco mitochondria with apparently low efficiency. Consistent with the proposal of a conserved mechanism for protein import in plants and fungi, the tobacco RISP3 and RISP4 presequences can direct import and processing of a GUS fusion protein in yeast mitochondria. Plant presequences, however, direct mitochondrial import in yeast less efficiently than the yeast presequence, indicating subtle differences between the plant and yeast mitochondrial import machineries. Our studies show that import of RISP4 may not require positively charged amino acid residues and an amphipathic secondary structure; however, these structural properties may improve the efficiency of mitochondrial import.