516
Chaperones, Molecular
string of subunits (PapA) in a helical
cylinder. All these proteins are produced
in the cytosol of
E. coli
andexportedtothe
periplasm, and their subsequent assembly
into a complete pilus requires the action
of the PapD chaperone. In the absence of
PapD, the various subunits are degraded
by periplasmic proteases. PapD is not,
however, part of the assembled pilus, and
so it admirably fulFlls the deFnition of a
molecular chaperone.
Solving the structure of PapD, both
alone and in complexes with proteins
(PapK and PapE) whose folding it assists,
has demonstrated how this chaperone
works. The pilus subunits possess an Ig-
fold, which normally consists of seven
antiparallel beta
sheets. In the pilus
subunits, one of these helices is missing,
which exposes a deep cleft in the protein.
When the protein is in a complex with
PapD, an N-terminal beta sheet from
PapD Flls the space that would normally
be
occupied
by
this
missing
region,
thus stabilizing the protein by hiding
the
hydrophobic
residues
that
would
otherwise be exposed. This is referred
to as ‘‘donor strand complementation,’’
since the completion of the stable Ig-fold
is made possible by the ‘‘donation’’ of a
β
-strand from the chaperone. Indeed, it
is likely to be the case that the folding of
the individual pilus subunits only takes
place in the presence of PapD. Given
that this complex is now stabilized by
the pilus-chaperone interaction, how does
subsequent assembly of the pilus subunit
into the growing pilus take place? This
assembly happens by a process referred
to as ‘‘donor strand exchange,’’ where
the place occupied by the donated strand
of the PapD chaperone is taken by a
donated beta strand from the adjacent
p
i
lu
ssu
bun
i
t
.Th
i
sp
r
o
c
e
s
st
a
k
e
sp
l
a
c
e
at an outer membrane pore made up of
the protein PapC (sometimes referred to
as an ‘‘usher’’). PapD, complexed to the
various different pilus subunits, arrives
at the PapC, and donor strand exchange
takes place in order to add each new
pilus subunit to those already there, which
will thus be progressively extruded from
the outer membrane of the cell. The
order in which the subunits are found
is presumably determined by the relative
afFnities of the different pilus subunits
for each other, although this has not been
directly proven. Recent results show that
theroleofPapDinthisentireprocessisfar
from passive. PapD primes pilus subunits
for assembly by holding the groove in
an open conformation, which is closed
during donor strand exchange, effectively
locking the N-terminal extension of each
adjacent pilus subunit in place on its
neighbor. The interaction between PapD
and its substrates is highly speciFc and,
moreover, PapD can be said to be providing
steric information required for the correct
folding of these substrates.
6
Conclusion
This review has focused on the roles of
molecular chaperones in the cell and de-
scribed some instances of the way in which
these roles can be understood in terms of
the structures and reaction mechanisms of
individual molecular chaperones. It must
be emphasized that this is an area of re-
search that has expanded very rapidly over
the past decade and a half, and many
aspects of molecular chaperone biology
have had to be neglected in such a brief
discussion. More attention is now being
paid to the networks in which molecular
chaperones operate and how these overlap
with other key areas of the cell’s activity,
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