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Short communication

Dehydration of yeast: Changes in the intracellular content of Hsp70 family

proteins

Irina Guzhova a, Irina Krallish b, Galina Khroustalyova b, Boris Margulis a, Alexander Rapoport b,*a Laboratory of Cell Protective Mechanisms, Institute of Cytology, Russian Academy of Science, Saint Petersburg, Russiab Laboratory of Cell Biology, Institute of Microbiology and Biotechnology, University of Latvia, LV-1586 Riga, Latvia

1. Introduction

Yeasts as many other types of microorganisms can be subjected

to significant changes of humidity in nature. As the result they can

undergo multiple cycles of dehydration and subsequent rehydra-

tion in their life. During evolution they worked out a variety of

mechanisms which help to maintain their viability at their transfer

into non-active state of anhydrobiosis. This state is characterized

by a transient andreversible reduction of metabolism and also by a

variety of changes at biochemical and functional levels [1]. The

latter include condensation of chromatin, separation by mem-

branes of rather big parts of nucleus and damaged areas of

cytoplasm [24], synthesis of trehalose and polyols [57],

stabilization of molecular organization of intracellular membranes

[8], maintenance of redox homeostasis[9] and other.

Heat shock proteins belonging to Hsp70 family are established

to be the ubiquitous stress-responsive system in all living

organisms. The accumulation of Hsp70 signals that a cell or tissue

respond to an environmental or xenobiotic harmful factor, and in

most cases the increase of Hsp70 expression renders cells more

resistant to repetitive stressors. Intracellular functions of Hsp70

are based on its chaperonic activity that implies assembly, folding,

intracellular localization, secretion, and degradation of cellular

polypeptides [1012]. Protective power of Hsp70 thought to be

linked to its chaperonic activity is proved by studies on hundreds

cell and animal models.

Thegenome ofSaccharomyces cerevisiae yeast contains 14 genes

comprising multigene Hsp70 family proteins [13]. This protein

family includes mitochondrial proteins Ssc1 and Ssc1p [1417],

cytosolicproteins Ssa1, Ssa1p,Ssa2 andSsa4pwhich accumulate in

cell nucleus during yeast starvation[18]. As in other organism in

yeast Hsp70 chaperones facilitate endoplasmic reticulum-asso-

ciated degradation of defective proteins [19]. It is known also

that the cytosolic yeast Hsp70 supervises proteins involved in the

response to stress and protein synthesis [20]. Loss of mitochondrial

Hsp70 (Ssc1p) function causes aggregation of mitochondrial

polypeptides in yeast cells [21]. S. cerevisiae cells with Hsp70

knockout demonstrate abnormal nuclear distribution and aberrant

microtubule formation in M-phase [22]. A few factors inducing

Hsp70 expression in yeast include heat shock and oxidative stress;

Process Biochemistry 43 (2008) 11381141

A R T I C L E I N F O

Article history:

Received 20 December 2007

Received in revised form 9 April 2008

Accepted 22 May 2008

Keywords:

Hsp70

Anhydrobiosis

Dehydrationrehydration

Protective reactions

Yeast

Saccharomyces cerevisiae

Debaryomyces hansenii

A B S T R A C T

Yeast is known to experience in natural and industrial conditions cycles of dehydrationrehydration.

Several molecular mechanisms can be triggered in response to this and other environmental stressors

and to rescue yeast cells of the cytotoxic effect. Since heat shock proteins constitute one of the most

important systems of the response to stress we studied whether the pre-induced major stress protein,

Hsp70, can cope with yeast cell drying. To induce Hsp70 expression the cells of two yeast species,

Saccharomyces cerevisiae and Debaryomyces hansenii, were subjected to non-lethal heat shock. It was

found that during yeast culture growth Hsp70 accumulation occurred at the exponential growth phase,

andthere wasno marked changein theprotein level at thestationary phaseboth in aerobic andanaerobic

conditions. Interestingly, dehydration of sensitive to this kind of stressS. cerevisiae grown in anaerobic

conditions led to the increase of Hsp70 expression; to our knowledge this finding was presented for the

first time. Dehydration of yeast taken from the stationary growth phase did not cause the induction of

Hsp70 expression. Irrespective of the inducer, Hsp70 did not rescue yeast cells from dehydration stress

damages. This result well coincides with data of other groups found that Hsp70 in yeast possesses

chaperonic activity, and the latter does not impact to an increase in protective power of the protein

demonstrated in many other organisms.

2008 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +371 67034891; fax: +371 67227925.

E-mail address: rapoport@latnet.lv(A. Rapoport).

Contents lists available atScienceDirect

Process Biochemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p r o c b i o

1359-5113/$ see front matter 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2008.05.012

mailto:rapoport@latnet.lvhttp://www.sciencedirect.com/science/journal/13595113http://dx.doi.org/10.1016/j.procbio.2008.05.012http://dx.doi.org/10.1016/j.procbio.2008.05.012http://www.sciencedirect.com/science/journal/13595113mailto:rapoport@latnet.lv

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it is also noteworthy that high amount of the chaperone was found

in cells subjectedto deuteriumoxideor genetically resistant to low

temperatures[23]. The expression and role of Hsp70 in conditions

of dehydration and rehydration remains unexplored. The aim of

this study was to analyze the possible function of pre-established

Hsp70 in cells of two yeast strains subjected to drying as well as to

understand if dehydration stress itself leads to the synthesis of

Hsp70.

2. Materials and methods

2.1. Yeast strains and cultivation conditions

In this study we used yeastS. cerevisiae14 (Collection of the Laboratory of Cell

Biology, Institute of Microbiology and Biotechnology, University of Latvia) and

Debaryomyces hansenii (generous gift from Prof. L. Adler, Geteborg University,

Sweden). The latter strain was earlier found to be significantly more resistant to

dehydration. Yeast cells were cultivated in 750 ml flasks at 30 8C using shaker

(180 rpm) for aerobic conditions (S. cerevisiaeandD. hansenii) and without shaking

witha great excess of nutrient medium for anaerobicconditions (only S. cerevisiae).

Nutrient medium contained (in g l1): MgSO40.7; NaCl 0.5; (NH4)2SO43.7; KH2PO41.0; K2HPO4 0.13; molasses43 (till finalconcentration of glucose20 g l

1). pH value

of nutrient medium was adjusted to pH 5.0 using H2SO4.

2.2. Biomass harvesting and dehydration

Yeast cells at the exponential phase (for yeast grown in aerobic conditions) and

stationary phase (for yeast grown both in aerobic and anaerobic conditions) were

collected. To establish time points for the harvesting of the biomass (data not

present) direct counting of cell amount in Goryaev chamber and spectro-

photometric determination of suspensions optical density at 600 nm were

performed. Harvested biomass was washed and compressed with the aid of

vacuum filtration unit. A part of yeast biomass was used in further experiments as

native counterpart, the second part was dehydrated by convective method at

30 8C to residual humidity of 810%, and the third portion of biomass was used for

the experiments on heat shock. This part was subjected to heat shock and also was

dehydrated till the residual humidity of 810%. Biomass relative humidity was

measured of its weight after drying at 105 8C during 48 h.

2.3. Determination of cells viability

Viability of native and dehydrated cells was measured with the help of

fluorescent microscopy using specific probe primuline[24]. The use of this methodgives the possibility to reveal live organisms in which only cell wall fluoresces and

dead yeast which have bright green fluorescence of the whole cell.

2.4. Heat shock

Compressed biomass was put in 250 ml flask. 75 ml of pre-heated till 42 8C

filtered cultural liquid was added to the flask. Procedure of heat shock was made at

42 8C during 30 min. After heat stress yeast cells were transferred to fresh nutrient

medium in which they were kept 1 h at 30 8C.

2.5. Quantification of Hsp70 by immunoblotting

To measure Hsp70 content the method of Western blotting was employed using

protocol of Towbin et al. [25]. Briefly, yeast cells were subjected to disintegrationin

0.1 M K-potassium buffer (pH 7.0) with glass beads (diameter 300 mkm) during

10 min at 4000 rpm with refrigeration using the disintegrator SCP-100-MRE,

Innomed-Konsult AB, Sweden. The samples of total protein extract fromdisintegrated yeast cells and were mixed with sodium dodecylsulfate (SDS) and

2-mercaptoethanol to give final concentration 2% and 15 mM, respectively. Equal

amounts of the total protein, 50mg, were applied onto lanes of 10% polyacrylamide

gel. Electrophoresis was performed with a voltage gradient of 5 V cm1 and

currency 30 mA per gel slab. After the electrophoresis protein bands were

transferred onto Immobilon nitrocellulose membrane with the aid of the semi-dry

blotting apparatus (GE Healthcare, Russia) according to standard protocol [25].

The bands of Hsp70 were stained with the use of SPA-822 monoclonal antibody

known to recognize inducible component of the yeast Hsp70 family (StressGen,

Canada).

3. Results

The major goal of this study was to elucidate whether heat

precondition accompanying with the accumulation of Hsp70 stress

protein can protect yeast cells from the deleterious effect of

dehydration as well as to understand if dehydration stress leads to

the synthesis of Hsp70 proteins in yeast. To establish the

conditions of Hsp70 accumulation we studied the protein level

in control and stressed yeast cells.

Theanalysis of Hsp70 expression during the S. cerevisiae growth

was performed in samples taken each 4 h after the cells had been

seededin nutrient medium. This study was performed with the aid

of Western blotting and showed that the level of Hsp70 was

strongly elevated during first 8 h after inoculation that corre-

sponded to exponential growth phase (Fig. 1). Twelve hours after

inoculation the level of Hsp70 began to decline and 12 h later the

signal fully disappeared. The yeast entered stationary phase of

growthat time point 18 h after seeding, andthe reductionof Hsp70

level revealed that despite a strong lowering of cellular metabo-

lism the protein is subjected to proteolysis. Thus, the highest level

of Hsp70 can be attained in the middle of exponential phase and

this point was selected for further experiments on pre-conditional

stress designed to increase Hsp70 amount in cells.

Since dehydration by itself can induce stress response, we

measured Hsp70 amount in S. cerevisiae cells grown in aerobic

conditions, taken at theexponential growthphaseand subjectedto

dehydration. It was found that drying led to a complete reductionof Hsp70 level (Fig. 2A). Viability of these cells was also found to be

at very low level14.8 1.15% (Fig. 2C). Dehydration of the same

yeast taken at the stationary phase did not cause expression of Hsp70

(Fig. 2A). In these experiments viability of dehydrated cells was

65.4 0.65% that is ordinary value for this yeast grown and

dehydrated in standard conditions in our previous studies of

anhydrobiosis[1]. Finally, dehydration of yeast grown in anaerobic

conditions and taken from stationary growth phase led to the

accumulation synthesis of Hsp70 family proteins (Fig. 2B). It is

necessary to mention that this yeast was extremely sensitive to

dehydration and the maximal viability did not exceed 1%.

To check whether the same response to stress is typical for

various yeast species, we studied profile of Hsp70 expression in D.

hansenii cells that are extremely resistant to dehydration [26].Similar toS. cerevisae these cells were found to contain Hsp70 at

the exponential phase of growth and not at the stationary phase

(Fig. 3A). Dehydration of yeastD. hansenii taken from exponential

growth phase led to the reduction of Hsp70 content ( Fig. 3A). As

suggested the viability of dehydrated D. hansenii remained high

enough in contrast with S. cerevisiae, and comprised5560%. Lastly

dehydration of D. hansenii cells taken from stationary growth

Fig. 1. Hsp70 protein content in the cells of Saccharomyces cerevisiae during its

growth in aerobic conditions: (A) Hsp70 protein content at different phases of

culture growth; (B) yeast culture growth curve.

I. Guzhova et al./ Process Biochemistry 43 (2008) 11381141 1139

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phase was not accompanied with the synthesis of Hsp70 family

proteins (Fig. 3A).

To check whether enhanced amount of Hsp70 due to heat pre-

conditioning can cause the increase of the resistance of yeast cells

to dehydration we subjected yeast cells to heat shock at 42 8C

30 min prior drying. In both aerobic ( Fig. 2A) or anaerobic culture

(Fig. 2B) conditions heat shock led to elevation of Hsp70 in yeast

cellsS. cerevisiaebut did not increase cell viability (Fig. 2C). Heat

shock ofD. hanseniicells also did not result in the enhancement of

their survival despite the significant elevation of Hsp70 level

(Fig. 3A and B).

4. Discussion

Systematic investigations of main factors able to positively

influence yeast viability during its transition to the state of

anhydrobiosis reveal a number of intracellular protective systems

that function in these conditions. These systems can work at the

ultrastructural level as well as they can be associated with

synthesis of a number of protective substances. The latter include

Hsp70 chaperone whose protective activity in a variety of

organisms is convincingly established. In this study we questioned

whether enhanced level of Hsp70 can contribute to the protectionfrom the deleterious effect of dehydration.First we studied profiles

of Hsp70 expression in control and stressed S. cerevisiae and D.

hansenii cells. The results show that Hsp70 is synthesized over

exponential phase of growth in both yeast strains. The specific

feature of this stage of growth is the active metabolism and

intensive synthesis of different proteins. Since one of the most

important roles of Hsp70 is chaperonic activity one can suggest

that this property must be useful at this particular stage of yeast

growth[10]. Subsequently, the reduction of total cellular protein

synthesis at the stationary phase does not demand a necessity in

Hsp70 synthesis. Probably it is the main reason why Hsp70

expression was not found in S. cerevisiae cells in aerobic and

anaerobic conditions and in aerobic D. hansenii cells at the

stationary growth phase. We further analyzed the reaction of yeastcells to a moderate heat stress. In S. cerevisiaeheat shock at 42 8C

induced Hsp70 in both anaerobic and aerobic conditions however

in the latter case only at the exponential phase of growth. We

demonstrated Hsp70 induction in heat stressedD. hansenii taken

from both exponential and stationary growth phases. Since

dehydration is also a strong stressor, we checked whether it can

induce Hsp70 expression. The data show that this induction occurs

only in S. cerevisae living in anaerobic conditions and taken from

stationary growth phase. It is worth-mentioning that the cells in

these conditions are extremely sensitive to drying. Therefore one

can speculate that the synthesis of Hsp70 occurs only in surviving

part of cells that comprises about 1% of the whole cell population

but we suppose that it would be much more probable that these

proteins are synthesized in the cells at the early stages ofdehydration when cells are still viable. It can be concluded that

unfortunately also this protective reaction does not help them to

increase their viability rate.

Discussing these results one significant thing shouldbe taken in

mind: yeast dehydration is comparatively long process. At least at

its first stage,whencells are keeping a rest of water and which lasts

approximately 9 h, theprocess is associated with thedestructionof

a number of unnecessary proteins, and this is also a part of the

program preparation of the cells to dehydration [1]. Taking into

account chaperonic function of Hsp70 we assume that it

participates in the degradation of intracellular proteins at the

early stages of drying process and simultaneously in prevention of

total demolition of cells. Certainly, one can ask why there was no

Hsp70 synthesis in other S. cerevisiae yeast probes subjected to

Fig. 2. Hsp70 protein content in the cells of S. cerevisiae after heat shock and

dehydration treatments and viability of cells after dehydration: (A) yeast was

grown in aerobic conditions; (B) yeast was grown in anaerobic conditions and was

taken at stationary growth phase; (C) viability of yeast cells after dehydration

without heat shock () and after heat shock (+). Exp, exponential growth phase;

Stat, stationary growth phase; C, control (yeast which has not been subjected to

heat shock); HS, yeast subjected to heat shock; Compr, compressed (intact) yeast;

Dry, yeast subjected to dehydration.

Fig. 3.Hsp70 protein content in the Debaryomyces hansenii cells grown in aerobic

condition after heatshockand dehydrationtreatments (A)and viabilityof cellsafter

dehydration without heat shock () and after heat shock (+) (B). Exp, exponential

growth phase; Stat, stationary growth phase; C, control (yeast which has not been

subjected to heat shock); HS, yeast subjected to heat shock; Compr, compressed

(intact) yeast; Dry, yeast subjected to dehydration.

I. Guzhova et al./ Process Biochemistry 43 (2008) 113811411140

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dehydration, for instance ones taken from exponential phase. One

of possible explanationsmay be that these cells already contain the

amount of Hsp70 family proteins which is completely sufficient for

therealizationof both above mentionedtasksat theinitiative stage

of yeast drying.

The changes of Hsp70 content in organisms that experience

anhydrobiosis in natural conditions were reported for tardigrades

Milnesium tardigradum. It was shown that three isoforms (isoforms

1, 2, 3) of this protein were expressed at the stage of their

restoration from the anhydrobiosis, however only one of these

(isoform 2) wasexpressed also when tardigrades were subjectedto

drying, whereas being in the active state tardigrades contained

extremely low quantity of this Hsp70 [27]. Similar results were

obtained using another tardigrades species, Richtersius coronifer.

Total amount of Hsp70 family proteins in these organisms was low

before their transfer into anhydrobiosis conditions and increased

during the first hour after beginning of rehydration [28]. Generally,

these results resemble our data with the only notice: we have not

observed dehydration-induced Hsp70 expression but it is still

possible that such phenomenon can take place at the stage of yeast

reactivation and it has to be studied in the further investigations.

Major goal of this study was to analyze the reaction of yeast

cells with enhanced Hsp70 level on dehydration stress. It wasexpected that a moderate heat shock would contribute to the

increase of Hsp70 and cells would be more resistant to deleterious

effect of drying. However, the data show that there was no

difference in viability between cells pretreated with heat shock

and untreated, see Figs. 2 and 3. The lack of Hsp70-mediated

protection can be explained by two reasons. First is that the

amount of chaperone can be insufficient to meet the demands of a

proper cell response to dehydration. As was shown above for

anaerobic S. cerevisae responding to drying at the stationary phase,

only a few remaining alive cells keep therational amount of Hsp70.

The same can be in case of cells that experience two sequential

stresses, heat precondition and drying: only a small part of cell

population can survive that is able to keep its protective resources

including Hsp70 chaperone. Secondly, besides Hsp70 chaperoneyeast cells acquire a variety of protective mechanisms andfor some

specific insults they may be much more efficient than Hsp70. It is

worth-mentioning that the thermotolerance of yeast cells over-

expressing different members of SSA gene family was not higher

than in their control counterpart [29]. In summary we show that

Hsp70 can be induced in yeast by two environmental stressors,

heat shock anddehydration, however its synthesis can be ratheran

indicator of stress response than a part of protection mechanism.

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