The yeast mitochondrial protein import receptor Mas20p binds precursor proteins through electrostatic interaction with the positively charged presequence.

Protein import into yeast mitochondria is mediated by the four outer membrane receptors Mas70p, Mas37p, Mas20p, and Mas22p. These receptors may function as two subcomplexes: a Mas37p/Mas70p heterodimer and an acidic complex consisting of Mas20p and Mas22p. To assess the relative contribution of these subcomplexes to precursor binding, we allowed different precursors to bind to the surface of deenergized mitochondria, then reenergized the mitochondria and measured the chase of the bound precursors into the organelles. Productive binding of several precursors with a positively charged amino-terminal matrix targeting sequence, such as SU9-DHFR, hsp60, and mitochondrial cpn10, was strongly inhibited by salt, by low concentrations of a mitochondrial presequence peptide, and by a deletion of Mas20p, but was independent of Mas37p/Mas70p. In contrast, productive binding of the ADP/ATP carrier was not inhibited by salt, the presequence peptide, or a deletion of Mas20p, but was strongly dependent on Mas37p/Mas70p. The precursors of alcohol dehydrogenase III and the Rieske iron-sulfur protein had binding properties between these two extremes. The productively bound precursor of cpn10 could be cross-linked to Mas20p. We conclude that Mas20p binds mitochondrial precursor proteins through electrostatic interactions with the positively charged presequence, whereas Mas37p/Mas70p may recognize some feature(s) of the mature part of precursor proteins.