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J Biol Chem, Vol. 274, Issue 28, 19617-19622, July 9, 1999

N-terminal Tail Export from the Mitochondrial Matrix
ADHERENCE TO THE PROKARYOTIC "POSITIVE-INSIDE" RULE OF MEMBRANE PROTEIN TOPOLOGY

From the Institut für Physiologische Chemie der Universität München, Goethestrasse 33, 80336 München, Germany and ^§ Centre de Genetique Moleculaire CNRS, Université Pierre et Marie Curie, 91190 Gif-sur-Yvette, France

Export of N-terminal tails of mitochondrial inner membrane proteins from the mitochondrial matrix is a membrane potential-dependent process, mediated by the Oxa1p translocation machinery. The hydrophilic segments of these membrane proteins, which undergo export, display a characteristic charge profile where intermembrane space-localized segments bear a net negative charge, whereas those remaining in the matrix have a net positive one. Using a model protein, preSu9(1-112)-dihydrofolate reductase (DHFR), which undergoes Oxa1p-mediated N-tail export, we demonstrate here that the net charge of N- and C-flanking regions of the transmembrane domain play a critical role in determining the orientation of the insertion process. The N-tail must bear a net negative charge to be exported to the intermembrane space. Furthermore, a net positive charge of the C-terminal region supports this N-tail export event. These data provide experimental evidence that protein export in mitochondria adheres to the "positive-inside" rule, described for sec-independent sorting of membrane proteins in prokaryotes. We propose here that the importance of a charge profile reflects a need for specific protein-protein interactions to occur in the export reaction, presumably at the level of the Oxa1p export machinery.

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