Institute of Cell and Molecular Biology, University of Edinburgh, Scotland, UK.
Most proteins present in the mitochondrion are nuclear encoded, and are directed to the organelle by virtue of a targeting sequence at the N terminus of the precursor protein. Mitochondrial (mt) protein targeting appears to require several accessory proteins that recognise mt precursors both in the cytoplasm and at the mt surface. We describe here the use of yeast genetics to identify a protein that is required for mt protein targeting. Two yeast mutants (mts1 and mts2) were isolated as extragenic suppressors of a known targeting defect in the presequence of the beta-subunit of ATP synthase. We have cloned and sequenced the wild-type allele of one of these genes (MTS1) and shown that it encodes a member of a family of RNA-binding proteins that is essential for growth.