490
Chaperones, Molecular
which active biological material could be
formed. Molecular chaperones prevent or
reverse such interactions, which explains
why they are so important for normal
cellular function. Although the original
chaperone (nucleoplasmin) is the key to
chromatin assembly, the vast majority of
chaperones are involved in protein folding
and assembly, and it is only these that will
be considered in this chapter.
In order for a given protein to be
classiFed as a molecular chaperone, the
chaperone activity of the protein should
be experimentally demonstrable. Ideally,
this demonstration would be made both
in vitro
and
in vivo
, through a combination
of biochemical and genetic methods, but
in practice this is not always feasible. The
kinds of experiments that can demonstrate
molecular chaperone activity are discussed
below, in the sections on individual classes
of molecular chaperone.
2
The Need for Molecular Chaperones:
Protein Folding
In Vitro
and
In Vivo
All life depends upon the ability of pro-
teins to fold into speciFc conformations.
An understanding of the process by which
proteins succeed in folding from a linear
chain of amino-acyl residues to a function-
ing tertiary or quaternary structure is thus
central to an appreciation of all aspects of
cell and organismal biology. The fact that
this process occasionally goes wrong, and
in doing so can lead to many pathological
states, makes the need for such an under-
standing even more important. Most of the
initial studies in this Feld were done using
highly puriFed proteins, and these gave us
many important insights into the essential
features of protein folding. Studies of this
nature are still continuing. More recently,
however, it has been realized that protein
folding inside the cell is a more complex
process than that studied in the test tube,
and many key discoveries have been made
by looking at cellular protein folding in
more detail. These include the discovery of
molecular chaperones, which act in many
ways to help proteins overcome some of
the intrinsic problems caused by having
to fold up in the complex internal envi-
ronment of the cell. This initial section
will thus briefly review our current under-
standing of protein folding both
in vitro
and
in vivo
, and will discuss in general
terms some of the areas where molecu-
lar chaperones have been shown to be
important or essential.
The earliest work in the area of pro-
tein folding was done by AnFnsen, who
demonstrated that proteins will sponta-
neously adopt their native, active confor-
mation in aqueous solution, thus proving
that folded proteins are more thermody-
namically stable than unfolded proteins.
This conclusion was reached by taking
puriFed proteins, denaturing them in suit-
able solvents that caused them to adopt
an unfolded conformation, and then re-
moving the denaturants and observing
recovery of activity of the protein in ques-
tion. No energy source is required in these
types of experiments for proteins to refold,
thus proving that the folded and active
protein must be more stable than the un-
folded form.
Although this was an elegant demon-
stration of the thermodynamically spon-
taneous nature of protein folding, it was
subsequently pointed out by Levinthal that
there was an apparent kinetic problem
in protein folding. Because the number
of possible conformations that can be
adopted by a protein is very high indeed,
and it takes a Fnite time for proteins
to sample these different conformations,
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