NaV1.2 EFL domain allosterically enhances Ca2+ binding to sites I and II of WT and pathogenic calmodulin mutants bound to the channel CTD

Neuronal voltage-gated sodium channel NaV1.2 C-terminal domain (CTD) binds calmodulin (CaM) constitutively at its IQ motif. A solution structure (6BUT) and other NMR evidence showed that the CaM N domain (CaMN) is structurally independent of the C-domain (CaMC) whether CaM is bound to the NaV1.2IQp (1,901-1,927) or NaV1.2CTD (1,777-1,937) with or without calcium. However, in the CaM + NaV1.2CTD complex, the Ca2+ affinity of CaMN was more favorable than in free CaM, while Ca2+ affinity for CaMC was weaker than in the CaM + NaV1.2IQp complex. The CTD EF-like (EFL) domain allosterically widened the energetic gap between CaM domains. Cardiomyopathy-associated CaM mutants (N53I(N54I), D95V(D96V), A102V(A103V), E104A(E105A), D129G(D130G), and F141L(F142L)) all bound the NaV1.2 IQ motif favorably under resting (apo) conditions and bound calcium normally at CaMN sites. However, only N53I and A102V bound calcium at CaMC sites at [Ca2+] < 100 μM. Thus, they are expected to respond like wild-type CaM to Ca2+ spikes in excitable cells.

Backbone resonance assignments of complexes of apo human calmodulin bound to IQ motif peptides of voltage-dependent sodium channels NaV1.1, NaV1.4 and NaV1.7

Human voltage-gated sodium (Na_V) channels are critical for initiating and propagating action potentials in excitable cells. Nine isoforms have different roles but similar topologies, with a pore-forming α-subunit and auxiliary transmembrane β-subunits. Na_V pathologies lead to debilitating conditions including epilepsy, chronic pain, cardiac arrhythmias, and skeletal muscle paralysis. The ubiquitous calcium sensor calmodulin (CaM) binds to an IQ motif in the C-terminal tail of the α-subunit of all Na_V isoforms, and contributes to calcium-dependent pore-gating in some channels. Previous structural studies of calcium-free (apo) CaM bound to the IQ motifs of Na_V1.2, Na_V1.5, and Na_V1.6 showed that CaM binding was mediated by the C-domain of CaM (CaM_C), while the N-domain (CaM_N) made no detectable contacts. To determine whether this domain-specific recognition mechanism is conserved in other Na_V isoforms, we used solution NMR spectroscopy to assign the backbone resonances of complexes of apo CaM bound to peptides of IQ motifs of Na_V1.1, Na_V1.4, and Na_V1.7. Analysis of chemical shift differences showed that peptide binding only perturbed resonances in CaM_C; resonances of CaM_N were identical to free CaM. Thus, CaM_C residues contribute to the interface with the IQ motif, while CaM_N is available to interact elsewhere on the channel.

Backbone resonance assignments of complexes of human voltage-dependent sodium channel NaV1.2 IQ motif peptide bound to apo calmodulin and to the C-domain fragment of apo calmodulin

Human voltage-gated sodium channel Na_V1.2 has a single pore-forming α-subunit and two transmembrane β-subunits. Expressed primarily in the brain, Na_V1.2 is critical for initiation and propagation of action potentials. Milliseconds after the pore opens, sodium influx is terminated by inactivation processes mediated by regulatory proteins including calmodulin (CaM). Both calcium-free (apo) CaM and calcium-saturated CaM bind tightly to an IQ motif in the C-terminal tail of the α-subunit. Our thermodynamic studies and solution structure (2KXW) of a C-domain fragment of apo ¹³C,¹⁵N- CaM (CaM_C) bound to an unlabeled peptide with the sequence of rat Na_V1.2 IQ motif showed that apo CaM_C (a) was necessary and sufficient for binding, and (b) bound more favorably than calcium-saturated CaM_C. However, we could not monitor the Na_V1.2 residues directly, and no structure of full-length CaM (including the N-domain of CaM (CaM_N)) was determined. To distinguish contributions of CaM_N and CaM_C, we used solution NMR spectroscopy to assign the backbone resonances of a complex containing a ¹³C,¹⁵N-labeled peptide with the sequence of human Na_V1.2 IQ motif (Na_V1.2_IQp) bound to apo ¹³C,¹⁵N-CaM or apo ¹³C,¹⁵N-CaM_C. Comparing the assignments of apo CaM in complex with Na_V1.2_IQp to those of free apo CaM showed that residues within CaM_C were significantly perturbed, while residues within CaM_N were essentially unchanged. The chemical shifts of residues in Na_V1.2_IQp and in the C-domain of CaM were nearly identical regardless of whether CaM_N was covalently linked to CaM_C. This suggests that CaM_N does not influence apo CaM binding to Na_V1.2_IQp.

Solution NMR structures of the C-domain of Tetrahymena cytoskeletal protein Tcb2 reveal distinct calcium-induced structural rearrangements

Tcb2 is a calcium-binding protein that localizes to the membrane-associated skeleton of the ciliated protozoan Tetrahymena thermophila with hypothesized roles in ciliary movement, cell cortex signaling, and pronuclear exchange. Tcb2 has also been implicated in a unique calcium-triggered, ATP-independent type of contractility exhibited by filamentous networks isolated from the Tetrahymena cytoskeleton. To gain insight into Tcb2's structure-function relationship and contractile properties, we determined solution NMR structures of its C-terminal domain in the calcium-free and calcium-bound states. The overall architecture is similar to other calcium-binding proteins, with paired EF-hand calcium-binding motifs. Comparison of the two structures reveals that Tcb2-C's calcium-induced conformational transition differs from the prototypical calcium sensor calmodulin, suggesting that the two proteins play distinct functional roles in Tetrahymena and likely have different mechanisms of target recognition. Future studies of the full-length protein and the identification of Tcb2 cellular targets will help establish the molecular basis of Tcb2 function and its unique contractile properties. Proteins 2016; 84:1748-1756. © 2016 Wiley Periodicals, Inc.

Backbone and side-chain chemical shift assignments for the C-terminal domain of Tcb2, a cytoskeletal calcium-binding protein from Tetrahymena thermophila

Tcb2 is a putative calcium-binding protein from the membrane-associated cytoskeleton of the ciliated protozoan Tetrahymena thermophila. It has been hypothesized to participate in several calcium-mediated processes in Tetrahymena, including ciliary movement, cell cortex signaling, and pronuclear exchange. Sequence analysis suggests that the protein belongs to the calmodulin family, with N- and C-terminal domains connected by a central linker, and two helix-loop-helix motifs in each domain. However, its calcium-binding properties, structure and precise biological function remain unknown. Interestingly, Tcb2 is a major component of unique contractile fibers isolated from the Tetrahymena cytoskeleton; in these fibers, addition of calcium triggers an ATP-independent type of contraction. Here we report the (1)H, (13)C and (15)N backbone and side-chain chemical shift assignments of the C-terminal domain of the protein (Tcb2-C) in the absence and presence of calcium ions. (1)H-(15)N HSQC spectra show that the domain is well folded both in the absence and presence of calcium, and undergoes a dramatic conformational change upon calcium addition. Secondary structure prediction from chemical shifts reveals an architecture encountered in other calcium-binding proteins, with paired EF-hand motifs connected by a flexible linker. These studies represent a starting point for the determination of the high-resolution solution structure of Tcb2-C at both low and high calcium levels, and, together with additional structural studies on the full-length protein, will help establish the molecular basis of Tcb2 function and unique contractile properties.

Structural and binding studies of the C-terminal domains of yeast TFIIF subunits Tfg1 and Tfg2

The general transcription factor TFIIF plays essential roles at several steps during eukaryotic transcription. While several studies have offered insights into the structure/function relationship in human TFIIF, much less is known about the yeast system. Here, we describe the first NMR structural and binding studies of the C-terminal domains (CTDs) of Tfg1 and Tfg2 subunits of Saccharomyces cerevisiae TFIIF. We used the program CS-ROSETTA to determine the three-dimensional folds of these domains in solution, and performed binding studies with DNA and protein targets. CS-ROSETTA models indicate that the Tfg1 and Tfg2 C-terminal domains have winged-helix architectures, similar to the human homologs. We showed that both Tfg1 and Tfg2 CTDs interact with double-stranded DNA oligonucleotides, and mapped the DNA binding interfaces using solution NMR. Tfg1-CTD, but not Tfg2-CTD, also binds to yeast FCP1, an RNA polymerase II-specific phosphatase, and we delineated the interaction surface with the CTD of FCP1. Our results provide insights into the structural basis of yeast TFIIF function and the differential roles of Tfg1 and Tfg2 subunits during transcription.

A quantitative risk assessment of West Nile virus introduction into Barbados

OBJECTIVE

To present a quantitative risk assessment of West Nile (WNV) virus introduction into Barbados, West Indies.

DESIGN AND METHODS

Three possible modes were considered: a) WNV infected mosquitoes via air transport, by city of departure, b) WNV infected mosquitoes via marine transport and c) viraemic migratory, birds. We estimated the number of WNV infected migratory birds as the product of the proportion of migratory birds infected and the number of migratory birds entering Barbados in three taxonomic groups. We further estimated the number of days these birds would be infectious as: [formula: see text]. We then estimated the number (#) of infectious mosquito-days for mosquitoes entering Barbados via air transport as: # infected mosquitoes = (total flights per week/city) x (duration of WNV season) x (number of Culex mosquitoes aboard each flight) x (Culex mosquito WNV infection prevalence) x (vector competence index) x (days infectious). The number of infected mosquitoes entering Barbados via marine transport was estimated using a similar expression as for air transport, except that the number of airplanes and mosquitoes/airplane were substituted with the # of sea containers during a 22-week mosquito season and # of mosquitoes/container.

RESULTS

Migratory birds (approximately 69-101 infected birds/year) were associated with the highest introductory risk followed by mode (a) (approximately 2 infected mosquitoes/year) and mode (b) (0. 004 infected mosquitoes/year).

CONCLUSIONS

Migratory birds and mosquitoes via air are imminent threats for virus introduction. Impending co-circulation of West Nile virus and four strains of dengue virus may present new challenges for public health.

Species interactions can explain Taylor's power law for ecological time series

One of the few generalities in ecology, Taylor's power law, describes the species-specific relationship between the temporal or spatial variance of populations and their mean abundances. For populations experiencing constant per capita environmental variability, the regression of log variance versus log mean abundance gives a line with a slope of 2. Despite this expectation, most species have slopes of less than 2 (refs 2, 3-4), indicating that more abundant populations of a species are relatively less variable than expected on the basis of simple statistical grounds. What causes abundant populations to be less variable has received considerable attention, but an explanation for the generality of this pattern is still lacking. Here we suggest a novel explanation for the scaling of temporal variability in population abundances. Using stochastic simulation and analytical models, we demonstrate how negative interactions among species in a community can produce slopes of Taylor's power law of less than 2, like those observed in real data sets. This result provides an example in which the population dynamics of single species can be understood only in the context of interactions within an ecological community.