Chaperones, Molecular
the phenotype through which the pro-
tein was frst discovered, and which
may be quite specifc to a particu-
lar organism.
– Fourth, the name ‘‘molecular chap-
erone’’ is used in a very catholic
sense to re±er to any protein with the
properties listed in Sect. 1 above, but
these may range ±rom extremely broad-
spectrum chaperones, which act on a
large range o± substrates to those that
have evolved to act with only one specifc
– Fi±th, many proteins are un±ortunately
described as molecular chaperones with
only minimal supporting evidence ±rom
in vitro
in vivo
The Table 1 is not intended to be
comprehensive, but illustrates the major
known ±amilies o± molecular chaperones,
together with the names o± some o± their
better-studied members, their cellular lo-
cations, and a very brie± description o±
their properties. The individual molecu-
lar chaperone ±amilies shown in the Table
are discussed more ±ully in the ±ollow-
ing sections.
Mechanisms and Roles of the Major
Molecular Chaperones
The Hsp60 Family
Introduction to the Hsp60 Family
The Hsp60 ±amily is one o± the best
characterized o± the molecular chaperone
±amilies. It is a large ±amily consisting
o± highly similar proteins, all with a sub-
unit molecular mass o± around 60 kDa.
Sequence comparisons have enabled the
division o± the Hsp60 ±amily into two
groups: Group I and Group II. The Group
I proteins are ±ound in nearly all bacteria,
and also in mitochondria and chloroplasts.
chaea, and in the eukaryotic cytosol. All
these proteins have a remarkable multi-
meric structure: Group I proteins assem-
ble into a ‘‘double doughnut’’ structure
with seven subunits in each ring, and the
Group II proteins assemble into a similar
structure but with eight or nine subunits
in each ring, depending on the particular
protein and organism. Each ring encloses
a cavity or cage. Proteins in the Hsp60
±amily are o±ten re±erred to generically as
Cellular Roles of the Hsp60 Family
The Group I ±amily o± the Hsp60 proteins
has been the most intensively studied, par-
ticularly the protein ±rom
Escherichia coli
which is re±erred to as GroEL. Genetic
evidence shows that these proteins are
essential, both in prokaryotes and eukary-
otes, and they are ±ound in all organisms
so ±ar studied, with the exception o± some
mycoplasmas. They are present at ±airly
high levels in cells (
E. coli
is estimated
to contain around 2000 GroEL complexes
per cell under normal growth conditions)
and are ±urther induced by heat shock.
All o± them appear to require a cochap-
erone, called Hsp10, which is a smaller
protein o± typically around 10 kDa, which
itsel± assembles into a ring with seven-
±old symmetry. In
E. coli
, this protein is
called GroES, and in
E. coli
and most
other bacteria it is encoded in the same
operon as the GroEL protein and is also
I± the levels o± GroEL become too
low to sustain cellular growth, or i± a
temperature-sensitive mutant is shi±ted to
the nonpermissive temperature, a large
subset o± cellular proteins ±ails to ±old
previous page 1168 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1170 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off