Transport of proteins across mitochondrial membranes

The vast majority of proteins comprising the mitochondrion are
encoded by nuclear genes, synthesized on ribosomes in the cytosol,
and translocated into the various mitochondrial subcompartments.
During this process proteins must cross the lipid membranes of the
mitochondrion without interfering with the integrity or functions
of the organelle. In recent years an approach combining
biochemical, molecular, genetic, and morphological methodology has
provided insights into various aspects of this complex process of
intracellular protein sorting. In particular, a greater
understanding of the molecular specificity and mechanism of
targeting of mitochondrial preproteins has been reached, as a
protein complex of the outer membrane which facilitates recognition
and initial membrane insertion has been identified and
characterized. Furthermore, pathways and components involved in the
translocation of preproteins across the two mitochondrial membranes
are being dissected and defined. The energetics of translocation
and the processes of unfolding and folding of proteins during
transmembrane transfer are closely linked to the function of a host
of proteins known as heat-shock proteins or molecular chaperones,
present both outside and inside the mitochondrion. In addition, the
analysis of the process of folding of polypeptides in the
mitochondrial matrix has allowed novel and unexpected insights into
general pathways of protein folding assisted by folding factors.
Pathways of sorting of proteins to the four different mitochondrial
subcompartments — the outer membrane (OM), intermembrane space,
inner membrane (IM) and matrix — are only partly understood and
reveal an amazing complexity and variation. Many additional protein
factors are involved in these latter processes, a few of which have
been analyzed, such as cytochrome c heme lyase and cytochrome c 1
heme lyase, enzymes that catalyze the covalent addition of the heme
group to cytochrome c and c 1 preproteins, and the mitochondrial
processing peptidase which cleaves signal sequence after import of
preproteins into the matrix. Thus, the study of transport of
polypeptides through the mitochondrial membranes does not only
contribute to the understanding of how biological membranes
facilitate the penetration of macromolecules but also provides
novel insights into the structure and function of this organelle.
are being dissected and defined. The energetics of translocation
and the processes of unfolding and folding of proteins during
transmembrane transfer are closely linked to the function of a host
of proteins known as heat-shock proteins or molecular chaperones,
present both outside and inside the mitochondrion. In addition, the
analysis of the process of folding of polypeptides in the
mitochondrial matrix has allowed novel and unexpected insights into
general pathways of protein folding assisted by folding factors.
Pathways of sorting of proteins to the four different mitochondrial
subcompartments — the outer membrane (OM), intermembrane space,
inner membrane (IM) and matrix — are only partly understood and
reveal an amazing complexity and variation. Many additional protein
factors are involved in these latter processes, a few of which have
been analyzed, such as cytochrome c heme lyase and cytochrome c 1
heme lyase, enzymes that catalyze the covalent addition of the heme
group to cytochrome c and c 1 preproteins, and the mitochondrial
processing peptidase which cleaves signal sequences after import of
preproteins into the matrix. Thus, the study of transport of
polypeptides through the mitochondrial membranes does not only
contribute to the understanding of how biological membranes
facilitate the penetration of macromolecules but also provides
novel insights into the structure and function of this organelle.