Deletion of the receptor MOM19 strongly impairs import of cleavable preproteins into Saccharomyces cerevisiae mitochondria.

The mitochondrial outer membrane proteins MOM19 and MOM72 are thought to function as import receptors for nuclear encoded preproteins. Different views exist about the importance of each receptor in the import of cleavable and noncleavable preproteins into mitochondria. Here we cloned and sequenced MOM19 from Saccharomyces cerevisiae and constructed a gene disruption mutant. Yeast cells lacking MOM19 were unable to grow on nonfermentable carbon sources and were slow in growing on a fermentable medium, while the growth of yeast cells lacking MOM72 (Mas70p) was much less impaired. delta MOM19 cells accumulated considerable amounts of mitochondrial preproteins in vivo. The import of cleavable preproteins into isolated delta MOM19 mitochondria was strongly inhibited, while import of the noncleavable ADP/ATP carrier and phosphate carrier was only slightly inhibited. The reciprocal situation was found for protein import into delta MOM72 mitochondria. In particular, import of the cleavable precursor of cytochrome c1 into delta MOM72 mitochondria was, in agreement with a previous report (Hines, V., and Schatz, G. (1993) J. Biol. Chem. 268, 449-454), found to be partially inhibited, yet a much stronger inhibition of import was seen into delta MOM19 mitochondria. The direct comparison of protein import into yeast mutants of either receptor yields a unifying hypothesis on mitochondrial preprotein targeting; both receptors have an overlapping specificity, and MOM19 plays a major role for cleavable preproteins. Interestingly, the primary sequence of MOM19 predicts the presence of a tetratricopeptide motif that was also found in MOM72, in the peroxisomal membrane protein PAS8/PAS10, and in several proteins involved in RNA synthesis or mitosis.