Differential Recognition of Preproteins by the Purified Cytosolic Domains of the Mitochondrial Import Receptors Tom20, Tom22, and Tom70*
- From the Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Straße 7, D-79104 Freiburg, Germany
Abstract
The preprotein translocase of the outer mitochondrial membrane (Tom) is a multi-subunit complex required for specific recognition and membrane translocation of nuclear-encoded preproteins. We have expressed and purified the cytosolic domains of three postulated import receptors, Tom20, Tom22, and Tom70. Each receptor domain is able to bind mitochondrial preproteins but with different specificity. Tom20 binds both preproteins with N-terminal presequences and preproteins with internal targeting signals; the binding is enhanced by the addition of salt. Tom22 selectively recognizes presequence-carrying preproteins in a salt-sensitive manner. Tom70 preferentially binds preproteins with internal targeting information. A chemically synthesized presequence peptide competes with preproteins for binding to Tom20 and Tom22 but not to Tom70. We conclude that each of the three import receptors binds preproteins independently and by a different mechanism. Both Tom20 and Tom22 function as presequence receptors.
Footnotes
-
↵* This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 388, and the Fonds der Chemischen Industrie.The costs of publication of this article were defrayed in part by the payment of page charges. The 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. Tel.: 49-761-203-5224; Fax: 49-761-203-5261; E-mail: pfanner{at}ruf.uni-freiburg.de.
-
↵1 The abbreviations used are: MOPS, 4-morpholinepropanesulfonic acid; DHFR, dihydrofolate reductase; PiC, phosphate carrier; Ni-NTA, nickel nitrilotriacetate; PAGE, polyacrylamide gel electrophoresis; BSA, bovine serum albumin.
-
- Received April 22, 1997.
- Revision received June 5, 1997.
