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Journal of Bacteriology, September 1999, p. 5871-5875, Vol. 181, No. 18
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
A Single-Ring Mitochondrial Chaperonin
(Hsp60-Hsp10) Can Substitute for GroEL-GroES In Vivo
Kåre L.
Nielsen,1,
Neil
McLennan,2,
Millicent
Masters,2 and
Nicholas J.
Cowan1,*
Department of Biochemistry, New York
University Medical Center, New York, New York
10016,1 and Institute of Cell and
Molecular Biology, University of Edinburgh, Edinburgh EH9 3JR,
Scotland2
Received 23 March 1999/Accepted 10 June 1999
 |
ABSTRACT |
Chaperonins participate in the facilitated folding of a variety of
proteins in vivo. To see whether the same spectrum of target proteins
can be productively folded by the double-ring prokaryotic chaperonin
GroEL-GroES and its single-ring human mitochondrial homolog,
Hsp60-Hsp10, we expressed the latter in an Escherichia coli
strain engineered so that the groE operon is under strict regulatory control. We found that expression of Hsp60-Hsp10 restores viability to cells that no longer express GroEL-GroES, formally demonstrating that Hsp60-Hsp10 can carry out all essential in vivo
functions of GroEL-GroES.
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TEXT |
Chaperonins are ubiquitous,
essential, multisubunit ATPases in which the subunits form a
cylindrical structure (a ring) enclosing a central cavity. They are
thought to assist the folding of their target proteins by providing a
sequestered environment conducive to correct folding in which extended
proteins can fold while shielded from nonproductive interactions with
other proteins (reviewed in references 2, 3, and
7). Two classes of chaperonin have been defined on
the basis of sequence relationships and the requirement for
cochaperonin (9): class I chaperonins (found in prokaryotes
and the organelles descended from them), which function in conjunction
with a cochaperonin, and class II chaperonins (found in archaebacteria
and in the cytosol of eukaryotes), which have no requirement for
cochaperonin. Only one kind of chaperonin molecule generally exists in
any given cellular compartment; therefore, any chaperonin must be
capable of facilitating the correct folding of a range of target
proteins. For example, in Escherichia coli, the chaperonin
GroEL (together with its cochaperonin GroES) is thought to participate
in the facilitated folding of 2 to 7% of newly synthesized proteins
(4, 11). Because these substrates are likely to be diverse
in sequence, it follows that the binding of target protein by
chaperonin must depend on properties common to misfolded, unfolded, or
partially folded target molecules (such as, for example, exposed
hydrophobic surfaces) that are recognized by corresponding surfaces
lining the chaperonin's central cavity (1).
The wide range of target proteins bound by chaperonins suggests that
chaperonin-mediated folding could be so nonspecific that any chaperonin
might facilitate the folding of any target protein. However, this is
not the case. Class I chaperonins are incapable of facilitating the
productive folding of actins and tubulins, target proteins which can
proceed to the native state only via an interaction with the class II
chaperonin found in the cytosol of eukaryotes (18).
Conversely, the eukaryotic cytosolic chaperonin does not bind unfolded
malate dehydrogenase, which is recognized and folded by class I
chaperonins (2). These observations are examples of
chaperonin specificity, a phenomenon which probably reflects the
coevolution of chaperonins and their principal target proteins
(10).
Among class I chaperonins, a major difference exists in the mechanism
of the facilitated folding reaction. In E. coli, the chaperonin GroEL functions by a two-stroke engine action
whereby the binding and hydrolysis of ATP by one chaperonin
ring controls the release of the cochaperonin GroES from the opposite
ring (8, 12, 17, 19, 24). In contrast, the mitochondrial
homolog of GroEL-GroES, Hsp60-Hsp10, functions as a single ring,
without any transition through a double-ring intermediate
(15). Given this difference and the sequence
divergence between GroEL and human Hsp60 (21) (the two
proteins are 51% identical in sequence), we decided to explore
the possibility that these chaperonins might not recognize an
identical set of target proteins. Here we show that Hsp60-Hsp10
can replace GroEL-GroES in E. coli cells. Thus, despite
their differences in sequence and mechanism, Hsp60-Hsp10 can facilitate
the productive folding of all essential proteins that are
dependent on GroEL-GroES. However, the inability of Hsp60-Hsp10 to
support the maturation of GroE-dependent bacteriophage lambda and T4
proteins suggests that some specificity differences may indeed exist
between these two chaperonin systems.
Transformation into MGM100 of constructs engineered for the
expression of various chaperonins and cochaperonins.
MGM100, a
derivative of the K-12 strain MG1655 described in reference
14, has been engineered so that the expression of the groE operon, although remaining in a single copy on the
chromosome, is under the control of the inducible arabinose promotor
PBAD. Since expression from PBAD is between
100- and 1,000-fold higher on arabinose broth than on glucose broth
(6) and the amount of GroE produced by MGM100 growing on
arabinose is less than that produced by MG1655 from PgroE
(see below), the amount of GroE produced by MGM100 growing
on glucose will be negligible. Consistent with this, MGM100 is inviable
in the absence of exogenously supplied arabinose, confirming that GroEL
and GroES are essential for growth. To determine whether Hsp60-Hsp10
can substitute for GroEL-GroES or whether the apical, substrate binding
domain of Hsp60 can substitute for the cognate domain of GroEL, we
introduced into MGM100 a series of plasmid constructs in which the
groE promoter was used to express GroEL, Hsp60, or
chaperonins and cochaperonins together. Constructs included those
expressing Hsp60-Hsp10, GroEL-GroES, or one of two chimeric assemblies
of GroEL and Hsp60 together with Hsp10 (15). The
chimeras consist of an Hsp60-derived apical domain fused to either a
wild-type GroEL equatorial domain (Hsp60-GroEL) or to a mutant GroEL,
SR1 (23), engineered so that ring-ring interactions are
abolished (Hsp60-GroELSR1).
Plasmid transformants of MGM100 were selected on either glucose (no
production of chromosome-encoded chaperonin) or arabinose (to induce
chromosome-encoded expression of GroEL-GroES). In the presence of
glucose, normal progeny of uniform colony size were obtained with all
constructs expressing both a chaperonin and a suitable cochaperonin
(Table 1). It follows that Hsp60-Hsp10 and the chimeric proteins plus Hsp10 can substitute for GroEL-GroES in
all essential folding reactions and that the double-ringed, two-stroke
engine mechanism utilized by GroE is not required for folding essential
chaperonin-dependent proteins in E. coli. The failure of
plasmids expressing GroEL alone or Hsp60 alone to yield transformants
on glucose proves that the MGM100 chromosome is not supplying the
cochaperonin needed to support colony formation. In vitro, GroEL can
function with either GroES or Hsp10, but Hsp60 will fold target
proteins only in conjunction with its cognate cochaperonin, Hsp10
(22). Since GroEL coexpressed with Hsp10 can replace
GroEL-GroES in MGM100, we conclude that GroEL can also function with
Hsp10 in vivo.
MGM100(pHSP60-Hsp10) transformants selected on glucose were transferred
to broth, and their growth was compared with that
of MGM100(pBR325)
(Fig.
1). The latter strain, as
previously found
(
14), lyses when grown without arabinose.
In contrast, MGM100(pHsp60-Hsp10)
continues to grow exponentially at
the same rate on glucose as
MGM100(pBR325) on arabinose, indicating
that Hsp60-Hsp10 can support
normal exponential growth of
E. coli in the absence of endogenous
GroEL-GroES.
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FIG. 1.
Growth in LB plus 0.2% arabinose (open symbols) or
glucose (closed symbols) of MGM100 plasmid transformants at 37°C.
Cultures grown overnight on LB plus arabinose were diluted 1:1,000 into
LB plus arabinose or glucose. Shown are data for pBR325 (triangles) and
pHsp60-Hsp10 (circles). All cultures were diluted 10-fold into a
prewarmed medium 140, 225, and 325 min after inoculation to maintain
exponential growth, except for the MGM100(pBR325) glucose culture,
which was diluted only at 140 min.
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When selected on a medium containing arabinose (to allow the expression
of endogenous GroEL-GroES), transformants were obtained
for all the
constructs tested (Table
1). However, in contrast
to the uniform
progeny obtained with plasmids that did not encode
Hsp60, those
encoding either Hsp60-Hsp10 or Hsp60 alone gave rise
to progeny
displaying a range of colony sizes, many very small,
suggesting that
the coexpression of Hsp60 and GroEL can be deleterious.
This could be
either a consequence of the coassembly of subunits
derived from the two
chaperonins resulting in the generation of
partially functional or
nonfunctional mixed oligomers or a mutual
inhibition of assembly such
that reduced numbers of active chaperonin
molecules are formed.
Note that the plasmids encoding chimeric
chaperonins did not cause
growth inhibition on arabinose. If these
chimeric chaperonins
(
15) form mixed oligomers, these mixed
oligomers
must be
functional.
All plasmids were also transformed into MG1655, the
groE+ parent of MGM100. In contrast to MGM100
transformants, all MG1655
transformant colonies were of normal size.
Since the two strains
differ principally in that MG1655 can produce
more GroE (by responding
to stress
see below) than MGM100, the
intolerance of MGM100 for
the coproduction of GroE and Hsp60 is likely
to be due to the
limited amount of GroE that it
produces.
Hsp60-Hsp10 can support growth at temperatures up to 42°C, but
coproduction with GroEL-GroES inhibits growth.
To further explore
the ability of Hsp60-Hsp10 to support E. coli growth, we
streaked single colonies of the transformants onto a medium containing
either arabinose or glucose and incubated them at various temperatures.
All strains grew in the presence of arabinose at 37°C, and strains
expressing compatible chaperonin and cochaperonin pairs also grew in
the presence of glucose (Table 1). As expected, strains expressing
Hsp60 alone, GroEL alone, or no chaperonin failed to grow at any
temperature on glucose.
At 43°C, strains expressing GroEL together with either GroES or
Hsp10 from plasmids grew well on both media. However, strains
expressing Hsp60 (or chimeric chaperonins containing a GroEL-derived
equatorial domain) together with Hsp10 failed to grow on glucose
at
43°C, although growth of MGM100 synthesizing Hsp60-Hsp10 was
evident
at temperatures up to 42°C. Failure to grow at 43°C seems
unlikely
to be attributable to the mechanistic incompetence of
Hsp60-Hsp10,
because in in vitro folding assays (done as described
in reference
15), this chaperonin folds malate dehydrogenase
with
the same efficiency as GroEL-GroES at either 37 or 43°C (Fig.
2). It seems more likely that
insufficient Hsp60 is produced in
E. coli from our
constructs to supply the higher levels of chaperonin
needed for high
temperature growth. That higher amounts of chaperonin
are indeed
required at high temperatures is evident from the failure
of MGM100 to
grow at 43°C, even in the presence of arabinose.
Since MGM100 is
dependent on GroE expressed from P
BAD, a promoter
that is
not upregulated by temperature (
13), the failure of
MGM100
to grow at high temperatures indicates that more GroE is
needed to grow
at these temperatures than is produced from P
BAD.
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FIG. 2.
Recovery of malate dehydrogenase enzymatic activity
(expressed as a fraction of a native control) obtained in in vitro
folding reactions (15) done at 37 or 42°C in the presence
of either GroEL-GroES (black bars), Hsp60-Hsp10 (white bars), or no
chaperonin (gray bars).
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To determine whether MGM100 does indeed produce relatively low levels
of Hsp60 from these plasmid constructs, we performed
a Western blot
analysis on extracts of cells grown at 37°C (Fig.
3). Cell extracts were separated by
sodium dodecyl sulfate-polyacrylamide
gel electrophoresis and
transferred to nitrocellulose membranes,
and the blots were probed with
a polyclonal antiserum (Stressgen,
Inc.) that detects GroEL and Hsp60.
FtsZ, which is not affected
by GroE depletion, was measured by using an
anti-FtsZ antibody
so as to provide a measure of overall protein in the
extracts.
Bound antibody was detected with the chemiluminescence ECL
detection
kit (Amersham, Inc.). We found, firstly, that MGM100 induced
with
0.2% arabinose produces only one-third as much GroEL as that
produced
from the
groE promoter in MG1655, the parent of
MGM100. This would
not be increased at high temperatures
(
13) and probably accounts
for the MGM100 temperature
sensitivity. The presence of pGroES-GroEL
increases GroEL levels
20-fold. In contrast, MGM100(pHsp60-Hsp10)
produces less than twice as
much Hsp60 as MG1655 produces GroEL,
despite transcription from
identical promoters in the two cases
and the high copy number of
pHsp60-Hsp10. It is not clear whether
this is a consequence of a
relatively reduced translation of the
human protein (the gene contains
codons that are rare in
E. coli)
or of the instability of
Hsp60.
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FIG. 3.
Amounts of chaperonin 60 measured by Western blotting.
(A) MGM100 harboring pBR325 (labelled pBR), pGroES-GroEL (pGro),
or pHsp60- Hsp10 (pHsp), grown overnight in LB plus arabinose,
were diluted 1:2,000 into LB plus glucose (+G) or arabinose (+A) and
sampled after 3 h. MG1655 was grown in LB. All cultures were grown
at 37°C. Densitometry of the image shown in panel A and also of
others (not shown) on which the extracts were diluted were used to
determine protein concentrations. The values for Hsp60 were corrected
for the fact (see panel B) that it reacts less well with anti-GroEL.
(B) Indicated amounts of purified proteins were used to construct a
standard curve for sample measurement. Anti-GroEL detects GroEL with
about three times the sensitivity with which it detects Hsp60.
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The growth of MGM100(pHsp60-Hsp10) observable on glucose appears to be
inhibited by arabinose at temperatures between 40 and
42°C. This
behavior confirms the observation made above that subunits
derived from
GroEL and Hsp60 can interact, either to form inactive
mixed multimers
or to interfere with the assembly of multimers
of the other type.
Although stable mixed GroEL-Hsp60 multimers
form when the parent
chaperonins are mixed, dissociated, and allowed
to reassociate in vitro
(
16) we do not know whether they are
biologically active.
Since there is only just enough Hsp60 produced
on glucose to support
growth at higher temperatures, the loss
of Hsp60 to inactive mixed
multimers could reduce the supply to
below that critical level.
MG1655(pHsp60-Hsp10), on the other
hand, grows satisfactorily at all
temperatures tested. We infer
that this is because MG1655, which
produces GroE from its native
promoter and is therefore induced to
produce high levels of GroE
at 43°C, can form enough homo-oligomers
to support growth at high
temperatures, even in the presence of high
levels of
Hsp60.
In order to further examine the effect of Hsp60-Hsp10 production
on the colony growth rate, aliquots of serial dilutions of
MGM100(pBR325), MGM100(pHsp60-Hsp10), and
MGM100(pGroES-GroEL)
were plated on broth plus arabinose or
glucose and incubated at
37 or 42°C. Colonies were counted after 1 or
2 days (Table
2).
When GroE is supplied
from a plasmid, MGM100 grows rapidly and
forms an equal number of
colonies on both media at each temperature.
In contrast, when MGM100 is
relying on GroE expressed from the
P
BAD promoter, growth is
satisfactory at 37°C but poor at higher
temperatures; this is
indicated both by slowed colony growth and
reduced viability. When
Hsp60-Hsp10 (but not GroE) are available,
the colony growth rate is
reduced at both temperatures, but the
final viability is not. However,
when both the Hsp and GroE proteins
are produced, growth is slower at
both temperatures than growth
on glucose, and the final viability at
high temperatures, already
low because of insufficient GroE, is not
enhanced. We conclude
that MGM100 does better with Hsp60-Hsp10 alone,
compared to the
situation in which GroE is also expressed. Since this
incompatibility
between chaperonins is not observed during
exponential growth
in liquid, these growth limitations most
likely take effect during
the postexponential stages of
growth.
Hsp60-Hsp10 is limited (relative to GroEL-GroES) in its ability to
support bacteriophage growth.
Mutations in the groE
genes were originally identified because they prevented bacteriophage
growth. GroEL-GroES are required for bacteriophage 
head assembly
(5) and for bacteriophage T5 tail assembly (25).
GroEL, but not GroES, is needed for phage T4 head assembly; this is
because T4 encodes a protein, gp31, which substitutes for GroES in the
chaperonin cycle (20). In order to explore potential
differences in the abilities of the E. coli and
mitochondrial chaperonins to facilitate the folding of bacteriophage
proteins, we examined the growth of these three bacteriophages in
MGM100(pHsp60-Hsp10) as follows. To make plating cells, MGM100 with
plasmid was grown in selective conditions overnight in Luria broth (LB)
with arabinose or glucose and reinoculated into the same medium on the
following day. For T4, chilled late-log-phase cells were used. For ,
log-phase cells were centrifuged and resuspended in 0.01%
MgCl2; these cells were also used for T5. Phage dilutions were preincubated with 0.1 ml of plating cells, suspended in 2.5 ml of
BBL (1% trypticase, 0.5% NaCl)-0.7% top agar containing 0.2%
arabinose or glucose and plated on sugar-supplemented LB or BBL plates.
The results are presented in Table 3.
Note that plaques of reduced size are observed in all cases when both
Hsp60-Hsp10 and GroEL are present, providing further evidence of
incompatibility between the two chaperonins.
Hsp60-Hsp10 is clearly able to support the growth of T5, although the
efficiency of plating may be reduced slightly. The mitochondrial
chaperonin does not, however, support the growth of either
or
T4 on
LB. Reduced numbers of very small lambda plaques were obtained
on a
less rich medium (BBL) where cell growth is slow, indicating
that
Hsp60-Hsp10 can assist the folding of
proteins, albeit
not very
efficiently. T4 plaques were not obtained on either medium,
even when
10
8 phages were added to a single plate. Hsp60 is unable to
use GroES
as a cochaperonin, and it is possible that it also cannot
interact
with gp31. This may be the reason why T4 growth is not
supported
by the mitochondrial chaperonin, rather than because gp23,
the
T4 GroEL substrate, fails to interact with
Hsp60.
The data presented here formally demonstrate that the mitochondrial
homolog of GroEL-GroES, Hsp60-Hsp10, can replace GroEL-GroES
in growing
E. coli cells. It follows that there are no target
proteins
essential for growth that cannot be productively folded
by both these
chaperonins. We cannot exclude the possibility that
there are
differences in folding efficiency or in target specificity
for
inessential proteins. Indeed, the reduced efficiency with
which
Hsp60-Hsp10 supports the growth of phages and the reduced
efficiency of
colony formation of cells dependent on Hsp60-Hsp10
suggest that there
may be some such differences. We conclude that
the single-ring
mechanism of action used by Hsp60-Hsp10 can adequately
replace the
double-ring two-stroke mechanism characteristic of
GroEL-GroES. We also
find that the coexpression of GroES-GroEL
and Hsp60 affects growth in a
manner most easily explained by
the hypothesis that subunits derived
from the two chaperonins
can interact, either to inhibit the completion
of ring formation
or by coassembly to form functionally compromised
mixed
multimers.
| |
ACKNOWLEDGMENTS |
This work was supported by a grant (to N.J.C.) from the National
Institutes of Health. M.M. and N.M. thank the U.K. Medical Research
Council for financial support.
We also thank S. McAteer for technical assistance.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Biochemistry, New York University Medical Center, New York, NY 10016. Phone: (212) 263-5809. Fax: (212) 263-8166. E-mail:
CowanN01{at}mcrcr6.med.nyu.edu.
Present address: Laboratory of Biotechnology, Aalborg Universitet,
Aalborg, Denmark.
Present address: CJD Surveillance Unit, Department of Pathology,
University of Edinburgh, Western General Hospital, Edinburgh, Scotland.
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Journal of Bacteriology, September 1999, p. 5871-5875, Vol. 181, No. 18
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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Richardson, A., Schwager, F., Landry, S. J., Georgopoulos, C.
(2001). The Importance of a Mobile Loop in Regulating Chaperonin/ Co-chaperonin Interaction. HUMANS VERSUS ESCHERICHIA COLI. J. Biol. Chem.
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