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J. Biol. Chem., Vol. 279, Issue 11, 9803-9810, March 12, 2004

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Ribosomes Specifically Bind to Mammalian Mitochondria via Protease-sensitive Proteins on the Outer Membrane^*

James A. MacKenzie and R. Mark Payne¶

From the Section on Molecular Medicine, Department of Internal Medicine and Section on Cardiology, Department of Pediatrics, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1081

The interaction of ribosomes with specific components of membranes is one of the central themes to the co-translational targeting and import of proteins. To examine ribosome binding to mammalian mitochondria, we used ribosome-nascent chain complexes (RNCs) to follow the in vitro binding of ribosomes that correspond to the initial targeting stage of proteins. Mitochondria were found to contain a limited number of RNC binding sites on the outer membrane. It required more than twice the amount of non-translating ribosomes to inhibit RNC binding by one-half, indicating that RNCs have a competitive binding advantage. In addition, we found that RNCs bind mainly through the ribosomal component and not the nascent chain. RNCs bind via protease-sensitive proteins on the outer membrane, as well as by protease-insensitive components suggesting that two classes of receptors exist. We also show that binding is sensitive to cation conditions. Nearly all of the binding was inhibited in 0.5 M KCl, indicating that they interact with the membrane primarily through electrostatic interactions. In addition, disruption of RNC structure by removing magnesium causes the complete inhibition of binding under normal binding conditions indicating that it is the intact ribosome that is crucial for binding and not the nascent chain. These findings support the hypothesis that the outer mitochondrial membrane contains receptors specific for ribosomes, which would support the conditions necessary for co-translational import.

Received for publication, July 4, 2003 , and in revised form, November 14, 2003.

^* This work was supported by National Institutes of Health NIDDK Grant RO1 DK55765. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: Dept. of Pediatrics, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1081. Tel.: 336-716-4627; Fax: 336-716-0533; E-mail: mpayne{at}wfubmc.edu.

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