Aggregation, Protein
37
residues exposed to the solvent. A small
intermediate domain connects the two
large domains. The intermediate segments
have some flexibility allowing a hinge-
like opening of the apical domains, which
occurs upon nucleotide binding. These
movements are large and have been visual-
ized by three-dimensional reconstruction
from cryoelectron microscopy by Sebil and
her group.
GroES is a heptamer of 10 kDa subunits
forming a flexible dome-shaped structure
with
an
internal
cavity
large
enough
to accommodate proteins up to 70 kDa.
Each
subunit
is
folded
into
a
single
domain containing
β
-sheets and flexible
loop regions. The loop regions are critical
for the interactions between GroEL and
GroES. It was deduced from electron
microscopy studies that GroES binding
to GroEL induces large movements in
the apical GroEL domains. This provokes
a
signiFcant
increase
in
the
volume
of the central cavity in which protein
folding proceeds. NMR coupled with the
study of hydrogen-exchange techniques
has
indicated
that
small
proteins
are
essentially unfolded in their GroEL-bound
states. Mass spectroscopy has revealed
the presence of fluctuating elements of
secondary structure for several proteins.
In
a
way,
the
GroEL–GroES
system
recognizes nonnative proteins.
The reaction cycle of the GroEL–GroES
system
is
represented
in
±ig. 8.
The
nonnative
protein binds
to
the
apical
domain
of
the
upper
ring
of
GroEL
through hydrophobic interactions. Then,
the equatorial domain of the same ring
binds ATP, and GroES caps the upper
ring, sequestering the protein inside the
internal chamber in which the protein
folding proceeds. The binding of GroES
induces a conformational change in GroEL
and ATP hydrolysis, which is a cooperative
process that produces a conformational
change in the lower ring, allowing it
to bind a nonnative protein molecule.
This
promotes
subsequent
binding
of
ATP and GroES in the lower ring, and
the dissociation of the upper complex,
releasing the protein and ejecting GroES.
If the protein has not reached the native
state, it is subjected to a new cycle.
ADP
ADP
ATP
ATP
ATP
ADP
ADP
ATP
ATP
ADP
ATP
t
6 s
Inf
N
ADP
A
E
I
GroEL
GroES
(I)
(II)
(III)
(IV)
Fig. 8
The reaction cycle of GroEL–GroES. Inf
is the unfolded protein, N the folded one, A is
the apical domain, (in blue), I the intermediate
domain (in red) and E the equatorial domain (in
magenta). (Reproduced from Wang & Weissman
(1999)
Nat. Struct. Biol.
6
, 597, with permission.)
(See color plate p. xxii).
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