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J Biol Chem, Vol. 273, Issue 46, 30077-30080, November 13, 1998
From the Departments of Tubulin, a ubiquitous protein of eukaryotic
cytoskeleton, is a building block unit of microtubule. Although several
cellular processes are known to be mediated through the
tubulin-microtubule system, the participation of tubulin or microtubule
in protein folding pathway has not yet been reported. Here we show that
goat brain tubulin has some functions and features similar to many known molecular chaperones. Substoichiometric amounts of tubulin can
suppress the non-thermal and thermal aggregation of a number of
unrelated proteins such as insulin, equine liver alcohol dehydrogenase, and soluble eye lens proteins containing Although it is the amino acid sequence of a protein that
determines its final three-dimensional structure of the functional form, it is now well established that many proteins in vivo
as well as in vitro fail to fold correctly in the active
form without the aid from a class of proteins known as molecular
chaperones (1-4). During the process of their synthesis,
translocation, and even during normal functioning in the cell
(particularly under stress conditions) proteins are often subjected to
structural destabilization. Such conditions are likely to lead to an
increase in the propensity for proteins to aggregate, leading to the
ultimate death of the biopolymers. Chaperones rescue these unstable
conformers by binding to them and eventually through a concerted steps
of events, often involving appropriate co-factors and even other molecular chaperones, guide the substrate proteins to the correct folded structure (2, 3).
Molecular chaperones occur ubiquitously in both prokaryotic and
eukaryotic cytosol, endoplasmic reticulum, mitochondria,
archaebacterial cytosol, and chloroplasts (2, 5). They comprise several protein families, which are structurally unrelated. These classes of
proteins includes GroEL, BiP, GRP, TRAP, TRiC, DnaJ gene products, and
heat shock proteins such as HSP40, HSP70, HSP90, etc. Molecular chaperones are now known to perform diverse functions not only helping
substrate proteins to fold properly but also helping attain the correct
fate of the protein in vivo, be it proper oligomeric assembly, transport to a particular subcellular compartment, or disposal of unwanted protein by degradation (1, 5). Different chaperones thus have different functions and they act sequentially (4).
However, irrespective of their functions, their action requires a
common step of binding the unstable non-native conformation of the
substrate, preventing the off-pathway reaction that leads to protein aggregation.
The structural features of a protein that make it a chaperone are
poorly understood. However, a close observation of the molecular chaperones known up to today reveals some interesting facts. They have
a domain containing a bundle of hydrophobic residues suitably located
at the surface of the protein enabling it to bind lipophilic substances
such as 1-anilinonapthalene-8-sulfonic acid (ANS) or 4,4'-dianilino-1,1'-binaphthyl-5,5-disulfonic acid, dipotassium salt
(bis-ANS) (6-8). Most known chaperones, e.g. GroEL, DnaK, Tubulin is a cytoskeleton protein that polymerizes to form
microtubules. Folding of tubulin in cytoskeleton is assisted by the
chaperone TCP-1 ring complex, also known as TRiC (16-18). Tubulin in
general seems to have some of the essential characteristics of a
chaperone protein such as surface hydrophobic pockets to bind
lipophilic molecules (19, 20), a flexible hydrophilic tail especially
rich in acidic residues (21), and a heterodimeric assembly of two
subunits, Materials--
PIPES,1
EGTA, GTP, insulin, subtilisin, phenylmethylsulfonyl fluoride, dimethyl
sulfoxide (Me2SO), dithiothreitol (DTT), and alcohol
dehydrogenase from equine liver were obtained from Sigma. All other
reagents were of analytical grade.
Purification of Tubulin--
Tubulin was isolated from goat
brains by two cycles of GTP and temperature-dependent
assembly and disassembly in buffer containing 50 mM PIPES,
1 mM EGTA, and 0.5 mM MgCl2 (pH
7.0) followed by two further cycles in 1 M glutamate buffer
(22). The purified tubulin freed from microtubule-associated proteins
was extensively dialyzed against 10 mM phosphate buffer to
remove any trace of glutamate and stored in aliquots at Preparation of Lens Crystallin Fractions--
Freshly excised
bovine eyes were obtained from a local slaughterhouse. The lenses were
surgically removed and homogenized in 20 mM Tris-HCl (pH
7.6) containing 0.1 M NaCl and 0.02% (w/v) NaN3. A water-soluble fraction was obtained from the
supernatant by centrifuging the homogenate at 27,000 × g for 30 min at 4 °C. The supernatant was passed through
a 100-kDa polysulfone membrane using an Amicon stirred cell. The
filtrate thus collected was Assay of Protein Aggregation--
Insulin dissolved in a minimum
volume of 0.02 M NaOH was diluted to the required
concentration (0.3 mg/ml) in 100 mM phosphate buffer (pH
7.0). The reduction of insulin was initiated by adding 20 µl of 1 M DTT to 1 ml of the sample in the spectrophotometric cuvette, and the extent of aggregation of the insulin B chain was
measured as a function of time at 25 °C by monitoring the apparent
absorbance (scattering) at 360 nm in a Shimadzu UV-160 spectrophotometer. Thermally induced aggregation of alcohol
dehydrogenase and soluble lens protein fractions were also measured in
the same spectrophotometer using a thermostatic cell holder assembly
maintained at constant temperature through a circulating water bath.
Protein solution and buffer were mixed in the cuvette at room
temperature and then placed in the thermostatic cell holder, and
apparent absorbance was measured as function of time. For calculating
the molar ratio of protein:tubulin resulting in nearly complete
protection of aggregation, the molecular masses of tubulin, insulin,
alcohol dehydrogenase, and Preparation of Insulin and alcohol dehydrogenase were used as substrates to study
chaperone-like activity of many proteins including It is known that
COMMUNICATION
Chaperone-like Activity of Tubulin*
,,¶
Biochemistry and
§ Chemistry, Bose Institute, Calcutta 700054, India
ABSTRACT
Top
Abstract
Introduction
Procedures
Results & Discussion
References
- and 
-crystallins. This
chaperone-like activity of tubulin becomes more pronounced as
temperature increases. Aging of tubulin solution at 37 °C also enhances its chaperone-like activity. Tubulin loses its chaperone-like activity upon removal of its flexible hydrophilic C-terminal tail. These results suggest that both electrostatic and hydrophobic interactions are important in substrate binding by tubulin and that the
negatively charged C-terminal tails play a crucial role for its
chaperone-like activity.
INTRODUCTION
Top
Abstract
Introduction
Procedures
Results & Discussion
References
-crystallin, etc., also have a flexible hydrophilic tail, the absence of which leads to loss of chaperone activity (9, 10). Because
of these well separated hydrophilic and hydrophobic domains, many
chaperones exist as a micelle-like large oligomeric form. GroEL exists
as a 14-mer (9), and TriC is a ring complex having 8-9 subunits per
ring (11, 12). Proposed oligomeric structures of -crystallin that
have chaperone-like activity (13) are based on micellar architecture
(14, 15). The list of new chaperones discovered is increasing quite steadily.
and (Mr 50,000 each), which
can further polymerize into microtubules. These features prompted us to
test whether tubulin possesses any chaperone-like activity. The results
presented in this communication reveal for the first time that tubulin
has chaperone-like properties as it can prevent the aggregation of a
number of proteins unrelated in structure or sequence.
EXPERIMENTAL PROCEDURES
Top
Abstract
Introduction
Procedures
Results & Discussion
References
70 °C
until use. Protein concentrations were determined by the method of
Lowry et al. (23) using bovine serum albumin as standard.
-crystallin-depleted soluble lens
protein, containing a mixture of -crystallin and low molecular
weight -crystallin and other minor constituents of soluble lens proteins.
-crystallin-depleted soluble lens protein were taken as 100, 5.7, 80, and 60 kDa, respectively, of which the last
one is an average value for the protein mixtures and not unambiguous
(13).
ss--
Digestion
of tubulin with subtilisin was performed at 30 °C in 100 mM phosphate buffer with 1 mM GTP. Subtilisin
was taken in the ratio enzyme:protein = 1:100 (w/w). The reaction
was terminated by the addition of 1% by volume of 1% (w/v)
phenylmethylsulfonyl fluoride in Me2SO.
RESULTS AND DISCUSSION
Top
Abstract
Introduction
Procedures
Results & Discussion
References
-crystallin and
HSP proteins. Aggregation of both insulin and alcohol dehydrogenase is
suppressed when incubated in the presence of -crystallin or HSP
proteins (24, 25). Reduction of the disulfide bonds between the A and B
chains of insulin with dithiothreitol rapidly leads to the aggregation
of B chains. To investigate the function of goat brain tubulin, the
aggregation of insulin (0.3 mg/ml) in 100 mM phosphate
buffer (pH 7.0) was monitored at 25 °C in the absence and presence
of varied amounts of goat brain tubulin. As shown in Fig.
1A, curve
1, insulin lost its native conformation upon cleavage of its
disulfide bond at 25 °C. Denaturation was accompanied by
aggregation. Bovine serum albumin had virtually no effect on the
aggregation of insulin (data not shown). When the same experiment was
performed in the presence of goat brain tubulin, aggregation was
suppressed. At an insulin to tubulin weight ratio of 1:1, aggregation
was significantly reduced (Fig. 1A, curve
2). At an insulin to tubulin weight ratio of 1:8, the aggregation was almost completely suppressed (Fig. 1A,
curve 3). There was no evidence of any
aggregation at a 1:10 weight ratio (data not shown). This complete
prevention of aggregation corresponds to a stoichiometric ratio of
1:0.6 for insulin:tubulin. Thermal aggregation of alcohol dehydrogenase
was carried out at 37 °C. Tubulin showed a pronounced effect on the
prevention of aggregation of alcohol dehydrogenase, and at the alcohol
dehydrogenase to tubulin weight ratio of 1:1 corresponding to a
stoichiometric ratio of 1:0.8, 80% suppression of aggregation occurred
(Fig. 1B, curve 3).
View larger version (23K):
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Fig. 1.
Aggregation of insulin and alcohol
dehydrogenase in the absence and presence of brain tubulin.
A, aggregation of insulin (0.3 mg) B-chains was initiated
using DTT in 100 mM phosphate buffer (pH 7.0) at 25 °C
in a volume of 1 ml with: curve 1, only insulin;
curve 2, plus 0.3 mg of tubulin; curve
3, plus 2.4 mg of tubulin. B, aggregation of
alcohol dehydrogenase (0.4 mg/ml) in 50 mM phosphate buffer
(pH 7.0) at 37 °C in a volume of 1.0 ml with the following
conditions: curve 1, only alcohol dehydrogenase;
curve 2, plus 0.2 mg of tubulin; curve
3, plus 0.4 mg of tubulin; curve 4, 2 mg of tubulin. Aggregation was monitored by measuring absorbance at 360 nm.
-crystallin acts as a molecular chaperone on a
native mixture of lens proteins (13). Thus the absence of
-crystallin from the total soluble protein fraction of lens homogenate (consisting of -crystallin and low molecular weight -crystallin fractions and all other low molecular weight soluble components) caused thermal aggregation at 60 °C (13). Addition of
-crystallin to the soluble lens protein fraction prevented the
thermal aggregation. We have tested the effect of tubulin on the
thermal aggregation of soluble lens proteins devoid of -crystallin
at 60 °C (Fig. 2). Tubulin showed a
pronounced effect on the suppression of aggregation of soluble lens
protein fraction even at a weight ratio of lens protein:tubulin as low
as 1:0.2 (Fig. 2). Complete suppression of aggregation required the
lens proteins:tubulin weight ratio of 1:0.5, corresponding to a
stoichiometric ratio of 1:0.3.
View larger version (21K):
[in a new window]
Fig. 2.
Prevention of aggregation of
-crystallin-depleted soluble lens proteins by tubulin.
Aggregation of total soluble fraction of bovine eye lens proteins at
60 °C after the removal of -crystallin was done under the
following conditions: total volume of assay mixture, 1.0 ml;
curve 1, 0.3 mg of -crystallin-depleted
soluble eye lens protein; curve 2, plus 0.03 mg
of tubulin; curve 3, plus 0.04 mg of tubulin;
curve 4, plus 0.06 mg of tubulin.
Chaperone-like activity of many proteins become more pronounced at a
higher temperature. The heat shock protein, HSP90, and -crystallin
bind more effectively to unfolded substrate proteins in the thermally
modified form (7, 26-28). We have studied the prevention of thermal
aggregation of insulin at 25 °C by tubulin, which was first
incubated at 50 °C and then brought back to 25 °C. At an
insulin:tubulin ratio of 1:1 (w/w) the prevention of aggregation by
control insulin is 50% at 25 °C (Fig.
3A, curve 2). However, when tubulin preincubated at 50 °C was used
in the same weight ratio, almost complete prevention of aggregation
occurs (Fig. 3A, curve 3). We have
also tested the effectiveness of tubulin preincubated alone at 37 °C
for different periods of times (aged or thermally modified tubulin) to
prevent the aggregation of alcohol dehydrogenase. Results of such an
experiment are shown in Fig. 3B. Experiments were carried
out at an insulin to tubulin weight ratio of 1:1 (molar ratio,
~1:0.06) at 25 °C. Tubulins used were native and 1, 2, 3, and
4 h aged at 37 °C. Although native tubulin inhibits 50%
aggregation of insulin at 25 °C, prevention is much more with aged
tubulin (Fig. 3B). Thermal treatment of tubulin and its
aging at 37 °C are known to expose hydrophobic sites leading to
enhanced binding of hydrophobic probes (19, 20). It is suggested that
molecular chaperones suppress aggregation by providing appropriately
placed hydrophobic surfaces to the denaturing protein substrates. Our
observation of enhanced activity of thermally modified tubulin (Fig.
3B, inset) also indicates a direct correlation of
chaperone-like activity with exposure of surface hydrophobic groups.
|
One of the characteristics of many known chaperones is the existence of
a flexible tail region. GroEL is known to have flexible N- and
C-terminal residues, which protrude in the central cavity and because
of their flexibility are not resolved in the crystal structure (9). It
was shown by NMR spectroscopy that -crystallin and HSP25 have
unstructured, flexible, and solvent-exposed C-terminal extensions (25).
It was hypothesized that the polar and unstructured flexible C-terminal
end plays a critical role in substrate-chaperone interactions and also
functions as a solubilizer. In fact, mutation in the C-terminal end,
deletion of 17 residues from the C-terminal end (10), or enzymatic
truncation of the C terminus caused a marked reduction in
chaperone-like activity of -crystallin (29, 30). C termini of both
- and -subunits of tubulin are rich in acidic residues,
especially glutamic acid (21). Because of the highest negative charge
density, both termini are exposed to solvents, susceptible to
proteolysis, and thought to be highly flexible (31, 32). Subtilisin can
cleave both C termini when incubated with tubulin at 37 °C. Thus,
the obvious question is does the removal of C termini of tubulin change
its chaperone-like activity? The results of such an experiment are
shown in Fig. 4. Tubulin lost its
chaperone-like activity when the C termini of both subunits
(ss) are removed using subtilisin (Fig.
4). It is interesting to note that complete loss of chaperone-like activity needs removal of the C terminus from both the subunits. When
the digestion is partial (Fig. 4, curve 3),
tubulin retains some chaperone-like activity.
|
The functional significance of this finding is not clear at present. In eukaryotic cytosol, folding of proteins including tubulin and actin is mediated by the chaperonin TRiC (16-18). TRiC is known to be present in all eukaryotic cytosol (2). Although TRiC has a double cylindrical architecture very similar to that of GroEL, it is known to mediate the folding of only a very limited subset of proteins including tubulin and actin. This is in sharp contrast to the generally wide selectivity of GroEL (2). The relative abundance of TRiC in many cell types is quite low (5). There is also no evidence to date for the other abundant chaperones in the eukaryotic cytosol such as HSP90 to play any significant role in protein folding (2). It is therefore an open question as to how the majority of the proteins in the eukaryotic cytosol fold. Our finding of the chaperone-like activity of tubulin may have some role in the formation or maintenance of native conformation of cytosolic proteins. The answer to the question whether tubulin itself helps the proteins to fold or just holds the polypeptide temporarily before being taken up by TRiC or some other hitherto unknown chaperone to guide the substrates through folding awaits further study.
In summary, we have identified tubulin as having a chaperone-like
function. Like GroEL, HSPs, small HSPs, and other members of the
molecular chaperone family, tubulin is able to prevent the irreversible
aggregation of proteins under heat shock as well as other conditions.
Like other chaperones, its activity increases with a rise of
temperature. Its activity is sensitive to the deletion of its flexible
hydrophilic tail, a feature known also for other chaperones.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed: Dept. of Biochemistry, Bose Inst., Centenary Bldg., P 1/12, CIT Scheme, VIIM, Calcutta 700054, India. Tel.: 91 33 337 9544; Fax: 91 33 334 3886; E-mail: bablu{at}boseinst.ernet.in.
The abbreviations used are: PIPES, 1,4-piperazinediethanesulfonic acid; DTT, dithiothreitol.
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Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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