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Andreas Strub, Joo Hyun Lim, Nikolaus Pfanner, Wolfgang Voos
The Mitochondrial Protein Import Motor
Mitochondrial proteins are synthesized as precursor
proteins in the cytosol and are posttranslationally
imported into the organelle. A complex system of
translocation machineries recognizes and transports
the precursor polypeptide across the mitochondrial
membranes. Energy for the translocation process is
mainly supplied by the mitochondrial membrane potential
(??) and the hydrolysis of ATP. Mitochondrial
Hsp70 (mtHsp70) has been identified as the major
ATPase driving the membrane transport of the precursor
polypeptides into the mitochondrial matrix. Together
with the partner proteins Tim44 and Mge1,
mtHsp70 forms an import motor complex interacting
with the incoming preproteins at the inner face of the
inner membrane. This import motor complex drives
the movement of the polypeptides in the translocation
channel and the unfolding of carboxy-terminal parts of
the preproteins on the outside of the outer membrane.
Two models of the molecular mechanism of mtHsp70
during polypeptide translocation are discussed. In the
‘trapping’ model, precursor movement is generated
by Brownian movement of the polypeptide chain in the
translocation pore. This random movement is made
vectorial by the interaction with mtHsp70 in the matrix.
The detailed characterization of conditional mutants
of the import motor complex provides the basis for an
extended model. In this ‘pulling’ model, the attachment
of mtHsp70 at the inner membrane
Biological Chemistry, Walter de Gruyter
Print ISSN: 1431-6730
Volume: 381, 09/2000
Pages: 943 - 949