THE ESSENTIAL YEAST PROTEIN MIM44 (ENCODED BY MPI1) IS INVOLVED IN ANEARLY STEP OF PREPROTEIN TRANSLOCATION ACROSS THE MITOCHONDRIAL INNERMEMBRANE

The essential yeast gene MPI1 encodes a mitochondrial membrane protein that is possibly involved in protein import into the organelle (A. C. Maarse, J. Blom, L. A. Grivell, and M. Meijer, EMBO J. 11:3619-3628, 1992). For this report, we determined the submitochondrial location of the MPI1 gene product and investigated whether it plays a direct role in the translocation of preproteins. By fractionation of mitochondria , the mature protein of 44 kDa was localized to the mitochondrial inne r membrane and therefore termed MIM44. Import of the precursor of MIM4 4 required a membrane potential across the inner membrane and involved proteolytic processing of the precursor. A preprotein in transit acro ss the mitochondrial membranes was cross-linked to MIM44, whereas prep roteins arrested on the mitochondrial surface or fully imported protei ns were not cross-linked. When preproteins were arrested at two distin ct stages of translocation across the inner membrane, only preproteins at an early stage of translocation could be cross-linked to MIM44. Mo reover, solubilized MIM44 was found to interact with in vitro-synthesi zed preproteins. We conclude that MIM44 is a component of the mitochon drial inner membrane import machinery and interacts with preproteins i n an early step of translocation.