A preprotein
is stably arrested and accumulated in the GIP complex by Tom40 and
Tom22.4 Such a 100 kDa core complex probably contains a single channel
that retains the basic channel properties but is already open in
the absence of preproteins. In contrast, in the presence of Tom22,
the wild-type GIP complex contains tightly regulated channels (probably
three channels). Tom22 apparently represents a component of the machinery
that controls the gate. The cytosolic domains of Tom22 and Tom20
are believed to form the major part of a cis site, which mediates
the import of all preproteins known to use the general import machinery
of mitochondria. The preprotein is then routed through the Tom complex
translocation channel and transferred to a trans site on the intermembrane
space (IMS) side of the outer membrane. The inter-membrane space
exposed segment of Tom40 and the C-terminal tail of Tom22 may contribute
to the trans-site. Matrix-targeted proteins are further transferred
to the matrix through Product Information import machinery in the
inner membrane.
The TOM complex of mammalian mitochondria resembles
the fungal Tom complex, but is distinct from the plant TOM system.
Thus, while unique components of the mammalian mitochondrial import
system have been identified (e.g. TOM34 and metaxin), Tom22, and
Tom37 have not been identified in plant mitochondria.
back
|
