Aggregation, Protein
35
several highly conserved families of related
proteins. They can be divided into two
classes according to their size. Small chap-
erones are less than 200 kDa, whereas
large chaperones are more than 800 kDa.
During the past few years, a large amount
of biochemical, biophysical, and low- and
high-resolution structural data have pro-
vided mechanistic insights into the ma-
chinery of protein folding as assisted by
molecular chaperones.
Molecular chaperones are involved in
diverse cellular functions. The constitutive
members of the heat-shock protein family
(Hsp70) can stabilize nascent polypep-
tide chains during their elongation in
ribosomes. The large cylindrical chaper-
onins GroEL in bacteria, mitochondria,
and chloroplasts and the corresponding
TriC in eukaryotes and archaebacteria
provide a sequestered environment for
productive folding. Several chaperones are
stress-dependent; their expression is in-
duced under conditions such as high
temperatures, which provoke protein un-
folding and aggregation. The members of
the Hsp90 and Hsp100 families, as well
as small Hsp, play a role in preventing
protein aggregation under stress. Chap-
erone interactions are also important for
the translocation of polypeptide chains
into membranes.
Within cells, the nascent polypeptide
chain is synthesized sequentially on the
ribosome
by
a
vectorial
process.
For
many proteins, the rate of this process
is slower than the rate of folding. Syn-
thesis times range from 20 s for a 400
residue–polypeptide chain in
E. coli
at
37
Ct
o1
0t
im
e
sa
sl
o
n
gf
o
rs
u
c
ha
chaininaneukaryoticcell.Manyunfolded
proteins refold completely in 20 s under
the same conditions. Thus, there is the
possibility for the elongating polypeptide
either to misfold before completion or to
be degraded by proteolytic enzymes. Chap-
erones prevent such unfavorable events by
protecting the nascent chain. Hsp70 and
its prokaryotic homolog DnaK recognize
extended hydrophobic regions of the elon-
gating polypeptides. These interactions are
not speci±c. Hsp70 and DnaK interact with
most unfolded polypeptide chains that ex-
pose hydrophobic residues. They do not
recognize folded proteins. Binding and re-
lease of unfolded proteins from Hsp70
are ATP-dependent and require the pres-
ence of various cochaperones such as DnaJ
and GrpE. The basic mechanism of Hsp70
(DnaK in
E. coli
) is represented in Fig. 6.
In
E. coli
, DnaJ binds the nascent un-
folded polypeptide, U; then the complex
binds to the ATP-bound state of DnaK.
ATP is hydrolyzed in the ternary complex
DnaJ
ATP
DnaK
ATP
ADP
ATP
ATP
Or towards GroEL
ADP
GrpE
Pi; DnaJ
U
UN
Fig. 6
Schematic representation of the basic mechanism of DnaK (see text).
previous page 35 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 37 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off