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Molecular and Cellular Biology, November 1998, p. 6515-6524, Vol. 18, No. 11
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Preprotein Translocase of the Outer Mitochondrial
Membrane: Molecular Dissection and Assembly of the General Import
Pore Complex
Peter J. T.
Dekker,
Michael T.
Ryan,
Jan
Brix,
Hanne
Müller,
Angelika
Hönlinger, and
Nikolaus
Pfanner*
Institut für Biochemie und
Molekularbiologie, Universität Freiburg, D-79104 Freiburg,
Germany
Received 24 April 1998/Returned for modification 16 June
1998/Accepted 6 August 1998
The preprotein translocase of the outer mitochondrial membrane
(Tom) is a multisubunit machinery containing receptors and a
general import pore (GIP). We have analyzed the molecular architecture of the Tom machinery. The receptor Tom22 stably associates with Tom40,
the main component of the GIP, in a complex with a molecular weight of
~400,000 (~400K), while the other receptors, Tom20 and Tom70, are
more loosely associated with this GIP complex and can be found in
distinct subcomplexes. A yeast mutant lacking both Tom20 and Tom70 can
still form the GIP complex when sufficient amounts of Tom22 are
synthesized. Besides the essential proteins Tom22 and Tom40, the GIP
complex contains three small subunits, Tom5, Tom6, and Tom7. In mutant
mitochondria lacking Tom6, the interaction between Tom22 and Tom40 is
destabilized, leading to the dissociation of Tom22 and the
generation of a subcomplex of ~100K containing Tom40, Tom7, and Tom5.
Tom6 is required to promote but not to maintain a stable association
between Tom22 and Tom40. The following conclusions are suggested. (i)
The GIP complex, containing Tom40, Tom22, and three small Tom proteins,
forms the central unit of the outer membrane import machinery. (ii)
Tom20 and Tom70 are not essential for the generation
of the GIP complex. (iii) Tom6 functions as an assembly factor for
Tom22, promoting its stable association with Tom40.
*
Corresponding author. Mailing address: Institut
für Biochemie und Molekularbiologie, Universität
Freiburg, Hermann-Herder-Straße 7, D-79104 Freiburg, Germany. Phone:
49-761 203 5224. Fax: 49-761 203 5261. E-mail:
pfanner{at}ruf.uni-freiburg.de.
Molecular and Cellular Biology, November 1998, p. 6515-6524, Vol. 18, No. 11
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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