Human Hsp60 with Its Mitochondrial Import Signal Occurs in Solution as Heptamers and Tetradecamers Remarkably Stable over a Wide Range of Concentrations

Human Hsp60 with Its Mitochondrial Import Signal Occurs in Solution as Heptamers and Tetradecamers Remarkably Stable over a Wide Range of Concentrations
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  Human Hsp60 with Its Mitochondrial Import SignalOccurs in Solution as Heptamers and TetradecamersRemarkably Stable over a Wide Range of Concentrations Silvia Vilasi 1 , Rita Carrotta 1 , Maria Rosalia Mangione 1 , Claudia Campanella 2,3 , Fabio Librizzi 1 ,Loredana Randazzo 1 , Vincenzo Martorana 1 , Antonella Marino Gammazza 2,3 , Maria Grazia Ortore 4 ,Annalisa Vilasi 5 ,Gabriella Pocsfalvi 5 ,GiosalbaBurgio 6 ,Davide Corona 6 ,Antonio Palumbo Piccionello 1,6 ,GiovanniZummo 2 ,DonatellaBulone 1 ,EverlyConwaydeMacario 7 ,AlbertoJ.L.Macario 3,7 ,PierLuigiSanBiagio 1 * , Francesco Cappello 1,2,3 1 Institute of Biophysics, National Research Council, Palermo, Italy,  2 Department of Experimental Biomedicine and Clinical Neurosciences, University of Palermo, Palermo,Italy,  3 Euro-Mediterranean Institute of Science and Technology, Palermo, Italy,  4 Department of Life and Environmental Sciences and National Interuniversity Consortiumfor the Physical Sciences of Matter, Marche Polytechnic University, Ancona, Italy,  5 Institute of Biosciences and Bioresources, National Research Council, Napoli, Italy, 6 Department of biological chemical and pharmaceutical sciences and technologies, University of Palermo, Palermo, Italy,  7 Department of Microbiology andImmunology, School of Medicine, University of Maryland at Baltimore, and Institute of Marine and Environmental Technology, Columbus Center, Baltimore, Maryland,United States of America Abstract It has been established that Hsp60 can accumulate in the cytosol in various pathological conditions, including cancer andchronic inflammatory diseases. Part or all of the cytosolic Hsp60 could be naı¨ve, namely, bear the mitochondrial importsignal (MIS), but neither the structure nor the in solution oligomeric organization of this cytosolic molecule has still beenelucidated. Here we present a detailed study of the structure and self-organization of naı¨ve cytosolic Hsp60 in solution.Results were obtained by different biophysical methods (light and X ray scattering, single molecule spectroscopy andhydrodynamics) that all together allowed us to assay a wide range of concentrations of Hsp60. We found that Naı¨ve Hsp60in aqueous solution is assembled in very stable heptamers and tetradecamers at all concentrations assayed, without anytrace of monomer presence. Citation:  Vilasi S, Carrotta R, Mangione MR, Campanella C, Librizzi F, et al. (2014) Human Hsp60 with Its Mitochondrial Import Signal Occurs in Solution asHeptamers and Tetradecamers Remarkably Stable over a Wide Range of Concentrations. PLoS ONE 9(5): e97657. doi:10.1371/journal.pone.0097657 Editor:  Annalisa Pastore, National Institute for Medical Research, Medical Research Council, London, United Kingdom Received  March 10, 2014;  Accepted  April 21, 2014;  Published  May 15, 2014 Copyright:    2014 Vilasi et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permitsunrestricted use, distribution, and reproduction in any medium, provided the srcinal author and source are credited. Data Availability:  The authors confirm that all data underlying the findings are fully available without restriction. All data are included in the manuscript andsupporting files. Funding:  This work has been supported by Italian grant FIRB ‘‘Future in research’’ RBFR12SIPT MIND: "Multidisciplinary Investigations for the development of Neuro-protective Drugs". FC and AJLM were partially supported by Euro-Mediterranean Institute of Science and Technology (Italy). The funders had no role instudy design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests:  The authors have declared that no competing interests exist.* E-mail: Introduction Hsp60 is a molecular chaperone, highly conserved during evolution that assists protein folding in mitochondria [1,2]. It isencoded and transcribed by a nuclear gene and translated in thecytosol. The newly translated (‘‘naı¨ ve’’) polypeptide has amitochondrial import signal (MIS), i.e., a sequence of 26 aminoacids at the N-terminus that drives Hsp60 to the inside of mitochondria where the MIS is cleaved and the protein reachesthe final conformation (mtHsp60 or Cpn60) [3]. A peptide derivedfrom the signal sequence MIS of human Hsp60 has been found tobe present in human histocompatibility leukocyte antigen (HLA)-Eand to be involved in the detection mechanism of stressed cells [4].The mitochondrial import of Hsp60, similarly to that of otherprotein precursors, is a complex mechanism that depends on themitochondrial membrane potential, and involves several molecu-lar chaperones, such as Hsp70, in the matrix space [5,6] as well as in the cytosol [7,8]. Once Hsp60 is imported into mitochondria, itsfolding and self-assembly from monomers to oligomeric species ismediated by functional pre-existing Hsp60 complexes that catalysechaperonin folding in an ATP-dependent process [9].In the yeast mitochondrion, analogously to the bacterialhomolog GroEL, mtHsp60 self-assembles in ring-shaped hepta-meric quaternary structure, two of which associate to form abarrel-shaped tetradecamer, which is the ATP-driven functionalmacromolecular chaperoning complex [9 – 12]. Hsp60 has been found in the mitochondria of a variety of eukaryotic cells,including human cell lines [13 – 15]. However, differently from bacterial homologs that exist only as tetradecamers, the humanmtHsp60 seems to exist as a homo-oligomer of seven subunits[13]. Moreover,  in vitro  studies have shown that some mammalianmtHsp60, including those from human and Chinese hamster cells,when purified as recombinants from  E  .  coli  , occur mainly asheptameric rings, in equilibrium with very minor populations of monomers and double-ring tetradecamers [16 – 18] and a single PLOS ONE | 1 May 2014 | Volume 9 | Issue 5 | e97657  ring seems to be sufficient for productive chaperonin-mediatedfolding   in vivo  [19]. The ‘‘two-stroke engine’’ has been demon-strated to be necessary for productive facilitated folding in thedouble toroidal structure GroEL/GroES because the co-chaper-onin GroES release is induced by the transfer of allostericinformation between the two rings [20,21]. Instead, mtHsp60 canfunction as an efficient ‘‘one stroke engine’’, due to the very loweraffinity of Hsp60 to its co-chaperonin Hsp10 in respect to thebinding parameters characterizing the GroEL/GroES interaction[19]. Moreover, two of the four residues (R452, E461, S463 andV464) that are essential for the GroEL double ring formation, aredifferent in the corresponding Hsp60 positions [19]. Only in thepresence of ATP and/or its own 10 kDa co-chaperonin Hsp10,the mtHsp60 heptamers are able to dimerize, and to form doublerings [16,22,23].  In vitro , the protein is unstable and can rapidlydissociate into monomers if incubated at 3.3  m M concentration atlow temperatures (e.g., 4 u C) or in the presence of ATP [17,24].  ATP seems to play a dual role: it is necessary for mtHsp60oligomerization at higher protein concentrations (80.2  m M) andfavors dissociation into monomers al lower concentrations(3.3  m M) [24].Very recently, the solid state structure of the mammalianmitochondrial Hsp60-Hsp10 complex has been determined for thefirst time by X-ray diffraction methods. The structure appeared asa symmetrical ‘football’-shaped complex of the chaperonin withthe co-chaperonin [25].It has been shown that Hsp60 plays key roles outsidemitochondria, too [6,26 – 31]. In pathologic situations, such as cancer and autoimmune/inflammatory diseases, Hsp60 accumu-lates in the cytosol as demonstrated by various techniques [26,27]. From the cytosol, Hsp60 may reach other cellular compartments,such as the Golgi, secretory vesicles, and plasma membrane innormal [32,33] and tumor [34,35] cells. Each one of these locations may have specific implications for pathogenesis anddisease progression. For example, Hsp60 accumulating in thecytosol of tumor cells can prevent pro-caspase-3 activation, in turnblocking apoptosis [28] and, for this reason, Hsp60 has beenproposed as a good candidate target for anti-cancer therapy[36,37].The most accepted hypothesis is that cytosolic accumulation of Hsp60 could occur via a complex mitochondrial export mecha-nism [6], but nothing in this regard has ever been demonstrated inthe case of Hsp60 cytosolic increase detected in tumor cells orduring inflammatory diseases [26,27]. Rather, there is evidence that, in some cases, Hsp60 can reside and accumulate in thecytosol without being imported into mitochondria after itssynthesis, and, therefore bearing the signal sequence MIS [28][8]. An example is provided by LNCaP cellular systems in whichthe exposition to specific apoptosis inducers, such as serumstarvation or Dox treatment, causes Hsp60 accumulation withoutinvolving apparent mitochondrial release [28]. Moreover, anantibody against the signal sequence of Hsp60 was cross-reactedwith a protein that is stably present only in the cytoplasm of ratliver [8] and, even if not explicitly commented, two bands arepresent in the western blot (with anti-HSP60 monoclonalantibody) of intact cytoplasm fraction from adult cardiac myocytes[38]. Also, the cytosolic Hsp60 accumulation mechanisms mayoccur with or without mitochondrial release concomitantly [28], sothat in the cytosol the two types of 60 kDa chaperonin proteins,mtHsp60 and its precursor naı¨ ve form, could coexist.While several studies have been performed to assess stability,oligomeric structure, and folding activity of purified mammalianmtHsp60 [16 – 19,24], little is known about the chaperonin, possibly naı¨ ve that, as said above, could accumulate in the cytosolwith its MIS. From studies on GroEL structure [39] and GroELmutants [40] we can assume that the presequence fits inside thenaı¨ ve Hsp60 cavity. However, this occurrence was not directlydetected, neither it is known whether the additional residues areable to influence the protein oligomeric structure. Moreover, thereare few reports concerning the import system of Hsp60 intomitochondria. Besides the essential requirement of the N-terminalsequence [6], it has been proven that the import mechanisminvolves the cytoplasmatic Hsp70, presumably by keeping theprotein as a monomer [8].But, what happens to the Hsp60 that could accumulate in thecytosol without entering the mitochondria? What is its oligomericstate? Is it able to form oligomeric complexes, such as heptamersor tetradecamers, which for GroEL and mtHsp60 are consideredthe functional chaperonin forms? Shedding light on naı¨ ve Hsp60structure and oligomeric state  in vitro  could help to validate its rolein all cases, physiological or pathological, in which it is not knownif the protein accumulates in the cytosol in its native or matureform.In this study, we addressed some of these unresolved issuesconcerning the structural characteristics and oligomeric state of the cytosolic Hsp60. In order to pursue this goal, we investigatedthe oligomeric state and stability of Hsp60 naı¨ ve  in vitro  by abattery of biophysical methods applied over a concentration rangespanning from 10 nM to 79.5  m M. This range is wide enough tobe supposed to cover the Hsp60 concentrations found  in vivo  underphysiological conditions, and during the progression of diseasescharacterized by an increase of the chaperonin in the affectedtissues, as determined by semi-quantitative immunohistochemistry.Even if there are no direct measurements on the Hsp60concentrations in the cytosol and mitochondria in normal orpathologic conditions, it is known that Hsp60 levels in humanblood occurs with a wide range of concentrations, from 0.02 nMto 12.6  m M [41,42].The approach used in this study, based on biophysicalmethodologies, aimed to investigate the oligomeric structure andstability of naı¨ ve Hsp60 under cell-free conditions. This allowed usto elucidate the structural basis underlying Hsp60 functions, thusproviding information relevant to draw what may occur insidecells in the cases in which the precursor form of the proteinaccumulates in the cytosol, both at physiological and pathologicallevels. Material and Methods ATPase activity of recombinant proteins  As a step prior to the experiments, we verified that thechaperonins under analysis were able to hydrolyze ATP (FigureS1). The recombinant naı¨ ve Hsp60 was obtained from ATGen(Seongnam, South Korea) in stock solution at 16.0  m M (1 mg/ml)(buffer 20 mM Tris pH 8.0 and 10% glycerol (w/w)). LyophilizedGroEL was obtained from SIGMA (St. Louis, MO, USA). ATPaseassay was performed as previously described [43]. Briefly,recombinant naı¨ ve Hsp60 or GroEL was added to ATPase buffer(6.6 mM HEPES (pH 7.6), 0.66 mM EDTA, 0.66 mM 2-mercaptoethanol, 0.033% NP-40, 1.1 mM MgCl 2 , 33  m M ATP,5  m Ci (  c -33P) ATP-3000 mmol 2 1 (Ge Healthcare)), and 100 ng of plasmidic DNA was used as substrate. The buffer was used ascontrol and it was setup in parallel. The reactions were incubatedfor 30 min at 24 u C. Unreacted ATP and free y-phosphate wereseparated by thin layer chromatography, using TLC cellulose(Merck Millipore, Milan, Italy). ATP hydrolysis quantification wasdone with a Bio-Rad (Berkeley, CA, USA) Personal MolecularImager FX System. Oligomeric Form of Mitochondrial Hsp60 PrecursorPLOS ONE | 2 May 2014 | Volume 9 | Issue 5 | e97657  Sample preparation for biophysics experiments  All the experiments were conducted by diluting naı¨ ve Hsp60from stock solutions to reach the concentration of 4.8  m M anddissolving GroEL in 20 mM Tris pH 8.0 and 10% glycerol (w/w)at 4.8  m M, too. The solutions were filtered through a series0.22  m m membrane and 1 MDa Vivaspin filters with Polyether-sulfone membrane (Sartorius, Germany). Higher protein concen-trations were obtained with Vivaspin concentrators with Poly-ethersulfone membrane and 10 kDa molecular weight cut-offs(Sartorius, Germany). Protein concentration for each experimentwas determined by the area under a peak from the corresponding High Performance Liquid Chromatography (HPLC) chromato-gram. All chemicals were obtained from Sigma unless specifiedotherwise. Bovine serum albumin (BSA) was purchased fromSigma and dissolved in buffer 50 mM sodium phosphate buffer. Fluorescence correlation spectroscopy  Aliquots of naı¨ ve HSP60 and GroEL were labeled with TFP Alexa-488 in a 0.1 M sodium-bicarbonate reaction buffer atpH 9.0. To remove the un-reacted probe molecules, the sampleswere passed through PD MID TRAP G-25 columns and,eventually, separated via HPLC as described in the following paragraph. In the latter step we also measured the proteinconcentration (111 nM) and the efficiency of labelling (ca. 2 Alexamols/HSP60 mol). The samples were diluted 1:2 many times toreach sub-nanomolar concentrations.The samples were placed in 30  m L wells, previously washed withBSA solution to avoid protein adsorption, for Fluorescencecorrelation spectroscopy (FCS) measurements on a HamamatsuC1943 instrument. The excitation wavelength is at 473 nm, whilethe laser power was kept fixed for all measurements. Theexperiments were performed at 23 u C.Each measurement consisted of 30 repetitions of 3 second-long acquisitions. The repetitions were used to estimate the statisticalerror on each point of the correlation function. The latter iscomputed via software in real time as the time autocorrelation of the fluorescence intensity fluctuations. It contains information onthe fluorophore concentration and diffusion. If the convolution of the illuminated volume and the confocal detected region can bedescribed as a Gaussian with an asymmetry ratio of   k , then thecorrelation function for a single species is G   t ð Þ { 1 ~ G   0 ð Þ D  t , t D , k ð Þ where  D  t , t D , k ð Þ ~ 1 z t = t D ð Þ { 1 1 z k { 2 t = t D   { 1 = 2 with the diffusion time  t  D   inversely proportional to the diffusioncoefficient.To take into account for the small fraction of free probemolecules still present in the samples we fitted the data with thesum of two  D   terms, of which one has its the diffusion time  t  D  equal to that measured in a pure 10 nM Alexa-488 solution. Dynamic and static light scattering The samples were placed into a dust-free quartz cell withoutfurther filtering and kept at 4  u C and 20  u C in the thermostaticcell compartment of a Brookhaven Instruments BI200-SMgoniometer. The temperature was controlled within 0.1 u C using a thermostatic recirculating bath. The light scattered intensity andits autocorrelation function were measured at  h =90 u  by using aBrookhaven BI-9000 correlator and a 50 mW He–Ne laser tunedat a wavelength  l =632.8 nm.Due to their Brownian motion, particles moving in solution giverise to fluctuations in the intensity of the scattered light. Theautocorrelator measures the homodyne intensity–intensity corre-lation function that, for a Gaussian distribution of the intensityprofile of the scattered light, is related to the electric fieldcorrelation function:  g   2 ð Þ q , t ð Þ ~  A z Bg   1 ð Þ q , t ð Þ h i 2 where  A  and  B   are the experimental baseline and the opticalconstant, respectively. For polydisperse particles,  g  (1) (q,t)  is givenby:  g   1 ð Þ q , t ð Þ ~ ð  ? 0 G   C  ð Þ exp { C  t ð Þ d  C Here,  G  (  C  ) is the normalized number distribution function forthe decay constant  C  =q  2  D  T  , where  q=(4  p n/ l  )sin(  q /2)  is thescattering vector defining the spatial resolution with n, the solventrefractive index and  D  T  , the translational diffusion coefficient. Thehydrodynamic diameter  D  H   is calculated from  D  T   through theStokes–Einstein relationship: D T  ~ k  B  T  3 pg D H  where  k   B   is the Boltzmann constant,  T   is the absolute temperature,and  g  is the solvent viscosity. Number-weighted distributionfunctions  P   N   of the z-average hydrodynamic diameter  D  H   wereobtained by the analysis of the intensity autocorrelation functionswere analyzed by means of a CONTIN-likejavascript:void(0);smoothing-constrained regularization method [44].The scattered intensity  I  (  q   ) is given in terms of the Rayleigh ratio I  (  q   )/ I  s r  2 / V  s , where  I  s  is the intensity of the laser source,  V  s  is thescattering volume, and  r   is the distance of the detector from thesample. Absolute values for scattered intensity were corrected forthe scattering from buffer alone and normalized by the intensity of a toluene standard, whose Rayleigh ratio was taken as14 6 10 2 6 cm 2 1 at 632.8 nm. Absolute Rayleigh ratio  R  (  q   ) isrelated to the weight averaged molecular mass  M  w  of particles bythe relation:  R  (  q   )=  KcM  w P  (  q   ), with the instrumental factor  K  =4 p 2 n˜  2 (d n˜  /d c   ) 2 l 0 2 4  N   A  2 1 , where  c   is the mass concentration,  P  (  q   ) isthe  z  -averaged form factor,  n˜   is the medium refractive index,  l 0  isthe incident wavelength, and  N   A   is the Avogadro’s number [45].We calculated the average molecular mass  M  w  by taking (d n˜  /d c   )=0.18 cm 3 g  2 1 , and  P  (  q   )=1. The form factor is related to theaverage shape and size of scatterers. However, it is equal to 1 whenthe size of solutes is much smaller than  q  2 1 [46]. Blue native polyacrylamide gel electrophoresis Native gel electrophoresis was performed using NativePAGEBis-Tris Gels according to the manufacturer’s instructions(Invitrogen, Carlsbad, CA, USA). Hsp60 and GroEL proteinsamples were diluted in NativePAGE Sample Buffer (1x)containing 1% digitonin and 0.5% n-dodecyl- b -D-maltoside(DDM) detergent solutions at pH 7.2. Samples and molecularweight marker (NativeMark Unstained Protein Standard-Invitro-gen) were loaded on precasted 4–16% Novex NativePAGE Bis-Tris gels that resolve proteins in the molecular weight range of 15– 1,000 kDa. lectrophoresis was performed at 150 V constant voltage for 120 min, using XCell SureLock Mini-Cell (Invitrogen).Gels were stained by Coomassie G-250 staining. Gels were Oligomeric Form of Mitochondrial Hsp60 PrecursorPLOS ONE | 3 May 2014 | Volume 9 | Issue 5 | e97657  destained in 8% acetic acid until the desired background wasobtained and scanned, using Gel Doc XR (Bio-Rad) molecularimager. Protein molecular weights were determined by theQuantity One software (Bio-Rad). HPLC system and conditions Chromatographic separations were performed with an HPLCdevice (LC-2010 AT Prominence, Shimadzu, Kyoto, Japan),equipped with an UV-Vis photodiode array detector and a 20  m Lsample loop. The samples injected at 24  u C were eluted with aflow of 0.5 ml min 2 1 in the sample buffer (Tris - HCl pH 8 + 10%glycerol 20 mM) degassed by an in-line degasser filter (DGU20A5). The chromatographic separations were achieved using assize exclusion columns with different separation range: twoShodex 806 and 804 coupled in series. The area of thechromatographic peaks recorded at 280 nm, were normalizedand used to determine the concentrations of all samples studied inthis work. Small Angle X-ray scattering Small Angle X-ray scattering (SAXS) experiments wereperformed at the 5.2 beamline of Elettra Synchrotron in Trieste,Italy. Measurements were carried out at 20 u C, using a sealed1 mm diameter glass capillary enclosed within a thermostaticcompartment connected to an external circulation bath and athermal probe for temperature control. The sample-detectordistance was set to 2.72 m and the X-ray wavelength  l  was0.154 nm. Because the scattering vector is Q= 4 p  sin(  q /2)  l 2 1 (where  q  is the scattering angle), the investigated Q-range was0.06 , Q  , 3.0 nm 2 1 . SAXS profiles were recorded on an imageplate detector and for each SAXS measurement the acquisitiontime was 2 minutes. Each sample was measured three times inorder to improve the signal to noise ratio, waiting 5 min betweeneach measurement in order to avoid radiation damage. Raw datawere radially averaged, considering the beam center from rat tailx-ray diffraction pattern. Both protein solutions and buffers weremeasured, hence the proteins scattering cross section d S /d V (Q)was obtained by properly subtracting from the protein solutionscattering curves the empty cell and buffer contributions [47].Data analysis was performed by using the Guinier law [48],according to which the scattering curve, at low Q range, can beapproximated as d  S d  V ( Q ) ~ d  S d  V (0) e { R g  2 Q 23 The particle gyration radius R g  , is linked to sample shape anddimension, and provides an estimation of protein aggregation.Hsp60 was measured at concentrations c=23.8  m M and atc=79.5  m M, while GroEL at c=52.5  m M. The concentrationsof all samples were checked before and after SAXS experiments byHPLC chromatograms. Results Oligomeric structure of naı¨ve Hsp60 at micro-molarconcentrations To gain insight into the naı¨ ve Hsp60 oligomeric structure, weinvestigated the protein properties and multimeric self-assemblyequilibria by High Performance Liquid Chromatography (HPLC),dynamic and static light scattering (DLS and SLS, respectively),and blue native PAGE. All these techniques allowed us to explorenaı¨ ve Hsp60 at micro-molar concentrations in the range 0.8 to47.6  m M.We first used HPLC to determine if, similarly to mtHsp60 in thesame concentration range [16,17], naı ¨ ve Hsp60 occurs asheptameric/tetradecameric oligomers. To determine the influenceof the protomer concentration on the protein multimeric state insolution and the contribution of cooperativity to the oligomeriza-tion rate, we carried out size exclusion chromatography. Inparallel, we did the same with GroEL, which occurs as atetradecamer (798 kDa) made up of two heptameric rings [49]. Inaddition, we ran bovine serum albumin (BSA), a protein withmolecular weight, 66,5 kDa, very close to that of the naı¨ ve Hsp60monomer.The results shown in Figure 1 revealed that the naı¨ ve Hsp60retention time (R.T.) is, at all the concentrations tested,considerably lower than that of monomer BSA and higher withrespect to GroEL tetradecamer. As the molecular weight of BSAmonomer is 66,5 kDa [50], very similar to that of Hsp60 (63 kDa),we can safely assume that Hsp60 solution does not include freemonomers. On the other hand, the fact that R. T. of Hsp60 ishigher than that of the GroEL tetradecamer, suggests anequilibrium between different oligomers. Since the molecularweight of the GroEL monomer (57 kDa) is lower than that of naı¨ ve Hsp60 (63 kDa), the elution peak observed for the humanchaperonin cannot be attributed to the presence of tetradecamers(which should have manifested themselves with a lower retentiontime with respect to GroEL), but it rather suggests an equilibriumbetween heptamers and tetradecamers.The absence of monomeric forms of naı¨ ve Hsp60 resulting fromheptamer or tetradecamer destabilization was confirmed by DLS,a non-invasive technique, widely used to characterize thehydrodynamic radius of proteins and protein aggregates oroligomers in solution [51 – 54]. Again, measurements were performed by comparing naı¨ ve Hsp60 at various concentrationsand GroEL at a fixed concentration. Figure 2 shows theautocorrelation functions of the scattered light intensity atq=18.7  m m 2 1 (A), and the hydrodynamic diameter distributions, Figure 1. Size exclusion chromatography results.  Size exclusionchromatography experiments on the naı¨ve Hsp60 at differentconcentrations (0.8  m M: red, 1.6  m M: black, 2.2  m M: magenta, 6.4  m M:green, 16.0  m M: turquoise) compared with GroEL at 7.0  m M (blue) andBSA (dotted line). The vertical line drawn across the Hsp60 peak helpsto highlight the independency of the retention time from proteinconcentration. The value of the retention time is consistent with thatexpected for heptameric and tetradecameric species in equilibrium.doi:10.1371/journal.pone.0097657.g001Oligomeric Form of Mitochondrial Hsp60 PrecursorPLOS ONE | 4 May 2014 | Volume 9 | Issue 5 | e97657  represented as number-weighted distributions (B). The autocorre-lation functions were superimposable for all naı¨ ve Hsp60concentrations tested and GroEL. Moreover, results from num-ber-weighted distributions showed that, for each Hsp60 concen-tration, the most populated species had hydrodynamic diametersin the range 12–25 nm, consistent with the dimensions of heptamers and tetradecamers. The number distribution of GroELsize showed a peak in the same region. The number of largeraggregates, responsible for other decays present in autocorrelationfunctions, were negligible for all the samples under analysis. Although DLS did not distinguish between heptamers andtetradecamers, it excluded the presence of monomers for all of the Hsp60 concentrations tested.To gain further insight into the protein oligomeric equilibrium,naı¨ ve Hsp60 at various concentrations was also characterized bySLS. Figure 3 displays the intensity scattered at  q  =18.7  m m 2 1 in aconcentration range from 1.6 to 47.6  m M, in terms of the Rayleighratio R at 90 u  scattering angle, R 90 , divided by the proteinconcentration. In the same graph, the ideal straight lines for thetetradecameric (882 kDa) and heptameric (441 kDa) structures areshown. Data relative to naı¨ ve Hsp60 fell in the intermediate regionbetween the two ideal lines and just above the line fortetradecamers. In the same graph, we show the intensity valuesof only the species with hydrodynamic radius compatible withnaı¨ ve Hsp60 oligomers, excluding higher-size molecular aggre-gates. The data show that, in this case, the R 90 /c values fellbetween the intensity scattered by an ideal heptamer populationand another constituted of only tetradecamers, suggesting aheptamer/tetradecamer equilibrium throughout the concentra-tions range tested.These results were confirmed by repeating HPLC and scattering experiments at various times (1, 4, and 7 days from the momentstock samples were thawed just before testing) and temperatures Figure 2. Dynamic light scattering characterization.  Dynamic light scattering characterization of the naı¨ve Hsp60 at different concentrations(0.8  m M: black, 6.4  m M: green, 16.0  m M: turquoise, 27.0  m M: orange) compared with GroEL at 7.0  m M (blue). (A) Normalized intensity autocorrelationfunctions g (2) (t). (B) Number-weighted distribution functions P N  of the  z  -average hydrodynamic diameter D H  obtained by the analysis of theautocorrelation functions. At each concentration, the hydrodynamic diameter of Hsp60 is always compatible with that of heptamer/tetradecamerspecies.doi:10.1371/journal.pone.0097657.g002 Figure 3. Static light scattering characterization.  Scattered lightintensities from solutions of naı¨ve Hsp60 protein at differentconcentrations, expressed in terms of the Rayleigh ratio  R 90 u     at18.7  m m 2 1 normalized by the concentration values  c  . Together withthe total scattered intensity (empty circles), the intensity contributionby species with diameter size lower than 70 nm (filled circles) is alsoreported. The lines represent the dependence of   R 90 u     /c   on concentra-tion predicted for Hsp60 protein totally assembled in tetradecamers(solid) or heptamers (dashed). The experimental values of intensityscattered by Hsp60 species always fall between those predicted fortotally tetradecameric or heptameric populations.doi:10.1371/journal.pone.0097657.g003Oligomeric Form of Mitochondrial Hsp60 PrecursorPLOS ONE | 5 May 2014 | Volume 9 | Issue 5 | e97657
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