Dynamic Regulation of Function of the Mitochondrial TIM23 Preprotein Translocase

The vast majority of mitochondrial proteins are synthesized on
cytosolic ribosomes in the form of precursor proteins and
subsequently imported into mitochondria through the concerted
action of the translocases present in the outer and the inner
membrane. The TIM23 complex (translocase of the inner membrane)
mediates translocation of precursor proteins across or their
insertion into the mitochondrial inner membrane in a membrane
potential and ATP-dependent manner. The TIM23 complex consists of
eight essential subunits that can be assigned to two operationally
defined parts: the membrane embedded protein conducting channel
with the receptor and the import motor associated with the channel
at the matrix side of the inner membrane. The present study was
undertaken to gain insight into the dynamics of the TIM23
translocase during import of different types of preproteins. A
previously uncharacterized protein component of the TIM23
translocase was identified and termed Tim21. Results presented in
this study demonstrate that the TIM23 translocase switches between
translocation mode that facilitates import of proteins into the
matrix and insertion mode that allows lateral sorting of proteins
into the lipid bilayer. The TIM23 translocase adopts different
conformations in its various states of activity: when it was empty,
when it inserted preproteins into the inner membrane and when it
translocated preproteins targeted to the matrix. The
interconversion of the TIM23 translocase between the functional
states occurs primarily by conformational changes of the essential
components, whereas non-essential components Tim21 and Pam17 are
responsible for the fine tuning of these processes. A hypothesis
that describes the behavior of the TIM23 translocase is presented.