Chemistry Reference
In-Depth Information
with their bacterial homologues have yielded rich understanding of the processes of
protein folding and FeS cluster assembly, the co-chaperones regulating protein im-
port are unique to mitochondria and must have evolved some time after the original
endosymbiont began to assume the form of an organelle. Much of the understanding
of mitochondrial function comes from studies in yeast
Saccharomyces cerevisiae
,
and their co-chaperone complement will be examined in this chapter.
The Mitochondrial Homologue of DnaK and its
Co-Chaperones
The major mitochondrial Hsp70 chaperone (called Ssc1 in yeast) is regulated by
5 co-chaperones of the J protein family. These co-chaperones define the spectrum
of processes Ssc1 mediates, such as protein folding, import and degradation. Pro-
tein folding is a conserved process, mediated in the mitochondrial matrix in an
analogous way to the process in the bacterial cytoplasm, and two co-chaperones
regulating the process in mitochondria (Mdj1 and Mge1/Yge1) are mitochondrial
equivalents of the bacterial co-chaperones DnaJ and GrpE. Mdj1, a DnaJ homo-
logue, is the only type I J protein in mitochondria (Rowley et al.
1994
). Mge1, a
GrpE homologue is the only bacterial-type nucleotide exchange factor in eukary-
otes (Bolliger et al.
1994
).
The mitochondrial Ssc1/Mdj1/Mge1 chaperone system shares biochemical, as
well as functional properties with the bacterial DnaK/DnaJ/GrpE (Fig.
10.1
). Like
DnaJ, Mdj1 binds to and prevents the aggregation of unfolded firefly luciferase
in
in vitro
assays (Prip-Buus et al.
1996
). The unfolded substrate is delivered to
the major chaperone and a typical J/Hsp70 interaction follows: Mdj1, through its J
domain stimulates the ATPase activity of Ssc1, coupling it with substrate delivery
(Kubo et al.
1999
). Firefly luciferase folding is facilitated by Ssc1, but the substrate
is efficiently released only in the presence of Mge1, which affects ADP release
from the chaperone. Mge1 dimer is sensitive to reactive oxygen species, and may
act as an oxidative state sensor (Marada et al.
2013
). Nucleotide release factors in
other eukaryotic compartments are not of bacterial origin, and the presence of this
bacterial-type co-chaperone further underlines the prokaryotic nature of the mito-
chondrial chaperone system.
Mdj1 can both deliver substrate to DnaK, and stimulate the ATPase activity of
DnaK
in vitro
, resulting in a productive folding interaction (Deloche et al.
1997b
).
It rapidly binds to ATP bound form of Ssc1, and disassociates more slowly. The rate
of Mdj1 release from Ssc1 is significantly increased in the presence of a protein
substrate (Mapa et al.
2010
). If expressed in bacterial cells, Mge1 can replace GrpE.
Also functional in bacterial cells is a hybrid J protein composed of the glycine-rich
and zinc finger domains of DnaJ fused to the J domain of Mdj1 (Deloche et al.
1997a
). Despite this apparent similarity between the co-chaperones of mitochon-
dria and bacteria, the two chaperone machines are not completely equivalent. Ssc1
can not, even in the presence of Mdj1, complement the loss of DnaK
in vivo
, and
Search WWH ::
Custom Search