ATP-bound states of GroEL captured by cryo-electron microscopy

Three-dimensional analysis of single particles by electron microscopy: sample preparation and data acquisition

Electron microscopy of single particles has recently become a very popular field in both biological and material sciences. It might be difficult for a novice researcher new to this field to know how to start tackling a new project. This chapter is designed to serve as a guideline for anyone starting a new project to determine a three-dimensional structure using single-particle techniques. The chapter describes the basic techniques necessary to prepare the samples and acquire the data to calculate a three-dimensional reconstruction in easy-to-understand, step-by-step instructions. It starts with the basic preparation of support films and the usage of a variety of staining techniques needed to assess the quality of the sample and the viability of the project. It ends with a detailed description of vitreous ice preparations designed to acquire high-resolution structural information. Guidelines and tips are given on how to record the best images with an electron microscope. Although this chapter is geared to researchers new to the field, experts might find it not only useful as a reference but also valuable because of the number of practical tips included.

Probing the sequence of conformationally induced polarity changes in the molecular chaperonin GroEL with fluorescence spectroscopy

Hydrophobic interactions play a major role in binding non-native substrate proteins in the central cavity of the bacterial chaperonin GroEL. The sequence of local conformational changes by which GroEL and its cofactor GroES assist protein folding can be explored using the polarity-sensitive fluorescence probe Nile Red. A specific single-cysteine mutant of GroEL (Cys261), whose cysteine is located inside the central cavity at the apical region of the protein, was covalently labeled with synthetically prepared Nile Red maleimide (NR). Bulk fluorescence spectra of Cys261-NR were measured to examine the effects of binding of the stringent substrate, malate dehydrogenase (MDH), GroES, and nucleotide on the local environment of the probe. After binding denatured substrate, the fluorescence intensity increased by 32 +/- 7%, suggesting enhanced hydrophobicity at the position of the label. On the other hand, in the presence of ATP, the fluorescence intensity decreased by 13 +/- 3%, implying increased local polarity. To explore the sequence of local polarity changes, substrate, GroES, and various nucleotides were added in different orders; the resulting changes in emission intensity provide insight into the sequence of conformational changes occurring during GroEL-mediated protein folding.

On the brotherhood of the mitochondrial chaperones mortalin and heat shock protein 60

The heat shock chaperones mortalin/mitochondrial heat shock protein 70 (mtHsp70) and Hsp60 are found in multiple subcellular sites and function in the folding and intracellular trafficking of many proteins. The chaperoning activity of these 2 proteins involves different structural and functional mechanisms. In spite of providing an excellent model for an evolutionarily conserved molecular "brotherhood", their individual functions, although overlapping, are nonredundant. As they travel to various locations, both chaperones acquire different binding partners and exert a more divergent involvement in tumorigenesis, cellular senescence, and immunology. An understanding of their functional biology may lead to novel designing and development of therapeutic strategies for cancer and aging.

The asymmetric ATPase cycle of the thermosome: elucidation of the binding, hydrolysis and product-release steps

Using a combination of intrinsic fluorescence to report ATP-induced rearrangements, quenched-flow to measure ATP hydrolysis "on-enzyme" and optical methods to probe the kinetics of product release, we have begun to dissect the process of energy transduction in the thermosome, a type II chaperonin from Thermoplasma acidophilum. Stoichiometric measurements of ATP binding reveal the tight association of eight nucleotide molecules per hexa-decamer, implying the filling of only one ring owing to strong negative cooperativity. After binding, we show that these eight ATP molecules are hydrolysed over the next 50 s, after which hydrolysis slows down markedly during the establishment of the steady state in the ATPase reaction, demonstrating that the kinetic system is off-rate limited. Looking in more detail, this rapid first-turnover can be dissected into two phases; the first occurring with a half-time of 0.8 s, the second with a half-time of 14 s, possibly reflecting the differential behaviour of the four alpha and four beta subunits in a single thermosome ring. To investigate the post-hydrolytic events, we used two heat-stable enzyme-linked optical assays to measure the rate of evolution of ADP and of phosphate from the thermosome active site. Neither product showed a rapid dissociation phase prior to the establishment of the steady state, showing that both are released slowly at a rate that limits the cycle. These data highlight the importance of the highly populated thermosome/ADP/Pi complex in the molecular mechanism.

Automated segmentation of molecular subunits in electron cryomicroscopy density maps

Electron cryomicroscopy (cryoEM) is capable of imaging large macromolecular machines composed of multiple components. However, it is currently only possible to achieve moderate resolution at which it may be possible to computationally extract the individual components in the machine. In this work, we present application details of an automated method for detecting and segmenting the components of a large machine in an experimentally determined density map. This method is applicable to object with and without symmetry and takes advantage of global and local symmetry axes if present. We have applied this segmentation algorithm to several cryoEM data sets already deposited in EMDB with various complexities, symmetries and resolutions and validated the results using manually segmented density and available structures of the components in the PDB. As such, automated segmentation could become a useful tool for the analysis of the ever-increasing number of structures of macromolecular machines derived from cryoEM.

Elucidation of steps in the capture of a protein substrate for efficient encapsulation by GroE

We have identified five structural rearrangements in GroEL induced by the ordered binding of ATP and GroES. The first discernable rearrangement (designated T --> R(1)) is a rapid, cooperative transition that appears not to be functionally communicated to the apical domain. In the second (R(1) --> R(2)) step, a state is formed that binds GroES weakly in a rapid, diffusion-limited process. However, a second optical signal, carried by a protein substrate bound to GroEL, responds neither to formation of the R(2) state nor to the binding of GroES. This result strongly implies that the substrate protein remains bound to the inner walls of the initially formed GroEL.GroES cavity, and is not yet displaced from its sites of interaction with GroEL. In the next rearrangement (R(2).GroES --> R(3).GroES) the strength of interaction between GroEL and GroES is greatly enhanced, and there is a large and coincident loss of fluorescence-signal intensity in the labeled protein substrate, indicating that there is either a displacement from its binding sites on GroEL or at least a significant change of environment. These results are consistent with a mechanism in which the shift in orientation of GroEL apical domains between that seen in the apo-protein and stable GroEL.GroES complexes is highly ordered, and transient conformational intermediates permit the association of GroES before the displacement of bound polypeptide. This ensures efficient encapsulation of the polypeptide within the GroEL central cavity underneath GroES.

Glu257 in GroEL is a sensor involved in coupling polypeptide substrate binding to stimulation of ATP hydrolysis

The ATPase activity of many types of molecular chaperones is stimulated by polypeptide substrate binding via molecular mechanisms that are, for the most part, unknown. Here, we report that such stimulation of the ATPase activity of GroEL is abolished when its conserved apical domain residue Glu257 is replaced by alanine. This mutation is also found to convert the ATPase profile of GroEL, a group I chaperonin, into one that is characteristic of group II chaperonins. Steady-state and transient kinetic analysis indicate that both effects are due, at least in part, to a reduction of the affinity of GroEL for ADP. This finding indicates that nonfolded proteins stimulate ATP hydrolysis by accelerating the off-rate of the ADP formed, thereby allowing more rapid cycles of ATP binding and hydrolysis.

Multiple Structural Transitions of the GroEL Subunit Are Sensitive to Intermolecular Interactions with Cochaperonin and Refolding Polypeptide

In this study we attempted to determine the specific roles of the numerous conformational changes that are observed in the bacterial chaperonin GroEL, by performing stopped-flow experiments on GroEL R231W in the presence of a refolding substrate protein. The apparent rate of one kinetic phase was decreased by approximately 25% in the presence of prebound unfolded malate dehydrogenase while another phase was suppressed completely under the same conditions, reflecting different effects of the unfolded protein on multiple structural transitions within GroEL. The addition of cochaperonin GroES counteracts the effect of the bound substrate protein in the former case, but had no effect on the latter, more extensive suppression. Using a chemically modified form of GroEL R231W which is incapable of releasing substrate proteins at low temperatures, we identified a conformational transition that is implicated in the release of substrate proteins. Parts of the actual process of substrate protein release were also observed through fluorescence resonance energy transfer experiments involving GroEL and labeled substrate protein. Analysis of the energy transfer data revealed an interesting relationship between substrate protein displacement and a specific structural transition in the GroEL apical domain.

Coarse-grained normal mode analysis in structural biology

The realization that experimentally observed functional motions of proteins can be predicted by coarse-grained normal mode analysis has renewed interest in applications to structural biology. Notable applications include the prediction of biologically relevant motions of proteins and supramolecular structures driven by their structure-encoded collective dynamics; the refinement of low-resolution structures, including those determined by cryo-electron microscopy; and the identification of conserved dynamic patterns and mechanically key regions within protein families. Additionally, hybrid methods that couple atomic simulations with deformations derived from coarse-grained normal mode analysis are able to sample collective motions beyond the range of conventional molecular dynamics simulations. Such applications have provided great insight into the underlying principles linking protein structures to their dynamics and their dynamics to their functions.

Allosteric signaling of ATP hydrolysis in GroEL-GroES complexes

The double-ring chaperonin GroEL and its lid-like cochaperonin GroES form asymmetric complexes that, in the ATP-bound state, mediate productive folding in a hydrophilic, GroES-encapsulated chamber, the so-called cis cavity. Upon ATP hydrolysis within the cis ring, the asymmetric complex becomes able to accept non-native polypeptides and ATP in the open, trans ring. Here we have examined the structural basis for this allosteric switch in activity by cryo-EM and single-particle image processing. ATP hydrolysis does not change the conformation of the cis ring, but its effects are transmitted through an inter-ring contact and cause domain rotations in the mobile trans ring. These rigid-body movements in the trans ring lead to disruption of its intra-ring contacts, expansion of the entire ring and opening of both the nucleotide pocket and the substrate-binding domains, admitting ATP and new substrate protein.

Electron Paramagnetic Resonance and Fluorescence Studies of the Conformation of Aspartate Aminotransferase Bound to GroEL

The interaction of the precursor to mitochondrial aspartate aminotransferase (pmAAT) with GroEL has been studied by electron paramagnetic resonance (EPR) and fluorescence spectroscopy. In the native protein, the spin probe was immobilized when attached to Cys166 at the domain interface, but was fully mobile when introduced at Cys(-19) in the N-terminal presequence peptide. Unfolding of the protein resulted in a highly mobile EPR spectrum for probes introduced at either site. However, the nitroxide group in GroEL-bound pmAAT showed either intermediate or high mobility depending on the spin probe used. Power saturation experiments indicated that the accessibility of the nitroxide side chain to Ni(EDDA) in the GroEL-pmAAT complex was higher than in the native state when in position 166 but lower when at position -19. Similar results were obtained in fluorescence quenching experiments. These data suggest that GroEL binds partly folded states of pmAAT with the presequence peptide probably in direct contact with GroEL. GroES and ATP, but not AMP-PNP or ADP, support refolding of pmAAT. During refolding, the rate of recovery of the native spectroscopic properties of labeled Cys166 is nearly identical to the rate-limiting reactivation step. Thus, correct docking of the large and small domains of pmAAT may be a key structural event in the regain of catalytic activity.

Is allostery an intrinsic property of all dynamic proteins?

Allostery involves coupling of conformational changes between two widely separated binding sites. The common view holds that allosteric proteins are symmetric oligomers, with each subunit existing in "at least" two conformational states with a different affinity for ligands. Recent observations such as the allosteric behavior of myoglobin, a classical example of a nonallosteric protein, call into question the existing allosteric dogma. Here we argue that all (nonfibrous) proteins are potentially allosteric. Allostery is a consequence of re-distributions of protein conformational ensembles. In a nonallosteric protein, the binding site shape may not show a concerted second-site change and enzyme kinetics may not reflect an allosteric transition. Nevertheless, appropriate ligands, point mutations, or external conditions may facilitate a population shift, leading a presumably nonallosteric protein to behave allosterically. In principle, practically any potential drug binding to the protein surface can alter the conformational redistribution. The question is its effectiveness in the redistribution of the ensemble, affecting the protein binding sites and its function. Here, we review experimental observations validating this view of protein allostery.

Allosteric regulation of chaperonins

Chaperonins are molecular machines that facilitate protein folding by undergoing energy (ATP)-dependent movements that are coordinated in time and space by complex allosteric regulation. Recently, progress has been made in describing the various functional (allosteric) states of these machines, the pathways by which they interconvert, and the coupling between allosteric transitions and protein folding reactions. However, various mechanistic issues remain to be resolved.

Crystal structure of wild-type chaperonin GroEL

The 2.9A resolution crystal structure of apo wild-type GroEL was determined for the first time and represents the reference structure, facilitating the study of structural and functional differences observed in GroEL variants. Until now the crystal structure of the mutant Arg13Gly, Ala126Val GroEL was used for this purpose. We show that, due to the mutations as well as to the presence of a crystallographic symmetry, the ring-ring interface was inaccurately described. Analysis of the present structure allowed the definition of structural elements at this interface, essential for understanding the inter-ring allosteric signal transmission. We also show unambiguously that there is no ATP-induced 102 degrees rotation of the apical domain helix I around its helical axis, as previously assumed in the crystal structure of the (GroEL-KMgATP)(14) complex, and analyze the apical domain movements. These results enabled us to compare our structure with other GroEL crystal structures already published, allowing us to suggest a new route through which the allosteric signal for negative cooperativity propagates within the molecule. The proposed mechanism, supported by known mutagenesis data, underlines the importance of the switching of salt bridges.

Automated molecular microscopy: the new Leginon system

We report here on the current state of our efforts in automated molecular microscopy. Our primary automated data acquisition software system, Leginon, has been completely redesigned over the past two years. The new distributed system has been developed using the Python programming language and is compatible with both Linux and Windows operating systems. The new flexible architecture was designed to allow for the development of customized data collection protocols, several of which are described here. The system has been used to acquire data for approximately 150 experiments and we have demonstrated the capacity for high throughput data acquisition by acquiring images of more than 100,000 particles in a single session at the microscope.

Electron cryomicroscopy of single particles at subnanometer resolution

Electron cryomicroscopy and single-particle reconstruction have advanced substantially over the past two decades. There are now numerous examples of structures that have been solved using this technique to better than 10 A resolution. At such resolutions, direct identification of alpha helices is possible and, often, beta-sheet-containing regions can be identified. The most numerous subnanometer resolution structures are the icosahedral viruses, as higher resolution is easier to achieve with higher symmetry. Important non-icosahedral structures solved to subnanometer resolution include several ribosome structures, clathrin assemblies and, most recently, the Ca2+ release channel. There is now hope that, in the next few years, this technique will achieve resolutions approaching 4 A, permitting a complete trace of the protein backbone without reference to a crystal structure.

Combining electron microscopy and comparative protein structure modeling

Recently, advances have been made in methods and applications that integrate electron microscopy density maps and comparative modeling to produce atomic structures of macromolecular assemblies. Electron microscopy can benefit from comparative modeling through the fitting of comparative models into electron microscopy density maps. Also, comparative modeling can benefit from electron microscopy through the use of intermediate-resolution density maps in fold recognition, template selection and sequence-structure alignment.

Ionic interactions at both inter-ring contact sites of GroEL are involved in transmission of the allosteric signal: a time-resolved infrared difference study

The biological activity of the double-ring chaperonin GroEL is regulated by complex allosteric interactions, which include positive intra-ring and negative inter-ring cooperativity. To further characterize inter-ring communication, the nucleotide-induced absorbance changes in the vibrational spectrum of the chaperonin GroEL, of two single-point mutants suppressing one inter-ring ionic contact (E461K and E434K) and of a single-ring version of this protein, were investigated by time-resolved infrared difference spectroscopy. Interaction of the nucleotide with the proteins was triggered by its photochemical release from a biologically inactive caged precursor [P3-1-(2-nitro) phenylethyl nucleotide]. The results indicate that (1) ATP binding to the protein induces a conformational change that affects concomitantly both intra-ring and inter-ring communication, and (2) the experimental absorbance changes are sensitive to the double-ring structure of the protein. The characterization of the single-point, inter-ring mutants demonstrates that ionic interactions at both contact sites are involved in the transmission of the allosteric signal. However, both mutations have different effects on the inter-ring interface. While that of E461K still retains ionic contacts sensitive to ATP binding, E434K shows spectroscopic features similar to those of the single-ring version of the protein, therefore suggesting that electrostatic interactions at these contact sites contribute differently to the stability of the inter-ring interface.

No evidence for a forced-unfolding mechanism during ATP/GroES binding to substrate-bound GroEL: no observable protection of metastable Rubisco intermediate or GroEL-bound Rubisco from tritium exchange

In tritium-hydrogen exchange experiments, the large GroEL substrate Rubisco was unfolded and exchanged in urea/acid/tritiated water, then diluted into either protic buffer or protic buffer containing GroEL. The respective Rubisco metastable folding intermediate or Rubisco-GroEL binary complex was then separated from residual tritium after varying times of exchange by centrifugation through P-10 or G-25 resin. No significant tritium was recovered in either case, in contrast to an earlier report. Thus, although the earlier-proposed forced unfolding mechanism for the action of GroEL on a bound polypeptide, occurring during ATP/GroES binding, remains an attractive hypothesis, the data here do not provide any indication that it is involved in the folding of Rubisco.

The 13Å Structure of a Chaperonin GroEL–Protein Substrate Complex by Cryo-electron Microscopy

The 13 angstroms resolution structures of GroEL bound to a single monomer of the protein substrate glutamine synthetase (GS(m)), as well as that of unliganded GroEL have been determined from a heterogeneous image population using cryo-electron microscopy (cryo-EM) coupled with single-particle image classification and reconstruction techniques. We combined structural data from cryo-EM maps and dynamic modeling, taking advantage of the known X-ray crystallographic structure and normal mode flexible fitting (NMFF) analysis, to describe the changes that occur in GroEL structure induced by GS(m) binding. The NMFF analysis reveals that the molecular movements induced by GS(m) binding propagate throughout the GroEL structure. The modeled molecular motions show that some domains undergo en bloc movements, while others show more complex independent internal movements. Interestingly, the substrate-bound apical domains of both the cis (GS(m)-bound ring) and trans (the opposite substrate-free ring) show counterclockwise rotations, in the same direction (though not as dramatic) as those documented for the ATP-GroEL-induced structure changes. The structural changes from the allosteric substrate protein-induced negative cooperativity between the GroEL rings involves upward concerted movements of both cis and trans equatorial domains toward the GS(m)-bound ring, while the inter-ring distances between the heptamer contact residues are maintained. Furthermore, the NMFF analysis identifies the secondary structural elements that are involved in the observed approximately 5 angstroms reduction in the diameter of the cavity opening in the unbound trans ring. Understanding the molecular basis of these substrate protein-induced structural changes across the heptamer rings provides insight into the origins of the allosteric negative cooperative effects that are transmitted over long distances (approximately 140 angstroms).

Protein structural transitions and their functional role

Living cells are a collection of molecular machines which carry out many of the functions essential for the cell's existence, differentiation and reproduction. Most, though not all, of these machines are made up of proteins. Because of their complexity, an understanding of how they work requires a synergistic combination of experimental and theoretical studies. In this paper we outline our studies of two such protein machines. One is GroEL, the chaperone from Escherichia coli, which aids in protein folding; the other is F(1)-ATPase, a motor protein which synthesizes and hydrolyses ATP.

Seeing GroEL at 6 A resolution by single particle electron cryomicroscopy

We present a reconstruction of native GroEL by electron cryomicroscopy (cryo-EM) and single particle analysis at 6 A resolution. alpha helices are clearly visible and beta sheet density is also visible at this resolution. While the overall conformation of this structure is quite consistent with the published X-ray data, a measurable shift in the positions of three alpha helices in the intermediate domain is observed, not consistent with any of the 7 monomeric structures in the Protein Data Bank model (1OEL). In addition, there is evidence for slight rearrangement or flexibility in parts of the apical domain. The 6 A resolution cryo-EM GroEL structure clearly demonstrates the veracity and expanding scope of cryo-EM and the single particle reconstruction technique for macromolecular machines.

Sequential ATP-induced allosteric transitions of the cytoplasmic chaperonin containing TCP-1 revealed by EM analysis

The eukaryotic cytoplasmic chaperonin containing TCP-1 (CCT) is a hetero-oligomeric complex that assists the folding of actins, tubulins and other proteins in an ATP-dependent manner. To understand the allosteric transitions that occur during the functional cycle of CCT, we imaged the chaperonin complex in the presence of different ATP concentrations. Labeling by monoclonal antibodies that bind specifically to the CCTalpha and CCTdelta subunits enabled alignment of all the CCT subunits of a given type in different particles. The analysis shows that the apo state of CCT has considerable apparent conformational heterogeneity that decreases with increasing ATP concentration. In contrast with the concerted allosteric switch of GroEL, ATP-induced conformational changes in CCT are found to spread around the ring in a sequential fashion that may facilitate domain-by-domain substrate folding. The approach described here can be used to unravel the allosteric mechanisms of other ring-shaped molecular machines.

The unfolding action of GroEL on a protein substrate

A molecular dynamics simulation of the active unfolding of denatured rhodanese by the chaperone GroEL is presented. The compact denatured protein is bound initially to the cis cavity and forms stable contacts with several of the subunits. As the cis ring apical domains of GroEL undergo the transition from the closed to the more open (ATP-bound) state, they exert a force on rhodanese that leads to the increased unfolding of certain loops. The contacts between GroEL and rhodanese are analyzed and their variation during the GroEL transition is shown. The major contacts, which give rise to the stretching force, are found to be similar to those observed in crystal structures of peptides bound to the apical domains. The results of the simulation show that multidomain interactions play an essential role, in accord with experiments. Implications of the results for mutation experiments and for the action of GroEL are discussed.

SPI-EM: towards a tool for predicting CATH superfamilies in 3D-EM maps

In this paper the theoretical framework used to build a superfamily probability in electron microscopy (SPI-EM) is presented. SPI-EM is a new tool for determining the homologous superfamily to which a protein domain belongs looking at its three-dimensional electron microscopy map. The homologous superfamily is assigned according to the domain-architecture database CATH. Our method follows a probabilistic approach applied to the results of fitting protein domains into maps of proteins and the computation of local cross-correlation coefficient measures. The method has been tested and its usefulness proven with isolated domains at a resolution of 8 A and 12 A. Results obtained with simulated and experimental data at 10 A suggest that it is also feasible to detect the correct superfamily of the domains when dealing with electron microscopy maps containing multi-domain proteins. The inherent difficulties and limitations that multi-domain proteins impose are discussed. Our procedure is complementary to other techniques existing in the field to detect structural elements in electron microscopy maps like alpha-helices and beta-sheets. Based on the proposed methodology, a database of relevant distributions is being built to serve the community.

Exploring the structural dynamics of the E.coli chaperonin GroEL using translation-libration-screw crystallographic refinement of intermediate states

Large rigid-body domain movements are critical to GroEL-mediated protein folding, especially apical domain elevation and twist associated with the formation of a folding chamber upon binding ATP and co-chaperonin GroES. Here, we have modeled the anisotropic displacements of GroEL domains from various crystallized states, unliganded GroEL, ATPgammaS-bound, ADP-AlFx/GroES-bound, and ADP/GroES bound, using translation-libration-screw (TLS) analysis. Remarkably, the TLS results show that the inherent motions of unliganded GroEL, a polypeptide-accepting state, are biased along the transition pathway that leads to the folding-active state. In the ADP-AlFx/GroES-bound folding-active state the dynamic modes of the apical domains become reoriented and coupled to the motions of bound GroES. The ADP/GroES complex exhibits these same motions, but they are increased in magnitude, potentially reflecting the decreased stability of the complex after nucleotide hydrolysis. Our results have allowed the visualization of the anisotropic molecular motions that link the static conformations previously observed by X-ray crystallography. Application of the same analyses to other macromolecules where rigid body motions occur may give insight into the large scale dynamics critical for function and thus has the potential to extend our fundamental understanding of molecular machines.

Substrate polypeptide presents a load on the apical domains of the chaperonin GroEL

A conundrum has arisen in the study of the structural states of the GroEL-GroES chaperonin machine: When either ATP or ADP is added along with GroES to GroEL, the same asymmetric complex, with one ring in a GroES-domed state, is observed by either x-ray crystallographic study or cryoelectron microscopy. Yet only ATP/GroES can trigger productive folding inside the GroES-encapsulated cis cavity by ejecting bound polypeptide from hydrophobic apical binding sites during attendant rigid body elevation and twisting of these domains. Here, we show that this difference occurs because polypeptide substrate in fact presents a load on the apical domains, and, although ATP can counter this load effectively, ADP cannot. We monitored apical domain movement in real time by fluorescence resonance energy transfer (FRET) between a fixed equatorial fluorophore and one attached to the mobile apical domain. In the absence of bound polypeptide, addition of either ATP/GroES or ADP/GroES to GroEL produced the same rapid rate and extent of decrease of FRET (t(1/2) < 1 sec), reflecting similarly rapid apical movement to the same end-state and explaining the results of the structural studies, which were all carried out in the absence of substrate polypeptide. But in the presence of bound malate dehydrogenase or rhodanese, whereas similar rapid and extensive FRET changes were observed with ATP/GroES, the rate of FRET change with ADP/GroES was slowed by >100-fold and the extent of change was reduced, indicating that the apical domains opened in a slow and partial fashion. These results indicate that the free energy of gamma-phosphate binding, measured earlier as 43 kcal per mol (1 cal = 4.184 J) of rings, is required for driving the forceful excursion or "power stroke" of the apical domains needed to trigger release of the polypeptide load into the central cavity.

A mutant chaperonin with rearranged inter-ring electrostatic contacts and temperature-sensitive dissociation

The chaperonin GroEL assists protein folding through ATP-dependent, cooperative movements that alternately create folding chambers in its two rings. The substitution E461K at the interface between these two rings causes temperature-sensitive, defective protein folding in Escherichia coli. To understand the molecular defect, we have examined the mutant chaperonin by cryo-EM. The normal out-of-register alignment of contacts between subunits of opposing wild-type rings is changed in E461K to an in-register one. This is associated with loss of cooperativity in ATP binding and hydrolysis. Consistent with the loss of negative cooperativity between rings, the cochaperonin GroES binds simultaneously to both E461K rings. These GroES-bound structures were unstable at higher temperature, dissociating into complexes of single E461K rings associated with GroES. Lacking the allosteric signal from the opposite ring, these complexes cannot release their GroES and become trapped, dead-end states.

Crystal Structure of the Native Chaperonin Complex from Thermus thermophilus Revealed Unexpected Asymmetry at the cis-Cavity

The chaperonins GroEL and GroES are essential mediators of protein folding. GroEL binds nonnative protein, ATP, and GroES, generating a ternary complex in which protein folding occurs within the cavity capped by GroES (cis-cavity). We determined the crystal structure of the native GroEL-GroES-ADP homolog from Thermus thermophilus, with substrate proteins in the cis-cavity, at 2.8 A resolution. Twenty-four in vivo substrate proteins within the cis-cavity were identified from the crystals. The structure around the cis-cavity, which encapsulates substrate proteins, shows significant differences from that observed for the substrate-free Escherichia coli GroEL-GroES complex. The apical domain around the cis-cavity of the Thermus GroEL-GroES complex exhibits a large deviation from the 7-fold symmetry. As a result, the GroEL-GroES interface differs considerably from the previously reported E. coli GroEL-GroES complex, including a previously unknown contact between GroEL and GroES.

Phi value analysis of an allosteric transition of GroEL based on a single-pathway model

There are currently two contradictory models for the kinetics of the ATP-induced GroEL allosteric transition occurring around 20 microM ATP. One model, proposed by Horovitz et al. demonstrates the existence of two parallel pathways for the allosteric transition and an abrupt ATP-dependent switch from one pathway to the other. The other model, which was proposed by the present authors, shows no need to assume the parallel pathways, and a combination of the transition-state theory and the Monod-Wyman-Changeux model of allostery can explain the kinetics as well as the equilibrium of the transition. The discrepancy appears to be due to whether we regard the transition as reversible or irreversible. Thus, here we have investigated the reversibility of the allosteric transition between 0 microM and 70 microM ATP by the use of a stopped-flow double-jump technique, which has allowed us to monitor the kinetics of the reverse reaction from the relaxed state at a high ATP concentration to the tense state at a low ATP concentration. The tryptophan fluorescence of a tryptophan-inserted variant of GroEL was used to follow the kinetics. As a result, the allosteric transition was shown to be a reversible process, supporting the validity of our model. We also show that the structural environment around the ATP-binding site of GroEL in the transition state is very similar to that in the relaxed state (Phi=0.9) by using a Phi value analysis in the kinetic Monod-Wyman-Changeux model, which is analogous to the mutational Phi value analysis in protein folding.

Kinetic analysis of ATP-dependent inter-ring communication in GroEL

Different concentrations of ATP were mixed rapidly with single-ring or double-ring forms of GroEL containing the Phe44-->Trp mutation and the time-resolved changes in fluorescence emission, upon excitation at 295 nm, were followed. Two kinetic phases that were previously found for double-ring GroEL are also observed in the case of the single-ring version: (i) a fast phase with a relatively large amplitude that is associated with the T-->R allosteric transition; (ii) and a slow phase with a smaller amplitude that is associated with ATP hydrolysis. In the case of weak intra-ring positive cooperativity, the rate constant corresponding to the T-->R allosteric switch of single-ring GroEL displays a bi-sigmoidal dependence on ATP concentration that may reflect parallel pathways of the allosteric transition. The slow phase is absent when double-ring or single-ring forms of GroEL are mixed with ADP or ATP without K(+), and it has a rate constant that is independent of ATP concentration. A third fast phase that is still unassigned is observed for both single-ring and double-ring GroEL when a large amount of data is collected. Finally, a fourth phase is observed in the case of double-ring GroEL that is found to be absent in the case of single-ring GroEL. This phase is here assigned to inter-ring communication by (i) determining its dependence on ATP concentration and (ii) based on its absence from single-ring GroEL and the Arg13-->Gly, Ala126-->Val GroEL mutant, which is defective in inter-ring negative cooperativity. The value of the rate constant corresponding to this phase is found to increase with increasing intra-ring positive cooperativity, with respect to ATP. This is the first report of the rate of ATP-mediated inter-ring communication in GroEL, in the presence of ATP alone, which is crucial for the cycling of this molecular machine between different functional states.

A kinetic analysis of the nucleotide-induced allosteric transitions in a single-ring mutant of GroEL

The function of GroE requires a complex system of allosteric communication driven by protein-nucleotide interactions. These rearrangements couple the binding and hydrolysis of ATP to an overall reaction cycle in which substrate proteins are bound, encapsulated and released. Positive cooperativity with respect to ATP binding occurs within one heptameric ring of GroEL, while negative cooperativity between the two rings generates an inherent asymmetry between the two rings. A previously engineered mutant of GroEL in which the ring-ring contacts are broken gives rise to a single-ring version of the wild-type chaperonin (SR1). We have studied the kinetics of the nucleotide-induced conformational changes in a single-tryptophan variant of SR1 (Y485W-SR1) and compared the resulting data with those we reported previously for the double-ring, single-tryptophan variant of wild-type GroEL (Y485W-GroEL). Remarkably, the parting of the rings does not have a major effect on the conformational changes occurring within the heptameric ring and a kinetic model is presented to describe the sequence of structural rearrangements that occur upon ATP binding to the SR1 molecule. The observation that both the ATP-induced and ADP-induced conformational rearrangements occur more rapidly in the SR1 than they do in wild-type GroEL, indicates that intra-ring conformational changes in the double-ring structure must overcome conformational constraints provided by the presence of the second ring. Lastly, the data presented here imply a role for inter-ring allostery in controlling the dissociation-association behaviour of the GroES co-protein in the overall reaction cycle.

Fast Fitting of Atomic Structures to Low-resolution Electron Density Maps by Surface Overlap Maximization

The complexities of X-ray crystallography and NMR spectroscopy for large protein complexes, and the comparative ease of approaches such as electron microscopy mean that low-resolution structures are often available long before their atomic resolution equivalents. To help bridge this gap in knowledge, we present 3SOM: an approach for finding the best fit of atomic resolution structures into lower-resolution density maps through surface overlap maximization. High-resolution templates (i.e. partial structures or models for multi-subunit complexes) and targets (lower-resolution maps) are initially represented as iso-surfaces. The latter are used first in a fast search for transformations that superimpose a significant portion of the target surface onto the template surface, which is quantified as surface overlap. The vast search space is reduced by considering key vectors that capture local surface information. The set of transformations with the highest surface overlap scores are then re-ranked by using more sophisticated scores including cross-correlation. We give a number of examples to illustrate the efficiency of the method and its restrictions. For targets for which partial complexes are available, the speed and performance of the method make it an attractive complement to existing methods, as many different hypotheses can be tested quickly on a single processor.

Stopped-flow Fluorescence Analysis of the Conformational Changes in the GroEL Apical Domain

GroEL undergoes numerous conformational alterations in the course of facilitating the folding of various proteins, and the specific movements of the GroEL apical domain are of particular importance in the molecular mechanism. In order to monitor in detail the numerous movements of the GroEL apical domain, we have constructed a mutant chaperonin (GroEL R231W) with wild type-like function and a fluorescent probe introduced into the apical domain. By monitoring the tryptophan fluorescence changes of GroEL R231W upon ATP addition in the presence and absence of the co-chaperonin GroES, we detected a total of four distinct kinetic phases that corresponded to conformational changes of the apical domain and GroES binding. By introducing this mutation into a single ring variant of GroEL (GroEL SR-1), we determined the extent of inter-ring cooperation that was involved in apical domain movements. Surprisingly, we found that the apical domain movements of GroEL were affected only slightly by the change in quaternary structure. Our experiments provide a number of novel insights regarding the dynamic movements of this protein.

Molecular architecture of the basal transcription factor B-TFIID

BTAF1 (formerly named TAF(II)170/TAF-172) is an essential, evolutionarily conserved member of the SNF2-like family of ATPase proteins and together with TATA-binding protein (TBP) forms the B-TFIID complex. BTAF1 has been proposed to play a key role in the dynamic regulation of TBP function in RNA polymerase II transcription. We have determined the structure of native B-TFIID purified from human cells by electron microscopy and by image analysis of single particles at a resolution of 28 A. B-TFIID is 15 x 9 nm in size and is organized into a large domain of about 170 kDa, which can be subdivided into two domains. Extending from this domain is a long thumb, which in turn is divided into subdomains of about 25, 15, and 35 kDa, the largest of which is located at the end of the thumb. Immunolabeling experiments localize the extreme carboxyl terminus of BTAF1 within the 170-kDa domain, whereas the amino terminus and TBP co-localize to the end of the protruding thumb. The central portion of BTAF1 localizes to the base of the thumb. Comparison of the native B-TFIID with its recombinant form shows that both share a similar domain organization. Collectively, these data provide the first structural model of the B-TFIID complex and map its key functional domains.

Crystal structures of the group II chaperonin from Thermococcus strain KS-1: steric hindrance by the substituted amino acid, and inter-subunit rearrangement between two crystal forms

The crystal structures of the group II chaperonins consisting of the alpha subunit with amino acid substitutions of G65C and/or I125T from the hyperthermophilic archaeum Thermococcus strain KS-1 were determined. These mutants have been shown to be active in ATP hydrolysis but inactive in protein folding. The structures were shown to be double-ring hexadecamers in an extremely closed form, which was consistent with the crystal structure of native alpha8beta8-chaperonin from Thermoplasma acidophilum. Comparisons of the present structures with the atomic structures of the GroEL14-GroES7-(ADP)7 complex revealed that the deficiency in protein-folding activity with the G65C amino acid substitution is caused by the steric hindrance of the local conformational change in an equatorial domain. We concluded that this mutant chaperonin with G65C substitution is deprived of the smooth conformational change in the refolding-reaction cycle. We obtained a new form of crystal with a distinct space group at a lower concentration of sulfate ion in the presence of nucleotide. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion. Such subunit rotation has never been characterized in group II chaperonins. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion.

Rapid refinement of crystallographic protein construct definition employing enhanced hydrogen/deuterium exchange MS

Crystallographic efforts often fail to produce suitably diffracting protein crystals. Unstructured regions of proteins play an important role in this problem and considerable advantage can be gained in removing them. We have developed a number of enhancements to amide hydrogen/high-throughput and high-resolution deuterium exchange MS (DXMS) technology that allow rapid identification of unstructured regions in proteins. To demonstrate the utility of this approach for improving crystallization success, DXMS analysis was attempted on 24 Thermotoga maritima proteins with varying crystallization and diffraction characteristics. Data acquisition and analysis for 21 of these proteins was completed in 2 weeks and resulted in the localization and prediction of several unstructured regions within the proteins. When compared with those targets of known structure, the DXMS method correctly localized even small regions of disorder. DXMS analysis was then correlated with the propensity of such targets to crystallize and was further used to define truncations that improved crystallization. Truncations that were defined solely on DXMS analysis demonstrated greatly improved crystallization and have been used for structure determination. This approach represents a rapid and generalized method that can be applied to structural genomics or other targets in a high-throughput manner.

Annealing function of GroEL: structural and bioinformatic analysis

The Escherichia coli chaperonin system, GroEL-GroES, facilitates folding of substrate proteins (SPs) that are otherwise destined to aggregate. The iterative annealing mechanism suggests that the allostery-driven GroEL transitions leading to changes in the microenvironment of the SP constitutes the annealing action of chaperonins. To describe the molecular basis for the changes in the nature of SP-GroEL interactions we use the crystal structures of GroEL (T state), GroEL-ATP (R state) and the GroEL-GroES-(ADP)(7) (R" state) complex to determine the residue-specific changes in the accessible surface area and the number of tertiary contacts as a result of the T-->R-->R" transitions. We find large changes in the accessible area in many residues in the apical domain, but relatively smaller changes are associated with residues in the equatorial domain. In the course of the T-->R transition the microenvironment of the SP changes which suggests that GroEL is an annealing machine even without GroES. This is reflected in the exposure of Glu386 which loses six contacts in the T-->R transition. We also evaluate the conservation of residues that participate in the various chaperonin functions. Multiple sequence alignments and chemical sequence entropy calculations reveal that, to a large extent, only the chemical identities and not the residues themselves important for the nominal functions (peptide binding, nucleotide binding, GroES and substrate protein release) are strongly conserved. Using chemical sequence entropy, which is computed by classifying aminoacids into four types (hydrophobic, polar, positively charged and negatively charged) we make several new predictions that are relevant for peptide binding and annealing function of GroEL. We identify a number of conserved peptide binding sites in the apical domain which coincide with those found in the 1.7 A crystal structure of 'mini-chaperone' complexed with the N-terminal tag. Correlated mutations in the HSP60 family, that might control allostery in GroEL, are also strongly conserved. Most importantly, we find that charged solvent-exposed residues in the T state (Lys 226, Glu 252 and Asp 253) are strongly conserved. This leads to the prediction that mutating these residues, that control the annealing function of the SP, can decrease the efficacy of the chaperonin function.

Role of the gamma-phosphate of ATP in triggering protein folding by GroEL-GroES: function, structure and energetics

Productive cis folding by the chaperonin GroEL is triggered by the binding of ATP but not ADP, along with cochaperonin GroES, to the same ring as non-native polypeptide, ejecting polypeptide into an encapsulated hydrophilic chamber. We examined the specific contribution of the gamma-phosphate of ATP to this activation process using complexes of ADP and aluminium or beryllium fluoride. These ATP analogues supported productive cis folding of the substrate protein, rhodanese, even when added to already-formed, folding-inactive cis ADP ternary complexes, essentially introducing the gamma-phosphate of ATP in an independent step. Aluminium fluoride was observed to stabilize the association of GroES with GroEL, with a substantial release of free energy (-46 kcal/mol). To understand the basis of such activation and stabilization, a crystal structure of GroEL-GroES-ADP.AlF3 was determined at 2.8 A. A trigonal AlF3 metal complex was observed in the gamma-phosphate position of the nucleotide pocket of the cis ring. Surprisingly, when this structure was compared with that of the previously determined GroEL-GroES-ADP complex, no other differences were observed. We discuss the likely basis of the ability of gamma-phosphate binding to convert preformed GroEL-GroES-ADP-polypeptide complexes into the folding-active state.

Conversion of the allosteric transition of GroEL from concerted to sequential by the single mutation Asp-155 -> Ala

The reaction cycle of the double-ring chaperonin GroEL is driven by ATP binding that takes place with positive cooperativity within each seven-membered ring and negative cooperativity between rings. The positive cooperativity within rings is due to ATP binding-induced conformational changes that are fully concerted. Herein, it is shown that the mutation Asp-155 --> Ala leads to an ATP-induced break in intra-ring and inter-ring symmetry. Electron microscopy analysis of single-ring GroEL particles containing the Asp-155 --> Ala mutation shows that the break in intra-ring symmetry is due to stabilization of allosteric intermediates such as one in which three subunits have switched their conformation while the other four have not. Our results show that eliminating an intra-subunit interaction between Asp-155 and Arg-395 results in conversion of the allosteric switch of GroEL from concerted to sequential, thus demonstrating that its allosteric behavior arises from coupled tertiary conformational changes.

Allosteric Changes in Protein Structure Computed by a Simple Mechanical Model: Hemoglobin T↔R2 Transition

Information on protein dynamics has been usually inferred from spectroscopic studies of parts of the proteins, or indirectly from the comparison of the conformations assumed in the presence of different substrates or ligands. While molecular simulations also provide information on protein dynamics, they usually suffer from incomplete sampling of conformational space, and become prohibitively expensive when exploring the collective dynamics of large macromolecular structures. Here, we explore the dynamics of a well-studied allosteric protein, hemoglobin (Hb), to show that a simple mechanical model based on Gaussian fluctuations of residues can efficiently predict the transition between the tense (T, unliganded) and relaxed (R or R2, O(2) or CO-bound) forms of Hb. The passage from T into R2 is shown to be favored by the global mode of motion, which, in turn is driven by entropic effects. The major difference between the dynamics of the T and R2 forms is the loss of the hinge-bending role of alpha(1)-beta(2) (or alpha(2)-beta(1)) interfacial residues at alpha Phe36-His45 and beta Thr87-Asn102 in the R2 form, which implies a decreased cooperativity in the higher affinity (R2) form of Hb, consistent with many experimental studies. The involvement of the proximal histidine beta His92 in this hinge region suggests that the allosteric propagation of the local structural changes (induced upon O(2) binding) into global ones occur via hinge regions. This is the first demonstration that there is an intrinsic tendency of Hb to undergo T-->R2 transition, induced by purely elastic forces of entropic origin that are uniquely defined for the particular contact topology of the T form.

GroEL stability and function. Contribution of the ionic interactions at the inter-ring contact sites

The chaperonin GroEL consists of a double ring structure made of identical subunits that display different modes of allosteric communication. The protein folding cycle requires the simultaneous positive intra-ring and negative inter-ring cooperativities of ATP binding. This ensures GroES binding to one ring and release of the ligands from the opposite one. To better characterize inter-ring allosterism, the thermal stability as well as the temperature dependence of the functional and conformational properties of wild type GroEL, a single ring mutant (SR1) and two single point mutants suppressing one interring salt bridge (E434K and E461K) were studied. The results indicate that ionic interactions at the two interring contact sites are essential to maintain the negative cooperativity for protein substrate binding and to set the protein thermostat at 39 degrees C. These electrostatic interactions contribute distinctly to the stability of the inter-ring interface and the overall protein stability, e.g. the E434K thermal inactivation curve is shifted to lower temperatures, and its unfolding temperature and activation energy are also lowered. An analysis of the ionic interactions at the inter-ring contact sites reveals that at the so called "left site" a network of electrostatic interactions involving three charged residues might be established, in contrast to what is found at the "right site" where only two oppositely charged residues interact. Our data suggest that electrostatic interactions stabilize protein-protein interfaces depending on both the number of ionic interactions and the number of residues engaged in each of these interactions. In the case of GroEL, this combination sets the thermostat of the protein so that the chaperonin distinguishes physiological from stress temperatures.

Objective evaluation of the relative modulation transfer function of densitometers for digitisation of electron micrographs

Digitisation of images recorded on film is a crucial part of data acquisition in electron microscopy, particularly for electron cryo-microscopy of biological specimens where the contrast and signal-to-noise ratio are low. A quantitative method to evaluate and compare the quality of densitometers, as measured by the modulation transfer function (MTF), is described here. The densitometer is modelled as a linear system, the output being the convolution of the input image and a point spread function. The MTF is the magnitude of the Fourier transform of the point spread function. The relative MTF describes the quality of signal transfer with spatial frequency. It is important that fine structural details in the micrograph are digitised with a high value for the MTF which does not vary with direction. A test pattern has been generated by projecting an electron image of a grid pattern onto film. The film is scanned and a computer program measures the intensities of the diffraction orders of the repeating pattern. Three different scanners are compared, one is a point scanner and the other two are line scanners. The test can be used to check if a scanner is set up optimally, and how it compares with another scanner.

Automated image acquisition and processing using a new generation of 4K x 4K CCD cameras for cryo electron microscopic studies of macromolecular assemblies

We have previously reported the development of AutoEM, a software package for semi-automated acquisition of data from a transmission electron microscope. In continuing efforts to improve the speed of structure determination of macromolecular assemblies by electron microscopy, we report here on the performance of a new generation of 4 K CCD cameras for use in cryo electron microscopic applications. We demonstrate that at 120 kV, and at a nominal magnification of 67000 x, power spectra and signal-to-noise ratios for the new 4 K CCD camera are comparable to values obtained for film images scanned using a Zeiss scanner to resolutions as high as approximately 1/6.5A(-1). The specimen area imaged for each exposure on the 4 K CCD is about one-third of the area that can be recorded with a similar exposure on film. The CCD camera also serves the purpose of recording images at low magnification from the center of the hole to measure the thickness of vitrified ice in the hole. The performance of the camera is satisfactory under the low-dose conditions used in cryo electron microscopy, as demonstrated here by the determination of a three-dimensional map at 15 A for the catalytic core of the 1.8 MDa Bacillus stearothermophilus icosahedral pyruvate dehydrogenase complex, and its comparison with the previously reported atomic model for this complex obtained by X-ray crystallography.

Folding with and without encapsulation by cis- and trans-only GroEL-GroES complexes

Although a cis mechanism of GroEL-mediated protein folding, occurring inside a hydrophilic chamber encapsulated by the co-chaperonin GroES, has been well documented, recently the GroEL-GroES-mediated folding of aconitase, a large protein (82 kDa) that could not be encapsulated, was described. This process required GroES binding to the ring opposite the polypeptide (trans) to drive release and productive folding. Here, we have evaluated this mechanism further using trans-only complexes in which GroES is closely tethered to one of the two GroEL rings, blocking polypeptide binding by that ring. In vitro, trans-only folded aconitase with kinetics identical to GroEL-GroES. Surprisingly, trans-only also folded smaller GroEL-GroES-dependent substrates, Rubisco and malate dehydrogenase, but at rates slower than the cis reaction. Remarkably, in vivo, a plasmid encoding a trans-only complex rescued a GroEL-deficient strain, but the colony size was approximately one-tenth that produced by wild-type GroEL-GroES. We conclude that a trans mechanism, involving rounds of binding to an open ring and direct release into the bulk solution, can be generally productive although, where size permits, cis encapsulation supports more efficient folding.