Mitochondrial Arrest on the Microtubule Highway-A Feature of Heart Failure and Diabetic Cardiomyopathy?

A pathophysiological consequence of both type 1 and 2 diabetes is remodelling of the myocardium leading to the loss of left ventricular pump function and ultimately heart failure (HF). Abnormal cardiac bioenergetics associated with mitochondrial dysfunction occurs in the early stages of HF. Key factors influencing mitochondrial function are the shape, size and organisation of mitochondria within cardiomyocytes, with reports identifying small, fragmented mitochondria in the myocardium of diabetic patients. Cardiac mitochondria are now known to be dynamic organelles (with various functions beyond energy production); however, the mechanisms that underpin their dynamism are complex and links to motility are yet to be fully understood, particularly within the context of HF. This review will consider how the outer mitochondrial membrane protein Miro1 (Rhot1) mediates mitochondrial movement along microtubules via crosstalk with kinesin motors and explore the evidence for molecular level changes in the setting of diabetic cardiomyopathy. As HF and diabetes are recognised inflammatory conditions, with reports of enhanced activation of the NLRP3 inflammasome, we will also consider evidence linking microtubule organisation, inflammation and the association to mitochondrial motility. Diabetes is a global pandemic but with limited treatment options for diabetic cardiomyopathy, therefore we also discuss potential therapeutic approaches to target the mitochondrial-microtubule-inflammatory axis.

Remodeling of the Purkinje Network in Congestive Heart Failure in the Rabbit

Supplemental Digital Content is available in the text.

Background:

Purkinje fibers (PFs) control timing of ventricular conduction and play a key role in arrhythmogenesis in heart failure (HF) patients. We investigated the effects of HF on PFs.

Methods:

Echocardiography, electrocardiography, micro-computed tomography, quantitative polymerase chain reaction, immunohistochemistry, volume electron microscopy, and sharp microelectrode electrophysiology were used.

Results:

Congestive HF was induced in rabbits by left ventricular volume- and pressure-overload producing left ventricular hypertrophy, diminished fractional shortening and ejection fraction, and increased left ventricular dimensions. HF baseline QRS and corrected QT interval were prolonged by 17% and 21% (mean±SEMs: 303±6 ms HF, 249±11 ms control; n=8/7; P=0.0002), suggesting PF dysfunction and impaired ventricular repolarization. Micro-computed tomography imaging showed increased free-running left PF network volume and length in HF. mRNA levels for 40 ion channels, Ca²⁺-handling proteins, connexins, and proinflammatory and fibrosis markers were assessed: 50% and 35% were dysregulated in left and right PFs respectively, whereas only 12.5% and 7.5% changed in left and right ventricular muscle. Funny channels, Ca²⁺-channels, and K⁺-channels were significantly reduced in left PFs. Microelectrode recordings from left PFs revealed more negative resting membrane potential, reduced action potential upstroke velocity, prolonged duration (action potential duration at 90% repolarization: 378±24 ms HF, 249±5 ms control; n=23/38; P<0.0001), and arrhythmic events in HF. Similar electrical remodeling was seen at the left PF-ventricular junction. In the failing left ventricle, upstroke velocity and amplitude were increased, but action potential duration at 90% repolarization was unaffected.

Conclusions:

Severe volume- followed by pressure-overload causes rapidly progressing HF with extensive remodeling of PFs. The PF network is central to both arrhythmogenesis and contractile dysfunction and the pathological remodeling may increase the risk of fatal arrhythmias in HF patients.

P3826Gene therapy for cardiac conduction system dysfunction in heart failure

Background and purpose

Heart failure (HF) is characterised by generalised dysfunction of the cardiac conduction system (CCS). Ion channel and structural remodelling in the CCS have been widely demonstrated in animal models of cardiovascular disease. As Purkinje fibres (PFs) are minute strands of tissue, little is known about their ultrastructure and remodelling in disease. Furthermore, given the role for microRNAs (miRs) in CCS molecular remodelling, we aimed to develop a tissue specific method for delivering therapeutic transgenes, such as miR sponges.

Methods

New Zealand rabbits were used for PF ultrastructural studies. HF was induced via pressure and volume overload. Free running PFs were processed for serial block face scanning electron microscopy (SBF-SEM). Manual contrast-based segmentation techniques were used on IMOD software to determine the 3D cellular ultrastructure. To target transgene expression to the CCS, adenoviral plasmids were cloned expressing a GFP reporter gene. GFP transcription was placed under control of the KCNE1 promoter, a K+ channel subunit expressed throughout the CCS, or the HCN4 promoter, a key pacemaker ion channel, to target the sinus node. The strong ubiquitous cytomegalovirus (CMV) promoter was used as a positive control. Adenovirus was produced using via transfection into the 293A cell line for viral packaging and amplification.

Results

Purkinje cells (PCs) formed a central core within PFs, encapsulated by an extensive collagen matrix. PCs were uninucleated and spindle shaped with an irregular membrane. Gap junctions were abundant and distributed along the lateral surface of cells, and there was a trend towards decreased expression in HF (p=0.0526, n=3 cells analysed per group). Hypertrophy and nuclear membrane breakdown were evident in HF PCs, the latter facilitating mitochondrial entry.

Using the CMV-GFP adenoviral construct, abundant GFP expression was conferred in ex vivo sinus node tissue, isolated sinus node myocytes, and neonatal ventricular rat cardiomyocytes (NRCMs). The KCNE1 promoter conferred relatively high GFP expression in NRCMs, greater than that from the HCN4 promoter. In isolated sinus node myocytes, the HCN4 promoter conferred greater transgene expression than in NRCMs. In ex vivo sinus node tissue, only the CMV construct was capable of driving significant GFP expression. Notably, expression was largely confined to the sinus node, with only sparse expression detected in the surrounding atrial muscle.

Conclusions

SBF-SEM revealed ultrastructure of free running PFs in situ, and uncovered novel structural changes in HF that are likely to be pro-arrhythmic. Preliminary data suggest that 1.2 kb and 0.8 kb fragments of the HCN4 promoter are capable of driving sinus node specific transgene expression. Further tests are warranted to confirm the utility of these promoters to express therapeutic transgenes, such as miR sponges to competitively inhibit miR activity in vitro and in vivo.

Acknowledgement/Funding

The British Heart Foundation

Cardiomyocyte damage control in heart failure and the role of the sarcolemma

The cardiomyocyte plasma membrane, termed the sarcolemma, is fundamental for regulating a myriad of cellular processes. For example, the structural integrity of the cardiomyocyte sarcolemma is essential for mediating cardiac contraction by forming microdomains such as the t-tubular network, caveolae and the intercalated disc. Significantly, remodelling of these sarcolemma microdomains is a key feature in the development and progression of heart failure (HF). However, despite extensive characterisation of the associated molecular and ultrastructural events there is a lack of clarity surrounding the mechanisms driving adverse morphological rearrangements. The sarcolemma also provides protection, and is the cell's first line of defence, against external stresses such as oxygen and nutrient deprivation, inflammation and oxidative stress with a loss of sarcolemma viability shown to be a key step in cell death via necrosis. Significantly, cumulative cell death is also a feature of HF, and is linked to disease progression and loss of cardiac function. Herein, we will review the link between structural and molecular remodelling of the sarcolemma associated with the progression of HF, specifically considering the evidence for: (i) Whether intrinsic, evolutionary conserved, plasma membrane injury-repair mechanisms are in operation in the heart, and (ii) if deficits in key 'wound-healing' proteins (annexins, dysferlin, EHD2 and MG53) may play a yet to be fully appreciated role in triggering sarcolemma microdomain remodelling and/or necrosis. Cardiomyocytes are terminally differentiated with very limited regenerative capability and therefore preserving cell viability and cardiac function is crucially important. This review presents a novel perspective on sarcolemma remodelling by considering whether targeting proteins that regulate sarcolemma injury-repair may hold promise for developing new strategies to attenuate HF progression.

Quantitative proteomics and single-nucleus transcriptomics of the sinus node elucidates the foundation of cardiac pacemaking

The sinus node is a collection of highly specialised cells constituting the heart’s pacemaker. The molecular underpinnings of its pacemaking abilities are debated. Using high-resolution mass spectrometry, we here quantify >7,000 proteins from sinus node and neighbouring atrial muscle. Abundances of 575 proteins differ between the two tissues. By performing single-nucleus RNA sequencing of sinus node biopsies, we attribute measured protein abundances to specific cell types. The data reveal significant differences in ion channels responsible for the membrane clock, but not in Ca²⁺ clock proteins, suggesting that the membrane clock underpins pacemaking. Consistently, incorporation of ion channel expression differences into a biophysically-detailed atrial action potential model result in pacemaking and a sinus node-like action potential. Combining our quantitative proteomics data with computational modeling, we estimate ion channel copy numbers for sinus node myocytes. Our findings provide detailed insights into the unique molecular make-up of the cardiac pacemaker.

The sinus node generates rhythmic heartbeat but the molecular basis of pacemaking is still under debate. Here, the authors combine quantitative proteomics and single-nucleus transcriptomics to characterize the molecular composition of the sinus node and provide insights into the underpinnings of pacemaking.

Three-dimensional electron microscopy techniques for unravelling mitochondrial dysfunction in heart failure and identification of new pharmacological targets

A hallmark of heart failure is mitochondrial dysfunction leading to a bioenergetics imbalance in the myocardium. Consequently, there is much interest in targeting mitochondrial abnormalities to attenuate the pathogenesis of heart failure. This review discusses (i) how electron microscopy (EM) techniques have been fundamental for the current understanding of mitochondrial structure–function, (ii) the paradigm shift in resolutions now achievable by 3‐D EM techniques due to the introduction of direct detection devices and phase plate technology, and (iii) the application of EM for unravelling mitochondrial pathological remodelling in heart failure. We further consider the tremendous potential of multi‐scale EM techniques for the development of therapeutics, structure‐based ligand design and for delineating how a drug elicits nanostructural effects at the molecular, organelle and cellular levels. In conclusion, 3‐D EM techniques have entered a new era of structural biology and are poised to play a pivotal role in discovering new therapies targeting mitochondria for treating heart failure.

Linked Articles

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc

3D Ultrastructure of the “Arrhythmogenic” Purkinje Fibre-ventricular Junction in Rabbit Hearts

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Calmodulin regulates Cav3 T-type channels at their gating brake

Calcium (Cav1 and Cav2) and sodium channels possess homologous CaM-binding motifs, known as IQ motifs in their C termini, which associate with calmodulin (CaM), a universal calcium sensor. Cav3 T-type channels, which serve as pacemakers of the mammalian brain and heart, lack a C-terminal IQ motif. We illustrate that T-type channels associate with CaM using co-immunoprecipitation experiments and single particle cryo-electron microscopy. We demonstrate that protostome invertebrate (LCav3) and human Cav3.1, Cav3.2, and Cav3.3 T-type channels specifically associate with CaM at helix 2 of the gating brake in the I-II linker of the channels. Isothermal titration calorimetry results revealed that the gating brake and CaM bind each other with high-nanomolar affinity. We show that the gating brake assumes a helical conformation upon binding CaM, with associated conformational changes to both CaM lobes as indicated by amide chemical shifts of the amino acids of CaM in 1H-15N HSQC NMR spectra. Intact Ca2+-binding sites on CaM and an intact gating brake sequence (first 39 amino acids of the I-II linker) were required in Cav3.2 channels to prevent the runaway gating phenotype, a hyperpolarizing shift in voltage sensitivities and faster gating kinetics. We conclude that the presence of high-nanomolar affinity binding sites for CaM at its universal gating brake and its unique form of regulation via the tuning of the voltage range of activity could influence the participation of Cav3 T-type channels in heart and brain rhythms. Our findings may have implications for arrhythmia disorders arising from mutations in the gating brake or CaM.

A computational model of spatio-temporal cardiac intracellular calcium handling with realistic structure and spatial flux distribution from sarcoplasmic reticulum and t-tubule reconstructions

Intracellular calcium cycling is a vital component of cardiac excitation-contraction coupling. The key structures responsible for controlling calcium dynamics are the cell membrane (comprising the surface sarcolemma and transverse-tubules), the intracellular calcium store (the sarcoplasmic reticulum), and the co-localisation of these two structures to form dyads within which calcium-induced-calcium-release occurs. The organisation of these structures tightly controls intracellular calcium dynamics. In this study, we present a computational model of intracellular calcium cycling in three-dimensions (3-D), which incorporates high resolution reconstructions of these key regulatory structures, attained through imaging of tissue taken from the sheep left ventricle using serial block face scanning electron microscopy. An approach was developed to model the sarcoplasmic reticulum structure at the whole-cell scale, by reducing its full 3-D structure to a 3-D network of one-dimensional strands. The model reproduces intracellular calcium dynamics during control pacing and reveals the high-resolution 3-D spatial structure of calcium gradients and intracellular fluxes in both the cytoplasm and sarcoplasmic reticulum. We also demonstrated the capability of the model to reproduce potentially pro-arrhythmic dynamics under perturbed conditions, pertaining to calcium-transient alternans and spontaneous release events. Comparison with idealised cell models emphasised the importance of structure in determining calcium gradients and controlling the spatial dynamics associated with calcium-transient alternans, wherein the probabilistic nature of dyad activation and recruitment was constrained. The model was further used to highlight the criticality in calcium spark propagation in relation to inter-dyad distances. The model presented provides a powerful tool for future investigation of structure-function relationships underlying physiological and pathophysiological intracellular calcium handling phenomena at the whole-cell. The approach allows for the first time direct integration of high-resolution images of 3-D intracellular structures with models of calcium cycling, presenting the possibility to directly assess the functional impact of structural remodelling at the cellular scale.

The organisation of the membrane and sub-cellular structures of cells in the heart closely controls the coupling between its electrical and mechanical function. Computational models of the cellular calcium handling system, which is responsible for this electro-mechanical coupling, have been developed in recent years to study underlying structure-function relationships. Previous models have been largely idealised in structure; we present a new model which incorporates experimental data describing the high-resolution organisation of the primary structures involved in calcium dynamics. Significantly, the structure of the intracellular calcium store is modelled for the first time. The model is shown to reproduce calcium dynamics in control cells in both normal and abnormal conditions, demonstrating its suitability for future investigation of structure-function relationships. Thus, the model presented provides a powerful tool for the direct integration of experimentally acquired structural data in healthy and diseased cells and assessment of the role of structure in regulating normal and abnormal calcium dynamics.

Targeting miR-423-5p Reverses Exercise Training-Induced HCN4 Channel Remodeling and Sinus Bradycardia

Supplemental Digital Content is available in the text.

Rationale:

Downregulation of the pacemaking ion channel, HCN4 (hyperpolarization-activated cyclic nucleotide gated channel 4), and the corresponding ionic current, I_f, underlies exercise training–induced sinus bradycardia in rodents. If this occurs in humans, it could explain the increased incidence of bradyarrhythmias in veteran athletes, and it will be important to understand the underlying processes.

Objective:

To test the role of HCN4 in the training-induced bradycardia in human athletes and investigate the role of microRNAs (miRs) in the repression of HCN4.

Methods and Results:

As in rodents, the intrinsic heart rate was significantly lower in human athletes than in nonathletes, and in all subjects, the rate-lowering effect of the HCN selective blocker, ivabradine, was significantly correlated with the intrinsic heart rate, consistent with HCN repression in athletes. Next-generation sequencing and quantitative real-time reverse transcription polymerase chain reaction showed remodeling of miRs in the sinus node of swim-trained mice. Computational predictions highlighted a prominent role for miR-423-5p. Interaction between miR-423-5p and HCN4 was confirmed by a dose-dependent reduction in HCN4 3′-untranslated region luciferase reporter activity on cotransfection with precursor miR-423-5p (abolished by mutation of predicted recognition elements). Knockdown of miR-423-5p with anti-miR-423-5p reversed training-induced bradycardia via rescue of HCN4 and I_f. Further experiments showed that in the sinus node of swim-trained mice, upregulation of miR-423-5p (intronic miR) and its host gene, NSRP1, is driven by an upregulation of the transcription factor Nkx2.5.

Conclusions:

HCN remodeling likely occurs in human athletes, as well as in rodent models. miR-423-5p contributes to training-induced bradycardia by targeting HCN4. This work presents the first evidence of miR control of HCN4 and heart rate. miR-423-5p could be a therapeutic target for pathological sinus node dysfunction in veteran athletes.

Post-Myocardial Infarction T-tubules Form Enlarged Branched Structures With Dysregulation of Junctophilin-2 and Bridging Integrator 1 (BIN-1)

BACKGROUND

Heart failure is a common secondary complication following a myocardial infarction (MI), characterized by impaired cardiac contraction and t-tubule (t-t) loss. However, post-MI nano-scale morphological changes to the remaining t-ts are poorly understood.

METHOD AND RESULTS

We utilized a porcine model of MI, using a nonlethal microembolization method to generate controlled microinfarcts. Using serial block face scanning electron microscopy, we report that post-MI, after mild left-ventricular dysfunction has developed, t-ts are not only lost in the peri-infarct region, but also the remnant t-ts form enlarged, highly branched disordered structures, containing a dense intricate inner membrane. Biochemical and proteomics analyses showed that the calcium release channel, ryanodine receptor 2 (RyR2), abundance is unchanged, but junctophilin-2 (JP2), important for maintaining t-t trajectory, is depressed (-0.5×) in keeping with the t-ts being disorganized. However, immunolabeling shows that populations of RyR2 and JP2 remain associated with the remodeled t-ts. The bridging integrator 1 protein (BIN-1), a regulator of tubulogensis, is upregulated (+5.4×), consistent with an overdeveloped internal membrane system, a feature not present in control t-ts. Importantly, we have determined that t-ts, in the remote region, are narrowed and also contain dense membrane folds (BIN-1 is up-regulated +3.4×), whereas the t-ts have a radial organization comparable to control JP2 is upregulated +1.7×.

CONCLUSIONS

This study reveals previously unidentified remodeling of the t-t nano-architecture in the post-MI heart that extends to the remote region. Our findings highlight that targeting JP2 may be beneficial for preserving the orientation of the t-ts, attenuating the development of hypocontractility post-MI.

Three-dimensional structure of the intercalated disc reveals plicate domain and gap junction remodeling in heart failure

The intercalated disc (ICD) orchestrates electrochemical and mechanical communication between neighboring cardiac myocytes, properties that are perturbed in heart failure (HF). Although structural data from transmission electron microscopy two-dimensional images have provided valuable insights into the domains forming the ICD, there are currently no three-dimensional (3D) reconstructions for an entire ICD in healthy or diseased hearts. Here, we aimed to understand the link between changes in protein expression in an ovine tachypacing-induced HF model and ultrastructural remodeling of the ICD by determining the 3D intercalated disc architecture using serial block face scanning electron microscopy. In the failing myocardium there is no change to the number of ICDs within the left ventricle, but there is an almost doubling of the number of discs with a surface area of <1.0 × 10(8)μm(2) in comparison to control. The 3D reconstructions further revealed that there is remodeling of the plicate domains and gap junctions with vacuole formation around and between the contributing membranes that form the ICDs in HF. Biochemical analysis revealed upregulation of proteins involved in stabilizing the adhesive and mechanical properties consistent with the morphological changes. Our studies here have shown that in tachypacing-induced HF mechanical stresses are associated with both structural and molecular alterations. To our knowledge, these data together provide novel, to our knowledge, insights as to how remodeling at the molecular and structural levels leads to impaired intercellular communication.

Targeting caveolin-3 for the treatment of diabetic cardiomyopathy

Diabetes is a global health problem with more than 550 million people predicted to be diabetic by 2030. A major complication of diabetes is cardiovascular disease, which accounts for over two-thirds of mortality and morbidity in diabetic patients. This increased risk has led to the definition of a diabetic cardiomyopathy phenotype characterised by early left ventricular dysfunction with normal ejection fraction. Here we review the aetiology of diabetic cardiomyopathy and explore the involvement of the protein caveolin-3 (Cav3). Cav3 forms part of a complex mechanism regulating insulin signalling and glucose uptake, processes that are impaired in diabetes. Further, Cav3 is key for stabilisation and trafficking of cardiac ion channels to the plasma membrane and so contributes to the cardiac action potential shape and duration. In addition, Cav3 has direct and indirect interactions with proteins involved in excitation-contraction coupling and so has the potential to influence cardiac contractility. Significantly, both impaired contractility and rhythm disturbances are hallmarks of diabetic cardiomyopathy. We review here how changes to Cav3 expression levels and altered relationships with interacting partners may be contributory factors to several of the pathological features identified in diabetic cardiomyopathy. Finally, the review concludes by considering ways in which levels of Cav3 may be manipulated in order to develop novel therapeutic approaches for treating diabetic cardiomyopathy.

Dependence of cardiac transverse tubules on the BAR domain protein amphiphysin II (BIN-1)

RATIONALE

Transverse tubules (t-tubules) regulate cardiac excitation-contraction coupling and exhibit interchamber and interspecies differences in expression. In cardiac disease, t-tubule loss occurs and affects the systolic calcium transient. However, the mechanisms controlling t-tubule maintenance and whether these factors differ between species, cardiac chambers, and in a disease setting remain unclear.

OBJECTIVE

To determine the role of the Bin/Amphiphysin/Rvs domain protein amphiphysin II (AmpII) in regulating t-tubule maintenance and the systolic calcium transient.

METHODS AND RESULTS

T-tubule density was assessed by di-4-ANEPPS, FM4-64 or WGA staining using confocal microscopy. In rat, ferret, and sheep hearts t-tubule density and AmpII protein levels were lower in the atrium than in the ventricle. Heart failure (HF) was induced in sheep using right ventricular tachypacing and ferrets by ascending aortic coarctation. In both HF models, AmpII protein and t-tubule density were decreased in the ventricles. In the sheep, atrial t-tubules were also lost in HF and AmpII levels decreased. Conversely, junctophilin 2 levels did not show interchamber differences in the rat and ferret nor did they change in HF in the sheep or ferret. In addition, in rat atrial and sheep HF atrial cells where t-tubules were absent, junctophilin 2 had sarcomeric intracellular distribution. Small interfering RNA-induced knockdown of AmpII protein reduced t-tubule density, calcium transient amplitude, and the synchrony of the systolic calcium transient.

CONCLUSIONS

AmpII is intricately involved in t-tubule maintenance. Reducing AmpII protein decreases t-tubule density, reduces the amplitude, and increases the heterogeneity of the systolic calcium transient.

Serial block face scanning electron microscopy for the study of cardiac muscle ultrastructure at nanoscale resolutions

Electron microscopy techniques have made a significant contribution towards understanding muscle physiology since the 1950s. Subsequent advances in hardware and software have led to major breakthroughs in terms of image resolution as well as the ability to generate three-dimensional (3D) data essential for linking structure to function and dysfunction. In this methodological review we consider the application of a relatively new technique, serial block face scanning electron microscopy (SBF-SEM), for the study of cardiac muscle morphology. Employing SBF-SEM we have generated 3D data for cardiac myocytes within the myocardium with a voxel size of ~15 nm in the X-Y plane and 50 nm in the Z-direction. We describe how SBF-SEM can be used in conjunction with selective staining techniques to reveal the 3D cellular organisation and the relationship between the t-tubule (t-t) and sarcoplasmic reticulum (SR) networks. These methods describe how SBF-SEM can be used to provide qualitative data to investigate the organisation of the dyad, a specialised calcium microdomain formed between the t-ts and the junctional portion of the SR (jSR). We further describe how image analysis methods may be applied to interrogate the 3D volumes to provide quantitative data such as the volume of the cell occupied by the t-t and SR membranes and the volumes and surface area of jSR patches. We consider the strengths and weaknesses of the SBF-SEM technique, pitfalls in sample preparation together with tips and methods for image analysis. By providing a 'big picture' view at high resolutions, in comparison to conventional confocal microscopy, SBF-SEM represents a paradigm shift for imaging cellular networks in their native environment.

Three-Dimensional Reconstruction of Cardiac Sarcoplasmic Reticulum Reveals a Continuous Network Linking Transverse-Tubules

Rationale:

The organization of the transverse-tubular (t-t) system and relationship to the sarcoplasmic reticulum (SR) underpins cardiac excitation–contraction coupling. The architecture of the SR, and relationship with the t-ts, is not well characterized at the whole-cell level. Furthermore, little is known regarding changes to SR ultrastructure in heart failure.

Objective:

The aim of this study was to unravel interspecies differences and commonalities between the relationship of SR and t-t networks within cardiac myocytes, as well as the modifications that occur in heart failure, using a novel high-resolution 3-dimensional (3D) imaging technique.

Methods and Results:

Using serial block face imaging coupled with scanning electron microscopy and image analysis, we have generated 3D reconstructions of whole cardiomyocytes from sheep and rat left ventricle, revealing that the SR forms a continuous network linking t-ts throughout the cell in both species. In sheep, but not rat, the SR has an intimate relationship with the sarcolemma forming junctional domains. 3D reconstructions also reveal details of the sheep t-t system. Using a model of tachypacing-induced heart failure, we show that there are populations of swollen and collapsed t-ts, patches of SR tangling, and disorder with rearrangement of the mitochondria.

Conclusions:

We provide the first high-resolution 3D structure of the SR network showing that it forms a cell-wide communication pipeline facilitating Ca 2+ diffusion, buffering, and synchronicity. The distribution of the SR within the cell is related to interspecies differences in excitation–contraction coupling, and we report the first detailed analysis of SR remodeling as a result of heart failure.

Characterization of the molecular architecture of human caveolin-3 and interaction with the skeletal muscle ryanodine receptor

Caveolin-3 (cav-3), an integral membrane protein, is a building block of caveolae as well as a regulator of a number of physiological processes by facilitating the formation of multiprotein signaling complexes. We report that the expression of cav-3 in insect (Sf9) cells induces caveola formation, comparable in size with those observed in native tissue. We have also purified the recombinant cav-3 determining that it forms an oligomer of ∼220 kDa. We present the first three-dimensional structure for cav-3 (using transmission electron microscopy and single particle analysis methods) and show that nine cav-3 monomers assemble to form a complex that is toroidal in shape, ∼16.5 nm in diameter and ∼ 5.5 nm in height. Labeling experiments and reconstitution of the purified cav-3 into liposomes have allowed a proposal for the orientation of the protein with respect to the membrane. We have identified multiple caveolin-binding motifs within the ryanodine receptor (RyR1) sequence employing a bioinformatic analysis. We have then shown experimentally that there is a direct interaction between recombinant cav-3 nonamers and purified RyR1 homotetramers that would imply that at least one of the predicted cav-3-binding sites is exposed within the fully assembled RyR1 structure. The cav-3 three-dimensional model provides new insights as to how a cav-3 oligomer can bind multiple partners in close proximity to form signaling complexes. Furthermore, a direct interaction with RyR1 suggests a possible role for cav-3 as a modifier of muscle excitation-contraction coupling and/or for localization of the receptor to regions of the sarcoplasmic reticulum.

Labelling of the 3D structure of the cardiac L-type voltage-gated calcium channel

Voltage-gated calcium channels (VGCCs) regulate calcium influx into all excitable cells. In the heart, the main calcium channels are the L-type VGCCs (LTCCs). These are localised to the sarcolemmal membrane, and are hetero-oligomeric complexes comprised of three non-covalently associated polypeptides; alpha1 (CaV1.2), alpha2delta and beta. We recently reported the 3D structure for a monomeric form of the cardiac LTCC1 using electron microscopy and single particle analysis. We also determined the first medium/low resolution structure of a T-type voltage gated calcium channel (CaV3.1) polypeptide. We identified the transmembrane and cytoplasmic domains of the T-type channel using labelling studies to determine the position of the C-terminus. By modelling of the CaV3.1 structure (comparable at these resolutions to CaV1.2) into the cardiac LTCC volume, we were able to delineate the subunit boundaries of the cardiac LTCC, leading to a proposal for a putative orientation of the LTCC with respect to the membrane bilayer. We have now extended these studies to include labelling of the extracellular alpha2 polypeptide using affinity purified antibodies raised against the Von Willebrand Factor A (VWA) domain and calmodulin-gold labelling of the C-terminus of CaV1.2. These data provide further support for the proposed orientation of the 3D structure of the cardiac LTCC.

Three-dimensional structure of CaV3.1: comparison with the cardiac L-type voltage-gated calcium channel monomer architecture

Calcium entry through voltage-gated calcium channels has widespread cellular effects upon a host of physiological processes including neuronal excitability, muscle excitation-contraction coupling, and secretion. Using single particle analysis methods, we have determined the first three-dimensional structure, at 23 A resolution, for a member of the low voltage-activated voltage-gated calcium channel family, CaV3.1, a T-type channel. CaV3.1 has dimensions of approximately 115x85x95 A, composed of two distinct segments. The cytoplasmic densities form a vestibule below the transmembrane domain with the C terminus, unambiguously identified by the presence of a His tag being approximately 65 A long and curling around the base of the structure. The cytoplasmic assembly has a large exposed surface area that may serve as a signaling hub with the C terminus acting as a "fishing rod" to bind regulatory proteins. We have also determined a three-dimensional structure, at a resolution of 25 A, for the monomeric form of the cardiac L-type voltage-gated calcium (high voltage-activated) channel with accessory proteins beta and alpha2delta bound to the ion channel polypeptide CaV1.2. Comparison with the skeletal muscle isoform finds a good match particularly with respect to the conformation, size, and shape of the domain identified as that formed by alpha2. Furthermore, modeling of the CaV3.1 structure (analogous to CaV1.2 at these resolutions) into the heteromeric L-type voltage-gated calcium channel complex volume reveals multiple interaction sites for beta-CaV1.2 binding and for the first time identifies the size and organization of the alpha2delta polypeptides.

Ca(2+)-calmodulin can activate and inactivate cardiac ryanodine receptors

BACKGROUND AND PURPOSE

Ca(2+)-calmodulin (Ca(2+)CaM) is widely accepted as an inhibitor of cardiac ryanodine receptors (RyR2); however, the effects of physiologically relevant CaM concentrations have not been fully investigated.

EXPERIMENTAL APPROACH

We investigated the effects of low concentrations of Ca(2+)CaM (50-100 nmol.L(-1)) on the gating of native sheep RyR2, reconstituted into bilayers. Suramin displaces CaM from RyR2 and we have used a gel-shift assay to provide evidence of the mechanism underlying this effect. Finally, using suramin to displace endogenous CaM from RyR2 in permeabilized cardiac cells, we have investigated the effects of 50 nmol.L(-1) CaM on sarcoplasmic reticulum (SR) Ca(2+)-release.

KEY RESULTS

Ca(2+)CaM activated or inhibited single RyR2, but activation was much more likely at low (50-100 nmol.L(-1)) concentrations. Also, suramin displaced CaM from a peptide of the CaM binding domain of RyR2, indicating that, like the skeletal isoform (RyR1), suramin directly competes with CaM for its binding site on the channel. Pre-treatment of rat permeabilized ventricular myocytes with suramin to displace CaM, followed by addition of 50 nmol x L(-1) CaM to the mock cytoplasmic solution caused an increase in the frequency of spontaneous Ca(2+)-release events. Application of caffeine demonstrated that 50 nmol x L(-1) CaM reduced SR Ca(2+) content.

CONCLUSIONS AND IMPLICATIONS

We describe for the first time how Ca(2+)CaM is capable, not only of inactivating, but also of activating RyR2 channels in bilayers in a CaM kinase II-independent manner. Similarly, in cardiac cells, CaM stimulates SR Ca(2+)-release and the use of caffeine suggests that this is a RyR2-mediated effect.

Small-angle X-ray scattering and NMR studies of the conformation of the PDZ region of SAP97 and its interactions with Kir2.1

The functional localization of potassium inward rectifiers is regulated by SAP97, a PDZ membrane-associated guanylate kinase protein. We describe here an investigation of the conformation of the PDZ domain region of SAP97 PDZ1-3. The NMR and SAXS data reveal conformational dynamics. The NMR data show minimal interdomain contacts, with the U3 linker region between PDZ2 and PDZ3 being largely unstructured. Shape analysis of the SAXS profiles revealed a dumbbell for the PDZ12 double domain. An overall elongated, asymmetric shape comprised of two to three distinct components characterizes the triple domain PDZ1-3. In addition, rigid body modeling shows that the representative average shape does not provide the full picture and that the data for the triple domain are consistent with large variations, suggesting significant conformational flexibility. However, the dynamics appears to be restricted as PDZ3 is located essentially within approximately 40 A from PDZ12. We also show that the Kir2.1 cytoplasmic domain interacts with all three PDZ domains but with a clear preference for PDZ2 even in the presence of the U3 region. We speculate that the restricted dynamics and preferential Kir2.1 binding to PDZ2 are features that enable SAP97 to function as a scaffold protein, allowing other proteins each to bind to the other two PDZ domains in sufficient proximity to yield productive channelosomes.

Solid-state NMR and functional measurements indicate that the conserved tyrosine residues of sarcolipin are involved directly in the inhibition of SERCA1

The transmembrane protein sarcolipin regulates calcium storage in the sarcoplasmic reticulum of skeletal and cardiac muscle cells by modulating the activity of sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs). The highly conserved C-terminal region ((27)RSYQY-COOH) of sarcolipin helps to target the protein to the sarcoplasmic reticulum membrane and may also participate in the regulatory interaction between sarcolipin and SERCA. Here we used solid-state NMR measurements of local protein dynamics to illuminate the direct interaction between the Tyr(29) and Tyr(31) side groups of sarcolipin and skeletal muscle Ca(2+)-ATPase (SERCA1a) embedded in dioleoylphosphatidylcholine bilayers. Further solid-state NMR experiments together with functional measurements on SERCA1a in the presence of NAc-RSYQY, a peptide representing the conserved region of sarcolipin, suggest that the peptide binds to the same site as the parent protein at the luminal face of SERCA1a, where it reduces V(max) for calcium transport and inhibits ATP hydrolysis with an IC(50) of approximately 200 microM. The inhibitory effect of NAc-RSYQY is remarkably sequence-specific, with the native aromatic residues being essential for optimal inhibitory activity. This combination of physical and functional measurements highlights the importance of aromatic and polar residues in the C-terminal region of sarcolipin for regulating calcium cycling and muscle contractility.

Apo calmodulin binding to the L-type voltage-gated calcium channel Cav1.2 IQ peptide

The influx of calcium through the L-type voltage-gated calcium channels (LTCCs) is the trigger for the process of calcium-induced calcium release (CICR) from the sarcoplasmic reticulum, an essential step for cardiac contraction. There are two feedback mechanisms that regulate LTCC activity: calcium-dependent inactivation (CDI) and calcium-dependent facilitation (CDF), both of which are mediated by calmodulin (CaM) binding. The IQ domain (aa 1645-1668) housed within the cytoplasmic domain of the LTCC Cav1.2 subunit has been shown to bind both calcium-loaded (Ca2+CaM ) and calcium-free CaM (apoCaM). Here, we provide new data for the structural basis for the interaction of apoCaM with the IQ peptide using NMR, revealing that the apoCaM C-lobe residues are most significantly perturbed upon complex formation. In addition, we have employed transmission electron microscopy of purified LTCC complexes which shows that both apoCaM and Ca2+CaM can bind to the intact channel.

Three-Dimensional Structure Determination of a Protein Supercomplex That Oxidizes Methane to Formaldehyde in Methylococcus capsulatus (Bath)

The oxidation of methane to methanol in methanotrophs is catalyzed by the enzyme methane monooxygenase (MMO). Two distinct forms of this enzyme exist, a soluble cytoplasmic MMO (sMMO) and a membrane-bound particulate form (pMMO). The active protein complex termed pMMO-C was purified recently from Methylococcus capsulatus (Bath). The complex consists of pMMO hydroxylase and an additional component pMMO-R, which was proposed to be the reductase for the pMMO complex. Further study of this complex has led here to the proposal that the pMMO-R is in fact methanol dehydrogenase, the subsequent enzyme in the methane oxidation pathway by methanotrophs. We describe here the biochemical and biophysical characterization of a stable purified complex of pMMO hydroxylase (pMMO-H) with methanol dehydrogenase (MDH) and report the first three-dimensional (3D) structure, determined by cryoelectron microscopy and single particle analysis to approximately 16 A resolution. The 3D structure reported here provides the first insights into the supramolecular organization of pMMO with MDH. These studies of pMMO-MDH complexes have provided further understanding of the structural basis for the particular functions of the enzymes in this system which might also be of relevance to the complete process of methane oxidation by methanotrophs under high copper concentration in the environment.

Studies of the aggregation of an amyloidogenic alpha-synuclein peptide fragment

The deposition of alpha-syn (alpha-synuclein) fibrils in Lewy bodies is a characteristic feature of individuals with neurodegenerative disorders. A peptide comprising the central residues 71-82 of alpha-syn [alpha-syn(71-82)] is capable of forming beta-sheet-rich, amyloid-like fibrils with similar morphologies to fibrils of the full-length protein, providing a useful model of pathogenic alpha-syn fibrils that is suitable for detailed structural analysis. We have studied the morphology and gross structural features of alpha-syn(71-82) fibrils formed under different conditions in order to obtain reliable conditions for producing fibrils for further structural investigations. The results indicate that the rate of aggregation and the morphology of the fibrils formed are sensitive to pH and temperature.

Applications of electron cryo-microscopy to cardiovascular research

Electron cryo-microscopy (cryo-EM) techniques have wide applications for the study of biological structures. The focus of this chapter is the use of cryo-EM and associated methods for the analysis of the three-dimensional (3D) structure of proteins and multicomponent macromolecular assemblies. Data evolving from these methods pertaining to the quaternary organization of proteins and protein-protein interactions, bridges an important gap between linear genomic information and understanding physiological function. This chapter provides general methods for examining two-dimensional crystalline arrays of proteins as well as single, randomly orientated protein complexes. It is significant that single-particle analysis of electron microscopy images has provided the only 3D data to-date for the two principal components of muscle excitation contraction coupling in the heart, namely the L-type voltage-gated calcium channel and the ryanodine receptor. This chapter describes approaches for identifying the extracellular and intracellular domains of the 3D structure of the L-type voltage-gated calcium channel and also incorporates general details for labeling and visualizing His-tagged proteins.

Characterization and structural analysis of an active particulate methane monooxygenase trimer from Methylococcus capsulatus (Bath)

The oxidation of methane to methanol in methanotrophs is catalyzed by the enzyme methane monooxygenase (MMO). Two distinct forms of this enzyme exist, a soluble cytoplasmic MMO (sMMO) and a membrane-bound particulate form (pMMO). We describe here the biochemical characterization of a stable and active purified pMMO hydroxylase (pMMO-H) and report a three-dimensional (3D) structure, determined by electron microscopy and single-particle analysis at 23 A resolution. Both biochemical and structural data indicate that pMMO hydroxylase is trimeric, with each monomer unit comprised of three polypeptides of 47, 26, and 23 kDa. Comparison of the recent crystal structure [Lieberman, R. L., and Rosenzweig, A. C. (2005) Nature 434, 177] of an uncharacterized pMMO-H complex with the three-dimensional (3D) structure determined here yielded a good match between the principal features and the organization of the enzyme monomers into trimers. The data presented here advance our current understanding of particulate methane monooxygenase function by the characterization of an active form of the enzyme and the corresponding 3D structure.

Structure of the Neisseria meningitidis Outer Membrane PilQ Secretin Complex at 12 Å Resolution

The bacterial pathogen Neisseria meningitidis expresses long, thin, retractile fibers (called type IV pili) from its cell surface and uses these adhesive structures to mediate primary attachment to epithelial cells during host colonization and invasion. PilQ is an outer membrane protein complex that is essential for the translocation of these pili across the outer membrane. Here, we present the structure of the PilQ complex determined by cryoelectron microscopy to 12 A resolution. The dominant feature of the structure is a large central cavity, formed by four arm features that spiral upwards from a squared ring base and meet to form a prominent cap region. The cavity, running through the center of the complex, is continuous and is effectively sealed at both the top and bottom. Analysis of the complex using self-orientation and by examination of two-dimensional crystals indicates a strong C4 rotational symmetry, with a much weaker C12 rotational symmetry, consistent with PilQ possessing true C4 symmetry with C12 quasi-symmetry. We therefore suggest that the complex is a homododecamer, formed by association of 12 PilQ polypeptide chains into a tetramer of trimers. The structure of the PilQ complex, with its large and well defined central chamber, suggests that it may not function solely as a passive portal in the outer membrane, but could be actively involved in mediating pilus assembly or modification.

L-type voltage-gated calcium channels: understanding function through structure

L-type voltage-gated calcium channels (VGCCs) are multisubunit membrane proteins that regulate calcium influx into excitable cells. Within the last two years there have been four separate reports describing the structure of the skeletal muscle VGCC determined by electron microscopy and single particle analysis methods. There are some discrepancies between the structures, as well as reports for both monomeric and dimeric forms of the channel. This article considers each of the VGCC structures in terms of similarities and differences with an emphasis upon translation of data into a biological context.

The Three-dimensional Structure of the Cardiac L-type Voltage-gated Calcium Channel

We describe here the first three-dimensional structure of the cardiac L-type voltage-gated calcium channel (VGCC) purified from bovine heart. The structure was determined by electron microscopy and single particle analysis of negatively stained complexes, using the angular reconstitution method. The cardiac VGCC can be isolated as a stable dimer, as reported previously for the skeletal muscle VGCC, with a central aqueous chamber formed by the two halves of the complex. Moreover, we demonstrate that the dimeric cardiac VGCC binds the dihydropyridine [3H]azidopine with a Kd approximately 310 pM. We have compared the cardiac VGCC structure with the skeletal muscle form, determined using the same reconstructive methodology, allowing us to identify common and distinct features of the complexes. By using antibody and lectin-gold labeling, we have localized the intracellular beta polypeptides and the extracellular glycosylation sites of the skeletal muscle VGCC, which can be correlated to the cardiac three-dimensional structure. From the data presented here the assignment of the orientation of the VGCC complexes with respect to the lipid bilayer is now possible. A difference between the cardiac and skeletal muscle ion channels is apparent in the putative transmembrane region, which would be consistent with the absence of the gamma subunit in the cardiac VGCC assembly.

Three-dimensional structure of the Neisseria meningitidis secretin PilQ determined from negative-stain transmission electron microscopy

The PilQ secretin from the pathogenic bacterium Neisseria meningitidis is an integral outer membrane protein complex which plays a crucial role in the biogenesis of type IV pili. We present here the first three-dimensional structure of this type of secretin at 2.5-nm resolution, obtained by single-particle averaging methods applied to the purified protein complex visualized in a negative stain. In projection, the PilQ complex is circular, with a donut-like appearance. When viewed from the side it has a rounded, conical profile. The complex was demonstrated to have 12-fold rotational symmetry, and this property was used to improve the quality of the density map by symmetry averaging. The dominant feature of the structure is a cavity, 10 nm deep, within the center of the molecule. The cavity is funnel-shaped in cross section, measures 6.5 nm in diameter at the top of the complex, and tapers to a closed point, effectively blocking formation of a continuous pore through the PilQ complex. These results suggest that the complex would have to undergo a conformational change in order to accommodate an assembled pilus fiber of diameter 6.5 nm running through the outer membrane.

3D structure of the skeletal muscle dihydropyridine receptor

The dihydropyridine receptors (DHPR) are L-type voltage-gated calcium channels that regulate the flux of calcium ions across the cell membrane. Here we present the three-dimensional (3D) structure at approximately 27A resolution of purified skeletal muscle DHPR, as determined by electron microscopy and single particle analysis. Here both biochemical and 3D structural data indicate that DHPR is dimeric. DHPR dimers are composed of two arch-shaped monomers approximately 210A across and approximately 75A thick, that interact very tightly at each end of the arch. The roughly toroidal structure of the two monomers encloses a cylindrical space of approximately 80A diameter, which is then closed on each side by two dome-shaped protein densities reaching over from each monomer arch. The dome-shaped domains have a length of approximately 80-90A and a maximum height of approximately 45A. Small orifices punctuate their exterior surface. The 3D structure disclosed here may have important implications for the understanding of DHPR Ca(2+) channel function. We also propose a model for its in vivo interactions with the calcium release channel at the junctional sarcoplasmic recticulum.

The location of plastocyanin in vascular plant photosystem I

We have studied the binding sites of the electron donor and acceptor proteins of vascular plant photosystem I by electron microscopy/crystallography. Previously, we identified the binding site for the electron acceptor (ferredoxin). In this paper we complete these studies with the characterization of the electron donor (plastocyanin) binding site. After cross-linking, plastocyanin is detected using Fourier difference analysis of two dimensionally ordered arrays of photosystem I located at the periphery of chloroplast grana. Plastocyanin binds in a small cavity on the lumenal surface of photosystem I, close to the center and with a slight bias toward the PsaL subunit of the complex. The recent release of the full coordinates for the cyanobacterial photosystem I reaction center has allowed a detailed comparison between the structures of the eukaryotic and prokaryotic systems. This reveals a very close homology, which is particularly striking for the lumenal side of photosystem I.

The location of the mobile electron carrier ferredoxin in vascular plant photosystem I

In this study, we present the location of the ferredoxin-binding site in photosystem I from spinach. Image analysis of negatively stained two-dimensional crystals indicates that the addition of ferredoxin and chemical cross-linkers do not significantly alter the unit cell parameters (for untreated photosystem I, a = 26.4 nm, b = 27.6 nm, and gamma = 90 degrees, space group p22(1)2(1) and for ferredoxin cross-linked photosystem I, a = 26.2 nm, b = 27.2 nm, and gamma = 90 degrees, space group p22(1)2(1)). Fourier difference analysis reveals that ferredoxin is bound on top of the stromal ridge principally interacting with the extrinsic subunits PsaC and PsaE. This location would be accessible to the stroma, thereby promoting efficient electron transfer away from photosystem I. This observation is significantly different from that of the ferredoxin binding site proposed for cyanobacteria. A model for the binding of ferredoxin in vascular plants is proposed and is discussed relative to observations in cyanobacteria.

A fragment of recombinant GABA(A) receptor alpha1 subunit forming rosette-like homo-oligomers

The type A gamma-aminobutyric acid (GABA(A)) receptor plays a major role in inhibitory synaptic transmission in the central nervous system. A fragment consisting of residues Cys166 to Leu296 of the alpha1 subunit of the GABA(A) receptor was overexpressed in Escherichia coli and was found to have stable beta-rich structures. Here, results from laser scattering, gel electrophoresis and electron microscopy demonstrated that this recombinant protein formed rosette-like homo-oligomers, mainly pentamers in solution. Therefore, the fragment apparently provides a valuable model system for studying the pentameric holoreceptor assembly. Non-reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis of the fragment showed that disulfide bonds formed between monomers contributed to the oligomerization of the fragment. The fact that this fragment alone could form pentamers in vitro strongly suggests that amino acid residues located within the Cys166-Leu296 region of the alpha1 subunit may contribute to the oligomerization of GABA(A) receptor in vivo.

Three-dimensional structure of higher plant photosystem I determined by electron crystallography

We describe the three-dimensional structure of higher plant photosystem I (PSI) as obtained by electron microscopy of two-dimensional crystals formed at the grana margins of thylakoid membranes. The negatively stained crystalline areas displayed unit cell dimensions a = 26.6 nm, b = 27.7 nm, and gamma = 90(o), and p22121 plane group symmetry consisting of two monomers facing upward and two monomers facing downward with respect to the membrane plane. Higher plant PSI shows several structural similarities to the cyanobacterial PSI complex, with a prominent ridge on the stromal side of the complex. The stromal ridge is resolved into at least three separate domains that are interpreted as representing the three well characterized stromal subunits, the psa C, D, and E gene products. The lumenal surface is relatively flat but exhibits a distinct central depression that may be the binding site for plastocyanin. Higher plant PSI is of dimensions 15-16 x 11-12.5 nm, and thus leaves a larger footprint in the membrane than its cyanobacterial equivalent (13 x 10.5 nm). It is expected that additional membrane-bound polypeptides will be present in the higher plant PSI. Both higher plant and cyanobacterial complexes span about 8-9 nm in the direction orthogonal to the membrane. This report represents the first three-dimensional structure for the higher plant PSI complex.

Two-dimensional crystals of photosystem I in higher plant grana margins

In this report, we present new structural data on the size, shape, and oligomeric form of higher plant photosystem I (PSI) formed within the thylakoid grana margins. We show that PSI complexes can be assembled into ordered molecular monolayers (two-dimensional crystals) using thylakoid membranes from a variety of higher plant sources. Digital image analysis of negatively stained two-dimensional crystals (a = 26.9 nm, b = 28.0 nm, gamma = 90 degrees, p22121 plane group) resulted in a projection map consisting of 4 monomers/unit cell. Higher plant PSI is slightly larger than its cyanobacterial equivalent but shows many similar features. Structural changes after urea and salt washing of the crystals supported the biochemical characterization and were mainly assigned to the stromal side of the complex where the psaC, psaD, and psaE gene products are known to be bound. Labeling with ferredoxin-colloidal gold complexes provided direct evidence for a segregated PSI population, with 5 nm diameter ferredoxin-gold particles enriched in the thylakoid grana margins and the two-dimensional crystals. This lateral segregation of photosynthetic complexes is important for the understanding of the kinetics of electron transfer between photosystem II and PSI in higher plants.

Arthrobacter D-xylose isomerase: partial proteolysis with thermolysin

The pattern and kinetics of partial proteolysis of Arthrobacter D-xylose isomerase tetramer was studied in order to determine the flexibility of surface loops that may control its stability. It was completely resistant to trypsin, chymotrypsin and elastase at 37 degrees C, but thermolysin cleaved specifically and quantitatively at Thr-347-Leu-348 between helices 10 and 11 to remove 47 residues from the C-terminus of each 43.3 kDa subunit. At high temperatures, helices 9 and 10 were removed from each 38 kDa subunit to give a 36 kDa tetramer. The kinetics of nicking by thermolysin indicated that the Thr-347-Leu-348 loop is locked at low temperatures, but 'melts' at 25 degrees C and is fully flexible above 34 degrees C. The flexibility appears to be associated with binding of Ca2+ ions at the active site, since Co2+, Mg2+ and xylitol protect in proportion to their ability to displace Ca2+. The missing C-terminal helices make many intersubunit contacts that appear in the structure to stabilize the tetramer, but the properties of the purified nicked proteins are almost indistinguishable from the native enzyme. Both the 38 kDa tetramer and the 36 kDa tetramer are identically active and dissociate similarly in urea or SDS to fully active dimers, but the nicked dimers are slightly less stable to urea at 62 degrees C. In the Mg2+ form the thermostability of the 38 kDa tetramer is identical with that of the native enzyme, but the 36 kDa tetramer has a slightly lower 'melting point' (70 degrees C versus 80 degrees C), which may be due to unravelling from the end of helix 8. Since elimination of all the C-terminal helices and many intersubunit contacts has so little effect, one can conclude that the 'weak point' that controls the protein's thermostability lies within the N-terminal beta-barrel domain.