Ca2+/CaM dependent protein kinase II (CaMKII)α and CaMKIIβ hub domains adopt distinct oligomeric states and stabilities

Ca2+ /calmodulin-dependent protein kinase II (CaMKII) is a multidomain serine/threonine kinase that plays important roles in the brain, heart, muscle tissue, and eggs/sperm. The N-terminal kinase and regulatory domain is connected by a flexible linker to the C-terminal hub domain. The hub domain drives the oligomeric organization of CaMKII, assembling the kinase domains into high local concentration. Previous structural studies have shown multiple stoichiometries of the holoenzyme as well as the hub domain alone. Here, we report a comprehensive study of the hub domain stoichiometry and stability in solution. We solved two crystal structures of the CaMKIIβ hub domain that show 14-mer (3.1 Å) and 16-mer (3.4 Å) assemblies. Both crystal structures were determined from crystals grown in the same drop, which suggests that CaMKII oligomers with different stoichiometries likely coexist. To further interrogate hub stability, we employed mass photometry and temperature denaturation studies of CaMKIIβ and CaMKIIα hubs, which highlight major differences between these highly similar domains. We created a dimeric CaMKIIβ hub unit using rational mutagenesis, which is significantly less stable than the oligomer. Both hub domains populate an intermediate during unfolding. We found that multiple CaMKIIβ hub stoichiometries are present in solution and that larger oligomers are more stable. CaMKIIα had a narrower distribution of molecular weight and was distinctly more stable than CaMKIIβ.

A domain-swapped CaMKII conformation facilitates linker-mediated allosteric regulation

Ca2+ signaling plays a key role in physiological processes such as memory formation and cardiac function. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is the primary kinase that responds to Ca2+ inputs in these cells. There are four CaMKII paralogs in mammals which are alternatively spliced in the variable linker region to create upwards of 70 different variants. In this study, we systematically studied different linker regions and determined that the position of charged residues within the linker region modulates the Ca2+/CaM sensitivity of the holoenzyme. We present an X-ray crystal structure of full-length CaMKIIδ that shows a domain-swapped conformation of the subunits within the dodecameric holoenzyme. In this structure, the kinase domain of one subunit is docked onto the hub domain of a different subunit, providing an additional interface within the holoenzyme. Mutations at the equatorial and lateral interfaces revealed that the kinase-hub interaction dissociates as the hub-hub interfaces are disturbed, which led alterations in the stoichiometry of CaMKII holoenzyme and Ca2+/CaM sensitivity. Molecular dynamics simulations of linker-containing domain-swapped and non-domain-swapped CaMKIIs reveal that the domain-swapped configuration facilitates an interaction between the calmodulin binding domain and the variable linker region, such that dynamic electrostatic forces between charges on these segments can modulate the equilibrium between the compact and extended conformational states of the holoenzyme. Small angle X-ray scattering data confirms that a negatively charged linker CaMKII holoenzyme adopts a more compact conformation compared to a positively charged linker. These data support a model where patches of charged linker residues interact with the calmodulin binding domain to allosterically regulate sensitivity to Ca2+/CaM. Our findings provide a new framework for understanding CaMKII structure and allosteric regulation by the variable linker region in Ca2+-sensitive cells.

Feed-forward stimulation of CAMK2 by the oncogenic pseudokinase PEAK1 generates a therapeutically 'actionable' signalling axis in triple negative breast cancer

The PEAK family of pseudokinases, comprising PEAK1-3, are signalling scaffolds that play oncogenic roles in several poor prognosis human cancers, including triple negative breast cancer (TNBC). However, therapeutic targeting of pseudokinases is challenging due to their lack of catalytic activity. To address this, we screened for PEAK1 effectors by affinity purification and mass spectrometry, identifying calcium/calmodulin-dependent protein kinase 2 (CAMK2)D and CAMK2G. PEAK1 promoted CAMK2D/G activation in TNBC cells via a novel feed-forward mechanism involving PEAK1/PLCγ1/Ca 2+ signalling and direct binding via a consensus CAMK2 interaction motif in the PEAK1 N-terminus. In turn, CAMK2 phosphorylated PEAK1 to enhance association with PEAK2, which is critical for PEAK1 oncogenic signalling. To achieve pharmacologic targeting of PEAK1/CAMK2, we repurposed RA306, a second generation CAMK2 inhibitor under pre-clinical development for treatment of cardiovascular disease. RA306 demonstrated on-target activity against CAMK2 in TNBC cells and inhibited PEAK1-enhanced migration and invasion in vitro . Moreover, RA306 significantly attenuated TNBC xenograft growth and blocked metastasis in a manner mirrored by CRISPR-mediated PEAK1 ablation. Overall, these studies establish PEAK1 as a critical cell signalling nexus, identify a novel mechanism for regulation of Ca 2+ signalling and its integration with tyrosine kinase signals, and identify CAMK2 as a therapeutically 'actionable' target downstream of PEAK1.

Role of CAMK2D in neurodevelopment and associated conditions

The calcium/calmodulin-dependent protein kinase type 2 (CAMK2) family consists of four different isozymes, encoded by four different genes-CAMK2A, CAMK2B, CAMK2G, and CAMK2D-of which the first three have been associated recently with neurodevelopmental disorders. CAMK2D is one of the major CAMK2 proteins expressed in the heart and has been associated with cardiac anomalies. Although this CAMK2 isoform is also known to be one of the major CAMK2 subtypes expressed during early brain development, it has never been linked with neurodevelopmental disorders until now. Here we show that CAMK2D plays an important role in neurodevelopment not only in mice but also in humans. We identified eight individuals harboring heterozygous variants in CAMK2D who display symptoms of intellectual disability, delayed speech, behavioral problems, and dilated cardiomyopathy. The majority of the variants tested lead to a gain of function (GoF), which appears to cause both neurological problems and dilated cardiomyopathy. In contrast, loss-of-function (LoF) variants appear to induce only neurological symptoms. Together, we describe a cohort of individuals with neurodevelopmental disorders and cardiac anomalies, harboring pathogenic variants in CAMK2D, confirming an important role for the CAMK2D isozyme in both heart and brain function.

Polymer-based microfluidic device for on-chip counter-diffusive crystallization and in situ X-ray crystallography at room temperature

Proteins are long chains of amino acid residues that perform a myriad of functions in living organisms, including enzymatic reactions, signalling, and maintaining structural integrity. Protein function is determined directly by the protein structure. X-ray crystallography is the primary technique for determining the 3D structure of proteins, and facilitates understanding the effects of protein structure on function. The first step towards structure determination is crystallizing the protein of interest. We have developed a centrifugally-actuated microfluidic device that incorporates the fluid handling and metering necessary for protein crystallization. Liquid handling takes advantage of surface forces to control fluid flow and enable metering, without the need for any fluidic or pump connections. Our approach requires only the simple steps of pipetting the crystallization reagents into the device followed by either spinning or shaking to set up counter-diffusive protein crystallization trials. The use of thin, UV-curable polymers with a high level of X-ray transparency allows for in situ X-ray crystallography, eliminating the manual handling of fragile protein crystals and streamlining the process of protein structure analysis. We demonstrate the utility of our device using hen egg white lysozyme as a model system, followed by the crystallization and in situ, room temperature structural analysis of the hub domain of calcium-calmodulin dependent kinase II (CaMKIIβ).

CaMKII binds both substrates and activators at the active site

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a signaling protein required for long-term memory. When activated by Ca2+/CaM, it sustains activity even after the Ca2+ dissipates. In addition to the well-known autophosphorylation-mediated mechanism, interaction with specific binding partners also persistently activates CaMKII. A long-standing model invokes two distinct S and T sites. If an interactor binds at the T-site, then it will preclude autoinhibition and allow substrates to be phosphorylated at the S site. Here, we specifically test this model with X-ray crystallography, molecular dynamics simulations, and biochemistry. Our data are inconsistent with this model. Co-crystal structures of four different activators or substrates show that they all bind to a single continuous site across the kinase domain. We propose a mechanistic model where persistent CaMKII activity is facilitated by high-affinity binding partners that kinetically compete with autoinhibition by the regulatory segment to allow substrate phosphorylation.

Arc self-association and formation of virus-like capsids are mediated by an N-terminal helical coil motif

Activity-regulated cytoskeleton-associated protein (Arc) is a protein interaction hub with diverse roles in intracellular neuronal signaling, and important functions in neuronal synaptic plasticity, memory, and postnatal cortical development. Arc has homology to retroviral Gag protein and is capable of self-assembly into virus-like capsids implicated in the intercellular transfer of RNA. However, the molecular basis of Arc self-association and capsid formation is largely unknown. Here, we identified a 28-amino-acid stretch in the mammalian Arc N-terminal (NT) domain that is necessary and sufficient for self-association. Within this region, we identified a 7-residue oligomerization motif, critical for the formation of virus-like capsids. Purified wild-type Arc formed capsids as shown by transmission and cryo-electron microscopy, whereas mutant Arc with disruption of the oligomerization motif formed homogenous dimers. An atomic-resolution crystal structure of the oligomerization region peptide demonstrated an antiparallel coiled-coil interface, strongly supporting NT-NT domain interactions in Arc oligomerization. The NT coil-coil interaction was also validated in live neurons using fluorescence lifetime FRET imaging, and mutation of the oligomerization motif disrupted Arc-facilitated endocytosis. Furthermore, using single-molecule photobleaching, we show that Arc mRNA greatly enhances higher-order oligomerization in a manner dependent on the oligomerization motif. In conclusion, a helical coil in the Arc NT domain supports self-association above the dimer stage, mRNA-induced oligomerization, and formation of virus-like capsids. DATABASE: The coordinates and structure factors for crystallographic analysis of the oligomerization region were deposited at the Protein Data Bank with the entry code 6YTU.

Heterogeneity in human hippocampal CaMKII transcripts reveals allosteric hub-dependent regulation

Calcium/calmodulin-dependent protein kinase II (CaMKII) plays a central role in Ca2+ signaling throughout the body. In the hippocampus, CaMKII is required for learning and memory. Vertebrate genomes encode four CaMKII homologs: CaMKIIα, CaMKIIβ, CaMKIIγ, and CaMKIIδ. All CaMKIIs consist of a kinase domain, a regulatory segment, a variable linker region, and a hub domain, which is responsible for oligomerization. The four proteins differ primarily in linker length and composition because of extensive alternative splicing. Here, we report the heterogeneity of CaMKII transcripts in three complex samples of human hippocampus using deep sequencing. We showed that hippocampal cells contain a diverse collection of over 70 CaMKII transcripts from all four CaMKII-encoding genes. We characterized the Ca2+/CaM sensitivity of hippocampal CaMKII variants spanning a broad range of linker lengths and compositions. The effect of the variable linker on Ca2+/CaM sensitivity depended on the kinase and hub domains. Moreover, we revealed a previously uncharacterized role for the hub domain as an allosteric regulator of kinase activity, which may provide a pharmacological target for modulating CaMKII activity. Using small-angle x-ray scattering and single-particle cryo-electron microscopy (cryo-EM), we present evidence for extensive interactions between the kinase and the hub domains, even in the presence of a 30-residue linker. Together, these data suggest that Ca2+/CaM sensitivity in CaMKII is homolog dependent and includes substantial contributions from the hub domain. Our sequencing approach, combined with biochemistry, provides insights into understanding the complex pool of endogenous CaMKII splice variants.

Characterization of CaMKIIα holoenzyme stability

Ca2+ /calmodulin-dependent protein kinase II (CaMKII) is a Ser/Thr kinase necessary for long-term memory formation and other Ca2+ -dependent signaling cascades such as fertilization. Here, we investigated the stability of CaMKIIα using a combination of differential scanning calorimetry (DSC), X-ray crystallography, and mass photometry (MP). The kinase domain has a low thermal stability (apparent Tm = 36°C), which is slightly stabilized by ATP/MgCl2 binding (apparent Tm = 40°C) and significantly stabilized by regulatory segment binding (apparent Tm = 60°C). We crystallized the kinase domain of CaMKII bound to p-coumaric acid in the active site. This structure reveals solvent-exposed hydrophobic residues in the substrate-binding pocket, which are normally buried in the autoinhibited structure when the regulatory segment is present. This likely accounts for the large stabilization that we observe in DSC measurements comparing the kinase alone with the kinase plus regulatory segment. The hub domain alone is extremely stable (apparent Tm  ~ 90°C), and the holoenzyme structure has multiple unfolding transitions ranging from ~60°C to 100°C. Using MP, we compared a CaMKIIα holoenzyme with different variable linker regions and determined that the dissociation of both these holoenzymes occurs at a higher concentration (is less stable) compared with the hub domain alone. We conclude that within the context of the holoenzyme structure, the kinase domain is stabilized, whereas the hub domain is destabilized. These data support a model where domains within the holoenzyme interact.

Functional implications of CaMKII alternative splicing

Ca²⁺ /calmodulin-dependent protein kinase II (CaMKII) is known to be a crucial regulator in the post-synapse during long-term potentiation. This important protein has been the subject of many studies centered on understanding memory at the molecular, cellular, and organismic level. CaMKII is encoded by four genes in humans, all of which undergo alternative splicing at the RNA level, leading to an enormous diversity of expressed proteins. Advances in sequencing technologies have facilitated the discovery of many new CaMKII transcripts. To date, newly discovered CaMKII transcripts have been incorporated into an ambiguous naming scheme. Herein, we review the initial experiments leading to the discovery of CaMKII and its subsequent variants. We propose the adoption of a new, unambiguous naming scheme for CaMKII variants. Finally, we discuss biological implications for CaMKII splice variants.

Live-cell FLIM-FRET using a commercially available system

Förster resonance energy transfer (FRET)-based sensors have been powerful tools in cell biologists' toolkit for decades. Informed by fundamental understanding of fluorescent proteins, protein-protein interactions, and the structural biology of reporter components, researchers have been able to employ creative design approaches to build sensors that are uniquely capable of probing a wide range of phenomena in living cells including visualization of localized calcium signaling, sub-cellular activity gradients, and tension generation to name but a few. While FRET sensors have significantly impacted many fields, one must also be cognizant of the limitations to conventional, intensity-based FRET measurements stemming from variation in probe concentration, sensitivity to photobleaching, and bleed-through between the FRET fluorophores. Fluorescence lifetime imaging microscopy (FLIM) largely overcomes the limitations of intensity-based FRET measurements. In general terms, FLIM measures the time, which for the reporters described in this chapter is nanoseconds (ns), between photon absorption and emission by a fluorophore. When FLIM is applied to FRET sensors (FLIM-FRET), measurement of the donor fluorophore lifetime provides valuable information such as FRET efficiency and the percentage of reporters engaged in FRET. This chapter introduces fundamental principles of FLIM-FRET toward informing the practical application of the technique and, using two established FRET reporters as proofs of concept, outlines how to use a commercially available FLIM system.

FRET-based sensor for CaMKII activity (FRESCA): A useful tool for assessing CaMKII activity in response to Ca2+ oscillations in live cells

Ca²⁺ oscillations and consequent Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activation are required for embryogenesis, as well as neuronal, immunological, and cardiac signaling. Fertilization directly results in Ca²⁺ oscillations, but the resultant pattern of CaMKII activity remains largely unclear. To address this gap, we first employed the one existing biosensor for CaMKII activation. This sensor, Camui, comprises CaMKIIα and therefore solely reports on the activation of this CaMKII variant. Additionally, to detect the activity of all endogenous CaMKII variants simultaneously, we constructed a substrate-based sensor for CaMKII activity, FRESCA (FRET-based sensor for CaMKII activity). To examine the differential responses of the Camui and FRESCA sensors, we used several approaches to stimulate Ca²⁺ release in mouse eggs, including addition of phospholipase Cζ cRNA, which mimics natural fertilization. We found that the Camui response is delayed or terminates earlier than the FRESCA response. FRESCA enables assessment of endogenous CaMKII activity in real-time by both fertilization and artificial reagents, such as Sr²⁺, which also leads to CaMKII activation. FRESCA's broad utility will be important for optimizing artificial CaMKII activation for clinical use to manage infertility. Moreover, FRESCA provides a new view on CaMKII activity, and its application in additional biological systems may reveal new signaling paradigms in eggs, as well as in neurons, cardiomyocytes, immune cells, and other CaMKII-expressing cells.

Reciprocal Activation within a Kinase-Effector Complex Underlying Persistence of Structural LTP

Long-term synaptic plasticity requires a mechanism that converts short Ca²⁺ pulses into persistent biochemical signaling to maintain changes in the synaptic structure and function. Here, we present a novel mechanism of a positive feedback loop, formed by a reciprocally activating kinase-effector complex (RAKEC) in dendritic spines, enabling the persistence and confinement of a molecular memory. We found that stimulation of a single spine causes the rapid formation of a RAKEC consisting of CaMKII and Tiam1, a Rac-GEF. This interaction is mediated by a pseudo-autoinhibitory domain on Tiam1, which is homologous to the CaMKII autoinhibitory domain itself. Therefore, Tiam1 binding results in constitutive CaMKII activation, which in turn persistently phosphorylates Tiam1. Phosphorylated Tiam1 promotes stable actin-polymerization through Rac1, thereby maintaining the structure of the spine during LTP. The RAKEC can store biochemical information in small subcellular compartments, thus potentially serving as a general mechanism for prolonged and compartmentalized signaling.

Molecular mechanism of activation-triggered subunit exchange in Ca(2+)/calmodulin-dependent protein kinase II

Activation triggers the exchange of subunits in Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), an oligomeric enzyme that is critical for learning, memory, and cardiac function. The mechanism by which subunit exchange occurs remains elusive. We show that the human CaMKII holoenzyme exists in dodecameric and tetradecameric forms, and that the calmodulin (CaM)-binding element of CaMKII can bind to the hub of the holoenzyme and destabilize it to release dimers. The structures of CaMKII from two distantly diverged organisms suggest that the CaM-binding element of activated CaMKII acts as a wedge by docking at intersubunit interfaces in the hub. This converts the hub into a spiral form that can release or gain CaMKII dimers. Our data reveal a three-way competition for the CaM-binding element, whereby phosphorylation biases it towards the hub interface, away from the kinase domain and calmodulin, thus unlocking the ability of activated CaMKII holoenzymes to exchange dimers with unactivated ones.

DOI:http://dx.doi.org/10.7554/eLife.13405.001

How does memory outlast the lifetime of the molecules that encode it? One enzyme that is found in neurons and has been suggested to help long-term memories to form is called CaMKII. Each CaMKII assembly is typically composed of 12 to 14 protein subunits associated in a ring and can exist in either an “unactivated” or “activated” state. In 2014, researchers showed that CaMKII assemblies can exchange subunits with each other. Importantly, an active CaMKII can mix with an unactivated CaMKII and share its activation state. CaMKII may use this mechanism to spread information to the next generation of proteins – thereby allowing activation to outlast the lifespan of the initially activated proteins. However the molecular mechanism that underlies this process was not clear.

Now, Bhattacharyya et al. – including some of the researchers involved in the 2014 work – address two questions about this mechanism. How do subunits exchange between CaMKII assemblies? And how does the activation of CaMKII initiate subunit exchange?

A closed-ring hub ties the subunits of CaMKII together, similar to the organization of the segments in an orange. To undergo subunit exchange, the hub must open up to release and accept subunits. Bhattacharyya et al. have now uncovered an intrinsic flexibility in the hub that is triggered by a short peptide segment in CaMKII. This segment, which is exposed in activated CaMKII but not in the unactivated form, can crack open the hub ring by binding between the hub subunits, like a finger separating the segments of an orange. This allows the hub to flex and expand, and once open, the hub’s flexibility allows room for subunits to be released or accepted.

Although this subunit exchange mechanism could be a powerful means for spreading the activated state throughout signaling pathways, the biological relevance of this phenomenon has not been clarified. However, the mechanistic framework provided by Bhattacharyya et al. may allow new experiments to be performed that test the consequences of subunit exchange in live cells and organisms. It could also enable investigations into the importance of subunit exchange in long-term memory.

DOI:http://dx.doi.org/10.7554/eLife.13405.002

Structural studies on the regulation of Ca2+/calmodulin dependent protein kinase II

Ca(2+)/calmodulin dependent protein kinase II (CaMKII) is a broadly distributed metazoan Ser/Thr protein kinase that is important in neuronal and cardiac signaling. CaMKII forms oligomeric assemblies, typically dodecameric, in which the calcium-responsive kinase domains are organized around a central hub. We review the results of crystallographic analyses of CaMKII, including the recently determined structure of a full-length and autoinhibited form of the holoenzyme. These structures, when combined with other data, allow informed speculation about how CaMKII escapes calcium-dependence when calcium spikes exceed threshold frequencies.

A mechanism for tunable autoinhibition in the structure of a human Ca2+/calmodulin- dependent kinase II holoenzyme

Calcium/calmodulin-dependent kinase II (CaMKII) forms a highly conserved dodecameric assembly that is sensitive to the frequency of calcium pulse trains. Neither the structure of the dodecameric assembly nor how it regulates CaMKII are known. We present the crystal structure of an autoinhibited full-length human CaMKII holoenzyme, revealing an unexpected compact arrangement of kinase domains docked against a central hub, with the calmodulin-binding sites completely inaccessible. We show that this compact docking is important for the autoinhibition of the kinase domains and for setting the calcium response of the holoenzyme. Comparison of CaMKII isoforms, which differ in the length of the linker between the kinase domain and the hub, demonstrates that these interactions can be strengthened or weakened by changes in linker length. This equilibrium between autoinhibited states provides a simple mechanism for tuning the calcium response without changes in either the hub or the kinase domains.

Structural characterization of two alternate conformations in a calbindin D₉k-based molecular switch

We have demonstrated that calbindin D(9k) can be converted into a calcium-sensing switch (calbindin-AFF) by duplicating the C-terminal half of the protein (residues 44-75) and appending it to the N-terminus (creating residues 44'-75'). This re-engineering results in a ligand-driven interconversion between two native folds: the wild-type structure (N) and a circularly permuted form (N'). The switch between N and N' is predicted to involve exchange of the 44-75 and 44'-75' segments, possibly linked to their respective folding and unfolding. Here we present direct structural evidence supporting the existence of N and N'. To isolate the N' and N conformations, we introduced the knockdown Ca(2+) binding mutation Glu → Gln at position 65 (E65Q mutant) or at the analogous position 65' (E65'Q mutant). E65Q and E65'Q are therefore expected to adopt conformations N' and N, respectively, in the presence of calcium. Though the amino acid sequences of E65Q and E65'Q differ at only these two positions, nuclear magnetic resonance resonance assignments, chemical shifts, and paramagnetic relaxation enhancement data reveal that they take on separate structures when bound to calcium. Both proteins are comprised of a well-folded domain and a disordered region. However, the segment that is disordered in E65Q (residues 44-75) is folded in E65'Q, and the region that is disordered in E65'Q (residues 44'-75') is structured in E65Q. The results demonstrate that the N' N' conformational change is mediated by a mutually exclusive folding reaction in which folding of one segment of the protein is coupled to unfolding of another segment, and vice versa.

Converting a protein into a switch for biosensing and functional regulation

Proteins that switch conformations in response to a signaling event (e.g., ligand binding or chemical modification) present a unique solution to the design of reagent-free biosensors as well as molecules whose biological functions are regulated in useful ways. The principal roadblock in the path to develop such molecules is that the majority of natural proteins do not change conformation upon binding their cognate ligands or becoming chemically modified. Herein, we review recent protein engineering efforts to introduce switching properties into binding proteins. By co-opting natural allosteric coupling, joining proteins in creative ways and formulating altogether new switching mechanisms, researchers are learning how to coax conformational changes from proteins that previously had none. These studies are providing some answers to the challenging question: how can one convert a lock-and-key binding protein into a molecular switch?

On the mechanism of protein fold-switching by a molecular sensor

Alternate frame folding (AFF) is a mechanism by which conformational change can be engineered into a protein. The protein structure switches from the wild-type fold (N) to a circularly-permuted fold (N'), or vice versa, in response to a signaling event such as ligand binding. Despite the fact that the two native states have similar structures, their interconversion involves folding and unfolding of large parts of the molecule. This rearrangement is reported by fluorescent groups whose relative proximities change as a result of the order-disorder transition. The nature of the conformational change is expected to be similar from protein to protein; thus, it may be possible to employ AFF as a general method to create optical biosensors. Toward that goal, we test basic aspects of the AFF mechanism using the AFF variant of calbindin D(9k). A simple three-state model for fold switching holds that N and N' interconvert through the unfolded state. This model predicts that the fundamental properties of the switch--calcium binding affinity, signal response (i.e., fluorescence change upon binding), and switching rate--can be controlled by altering the relative stabilities of N and N'. We find that selectively destabilizing N or N' changes the equilibrium properties of the switch (binding affinity and signal response) in accordance with the model. However, kinetic data indicate that the switching pathway does not require whole-molecule unfolding. The rate is instead limited by unfolding of a portion of the protein, possibly in concert with folding of a corresponding region.

Probing local structural fluctuations in myoglobin by size-dependent thiol-disulfide exchange

All proteins undergo local structural fluctuations (LSFs) or breathing motions. These motions are likely to be important for function but are poorly understood. LSFs were initially defined by amide hydrogen exchange (HX) experiments as opening events, which expose a small number of backbone amides to (1)H/(2)H exchange, but whose exchange rates are independent of denaturant concentration. Here, we use size-dependent thiol-disulfide exchange (SX) to characterize LSFs in single cysteine-containing variants of myoglobin (Mb). SX complements HX by providing information on motions that disrupt side chain packing interactions. Most importantly, probe reagents of different sizes and chemical properties can be used to characterize the size of structural opening events and the properties of the open state. We use thiosulfonate reagents (126-274 Da) to survey access to Cys residues, which are buried at specific helical packing interfaces in Mb. In each case, the free energy of opening increases linearly with the radius of gyration of the probe reagent. The slope and the intercept are interpreted to yield information on the size of the opening events that expose the buried thiol groups. The slope parameter varies by over 10-fold among Cys positions tested, suggesting that the sizes of breathing motions vary substantially throughout the protein. Our results provide insight to the longstanding question: how rigid or flexible are proteins in their native states?

A Ca2+-sensing molecular switch based on alternate frame protein folding

Existing strategies for creating biosensors mainly rely on large conformational changes to transduce a binding event to an output signal. Most molecules, however, do not exhibit large-scale structural changes upon substrate binding. Here, we present a general approach (alternate frame folding, or AFF) for engineering allosteric control into ligand binding proteins. AFF can in principle be applied to any protein to establish a binding-induced conformational change, even if none exists in the natural molecule. The AFF design duplicates a portion of the amino acid sequence, creating an additional "frame" of folding. One frame corresponds to the wild-type sequence, and folding produces the normal structure. Folding in the second frame yields a circularly permuted protein. Because the two native structures compete for a shared sequence, they fold in a mutually exclusive fashion. Binding energy is used to drive the conformational change from one fold to the other. We demonstrate the approach by converting the protein calbindin D(9k) into a molecular switch that senses Ca2+. The structures of Ca2+-free and Ca2+-bound calbindin are nearly identical. Nevertheless, the AFF mechanism engineers a robust conformational change that we detect using two covalently attached fluorescent groups. Biological fluorophores can also be employed to create a genetically encoded sensor. AFF should be broadly applicable to create sensors for a variety of small molecules.