Mas6p can be cross-linked to an arrested precursor and interacts with other proteins during mitochondrial protein import

Mas6p is an integral membrane protein of the yeast mitochondrial inner membrane, which is essential for mitochondrial protein import (1). To determine whether Mas6p is directly involved in recognizing precursors or translocating them across the inner membrane, we asked if Mas6p was in close proximity to precursor proteins being imported into mitochondria. We report here that Mas6p can be chemically cross-linked to an imported protein arrested in transit through the mitochondrial inner membrane. Antiserum to Mas6p specifically immunoprecipitates one of several different mitochondrial proteins that are cross-linked to blocked precursors. Our results strongly suggest that Mas6p physically interacts with precursors during their translocation into the matrix. In addition, at least two other mitochondrial proteins that are each cross-linked to arrested precursors can be coimmunoprecipitated along with Mas6p under non-denaturing conditions. These observations provide evidence for a complex of proteins including Mas6p, each of which interacts with mitochondrial precursors during import.