Abstract
Post-translational import of precursor proteins into yeast mitochondria is mediated by at least four protease-sensitive outer membrane proteins: Mas20p, Mas22p, Mas37p, and Mas70p. These "import receptors" recognize either the N-terminal targeting signal or some other feature of mitochondrial precursor proteins. The only exception to this general rule appeared to be the precursor to subunit Va of cytochrome c oxidase (COXVa). Although this precursor carries a typical N-terminal mitochondrial targeting sequence, its import into mitochondria has been suggested to be independent of the known import receptors. Here we show that if import into isolated yeast mitochondria is assayed under conditions in which binding of the COXVa precursor to mitochondria is rate-limiting, import is strongly inhibited by protease pretreatment of the mitochondria or by antibodies against Mas20p. Post-translational import of the COXVa precursor can thus proceed by the general, receptor-mediated pathway.
Highlights
Post-translational import of precursor proteins into yeast mitochondria is mediated by at least four protease-sensitive outer membrane proteins: Mas20p, Mas22p, Mas37p, and Mas70p
We show that if import into isolated yeast mitochondria is assayed under conditions in which binding of the COXVa precursor to mitochondria is rate-limiting, import is strongly inhibited by protease pretreatment ofthe mitochondria or by antibodies against Mas20p
One of the first steps of protein import into mitochondria is the binding of precursor proteins to several "import receptor" subunits on the cytosolic surface of the outer membrane (I, 2)
Summary
Precursors prebound to deenergized mitochondria were chased into mitochondria upon reenergization of the organelles; this "productive binding" was sensitive to the treatments mentioned above [9,10,11,12] These measurements are only valid if the receptor-mediated step is rate-limiting for the import process. Import of the precursor into the mitochondrial inner membrane uses the translocation channels of both mitochondrial membranes [16] and the translocase function of mhsp70 [17] but has been reported to bypass the usual import receptors on the mitochondrial surface This receptor independence was inferred from the observations that import of the COXVa precursor was not inhibited by trypsin pretreatment of mitochondria (I6) or by genetically deleting the Mas70p or Mas20p receptor subunits [17], the published evidence suggested to us that these import assays had been performed in the presence of a functional excess of mitochondria and that binding of the precursor to mitochondria might not have been rate-limiting. Post-translational import of the COXVa precursor into isolated yeast mitochondria can occur by the usual, receptor-mediated import pathway
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