A tamoxifen receptor within a voltage-gated sodium channel.

Voltage-gated sodium channels are targets for many analgesic and antiepileptic drugs whose therapeutic mechanisms and binding sites have been well characterized. We describe the identification of a previously unidentified receptor site within the NavMs voltage-gated sodium channel. Tamoxifen, an estrogen receptor modulator, and its primary and secondary metabolic products bind at the intracellular exit of the channel, which is a site that is distinct from other previously characterized sodium channel drug sites. These compounds inhibit NavMs and human sodium channels with similar potencies and prevent sodium conductance by delaying channel recovery from the inactivated state. This study therefore not only describes the structure and pharmacology of a site that could be leveraged for the development of new drugs for the treatment of sodium channelopathies but may also have important implications for off-target health effects of this widely used therapeutic drug.

Cannabidiol interactions with voltage-gated sodium channels.

Voltage-gated sodium channels are targets for a range of pharmaceutical drugs developed for the treatment of neurological diseases. Cannabidiol (CBD), the non-psychoactive compound isolated from cannabis plants, was recently approved for treatment of two types of epilepsy associated with sodium channel mutations. This study used high-resolution X-ray crystallography to demonstrate the detailed nature of the interactions between CBD and the NavMs voltage-gated sodium channel, and electrophysiology to show the functional effects of binding CBD to these channels. CBD binds at a novel site at the interface of the fenestrations and the central hydrophobic cavity of the channel. Binding at this site blocks the transmembrane-spanning sodium ion translocation pathway, providing a molecular mechanism for channel inhibition. Modelling studies suggest why the closely-related psychoactive compound tetrahydrocannabinol may not have the same effects on these channels. Finally, comparisons are made with the TRPV2 channel, also recently proposed as a target site for CBD. In summary, this study provides novel insight into a possible mechanism for CBD interactions with sodium channels.

Mechanism of ß-actin mRNA Recognition by ZBP1.

Zipcode binding protein 1 (ZBP1) is an oncofetal RNA-binding protein that mediates the transport and local translation of ß-actin mRNA by the KH3-KH4 di-domain, which is essential for neuronal development. The high-resolution structures of KH3-KH4 with their respective target sequences show that KH4 recognizes a non-canonical GGA sequence via an enlarged and dynamic hydrophobic groove, whereas KH3 binding to a core CA sequence occurs with low specificity. A data-informed kinetic simulation of the two-step binding reaction reveals that the overall reaction is driven by the second binding event and that the moderate affinities of the individual interactions favor RNA looping. Furthermore, the concentration of ZBP1, but not of the target RNA, modulates the interaction, which explains the functional significance of enhanced ZBP1 expression during embryonic development.

Noncanonical G recognition mediates KSRP regulation of let-7 biogenesis.

Let-7 is an important tumor-suppressive microRNA (miRNA) that acts as an on-off switch for cellular differentiation and regulates the expression of a set of human oncogenes. Binding of the human KSRP protein to let-7 miRNA precursors positively regulates their processing to mature let-7, thereby contributing to control of cell proliferation, apoptosis and differentiation. Here we analyze the molecular basis for KSRP-let-7 precursor selectivity and show how the third KH domain of the protein recognizes a G-rich sequence in the pre-let-7 terminal loop and dominates the interaction. The structure of the KH3-RNA complex explains the protein recognition of this noncanonical KH target sequence, and we demonstrate that the specificity of this binding is crucial for the functional interaction between the protein and the miRNA precursor.

KH domains with impaired nucleic acid binding as a tool for functional analysis.

In eukaryotes, RNA-binding proteins that contain multiple K homology (KH) domains play a key role in coordinating the different steps of RNA synthesis, metabolism and localization. Understanding how the different KH modules participate in the recognition of the RNA targets is necessary to dissect the way these proteins operate. We have designed a KH mutant with impaired RNA-binding capability for general use in exploring the role of individual KH domains in the combinatorial functional recognition of RNA targets. A double mutation in the hallmark GxxG loop (GxxG-to-GDDG) impairs nucleic acid binding without compromising the stability of the domain. We analysed the impact of the GDDG mutations in individual KH domains on the functional properties of KSRP as a prototype of multiple KH domain-containing proteins. We show how the GDDG mutant can be used to directly link biophysical information on the sequence specificity of the different KH domains of KSRP and their role in mRNA recognition and decay. This work defines a general molecular biology tool for the investigation of the function of individual KH domains in nucleic acid binding proteins.

Molecular basis of FIR-mediated c-myc transcriptional control.

The far upstream element (FUSE) regulatory system promotes a peak in the concentration of c-Myc during cell cycle. First, the FBP transcriptional activator binds to the FUSE DNA element upstream of the c-myc promoter. Then, FBP recruits its specific repressor (FIR), which acts as an on/off transcriptional switch. Here we describe the molecular basis of FIR recruitment, showing that the tandem RNA recognition motifs of FIR provide a platform for independent FUSE DNA and FBP protein binding and explaining the structural basis of the reversibility of the FBP-FIR interaction. We also show that the physical coupling between FBP and FIR is modulated by a flexible linker positioned sequentially to the recruiting element. Our data explain how the FUSE system precisely regulates c-myc transcription and suggest that a small change in FBP-FIR affinity leads to a substantial effect on c-Myc concentration.

Orientation of the central domains of KSRP and its implications for the interaction with the RNA targets.

KSRP is a multi-domain RNA-binding protein that recruits the exosome-containing mRNA degradation complex to mRNAs coding for cellular proliferation and inflammatory response factors. The selectivity of this mRNA degradation mechanism relies on KSRP recognition of AU-rich elements in the mRNA 3'UTR, that is mediated by KSRP's KH domains. Our structural analysis shows that the inter-domain linker orients the two central KH domains of KSRP-and their RNA-binding surfaces-creating a two-domain unit. We also show that this inter-domain arrangement is important to the interaction with KSRP's RNA targets.

Phosphorylation-mediated unfolding of a KH domain regulates KSRP localization via 14-3-3 binding.

The AU-rich element (ARE)-mediated mRNA-degradation activity of the RNA binding K-homology splicing regulator protein (KSRP) is regulated by phosphorylation of a serine within its N-terminal KH domain (KH1). In the cell, phosphorylation promotes the interaction of KSRP and 14-3-3zeta protein and impairs the ability of KSRP to promote the degradation of its RNA targets. Here we examine the molecular details of this mechanism. We report that phosphorylation leads to the unfolding of the structurally atypical and unstable KH1, creating a site for 14-3-3zeta binding. Using this site, 14-3-3zeta discriminates between phosphorylated and unphosphorylated KH1, driving the nuclear localization of KSRP. 14-3-3zeta -KH1 interaction regulates the mRNA-decay activity of KSRP by sequestering the protein in a separate functional pool. This study demonstrates how an mRNA-degradation pathway is connected to extracellular signaling networks through the reversible unfolding of a protein domain.

The sequence selectivity of KSRP explains its flexibility in the recognition of the RNA targets.

K-homology (KH) splicing regulator protein (KSRP) is a multi-domain RNA-binding protein that regulates different steps of mRNA metabolism, from mRNA splicing to mRNA decay, interacting with a broad range of RNA sequences. To understand how KSRP recognizes its different RNA targets it is necessary to define the general rules of KSRP-RNA interaction. We describe here a complete scaffold-independent analysis of the RNA-binding potential of the four KH domains of KSRP. The analysis shows that KH3 binds to the RNA with a significantly higher affinity than the other domains and recognizes specifically a G-rich target. It also demonstrates that the other KH domains of KSRP display different sequence preferences explaining the broad range of targets recognized by the protein. Further, KSRP shows a strong negative selectivity for sequences containing several adjacent Cytosines limiting the target choice of KSRP within single-stranded RNA regions. The in-depth analysis of the RNA-binding potential of the KH domains of KSRP provides us with an understanding of the role of low sequence specificity domains in RNA recognition by multi-domain RNA-binding proteins.

The structure of the C-terminal KH domains of KSRP reveals a noncanonical motif important for mRNA degradation.

The AU-rich element (ARE) RNA-binding protein KSRP (K-homology splicing regulator protein) contains four KH domains and promotes the degradation of specific mRNAs that encode proteins with functions in cellular proliferation and inflammatory response. The fourth KH domain (KH4) is essential for mRNA recognition and decay but requires the third KH domain (KH3) for its function. We show that KH3 and KH4 behave as independent binding modules and can interact with different regions of the AU-rich RNA targets of KSRP. This provides KSRP with the structural flexibility needed to recognize a set of different targets in the context of their 3'UTR structural settings. Surprisingly, we find that KH4 binds to its target AREs with lower affinity than KH3 and that KSRP's mRNA binding, and mRNA degradation activities are closely associated with a conserved structural element of KH4.

RNA polymerase II CTD phosphopeptides compete with RNA for the interaction with Pcf11.

In Saccharomyces cerevisiae, the cleavage/polyadenylation factor Pcf11 is an important regulatory factor required for recruiting the polyadenylation machinery to the elongating RNA polymerase II (RNAPII) and is necessary for correct transcriptional termination. The interaction with RNAPII is mediated by a CTD-interacting domain (CID) located in the N-terminal region of Pcf11 that binds in a phospho-dependent manner the heptad repeats in the RNAPII CTD. We have previously investigated this protein-protein interaction. We examine here the interaction of the CID with different RNA sequences and look at the effect of phosphopeptides derived from the CTD heptad repeats on the RNA-protein interaction. Our findings demonstrate that the CID displays weak RNA-binding activity, but with some degree of sequence preference, the RNA-protein and peptide-protein interfaces overlap and the CTD-derived phosphopeptides and RNA compete for the binding site. We propose that competition between the protein-peptide and the protein-RNA interaction is important mechanistically and required for the disengagement of polyadenylation factors from RNAPII.

The structure of the N-terminal domain of the fragile X mental retardation protein: a platform for protein-protein interaction.

FMRP, whose lack of expression causes the X-linked fragile X syndrome, is a modular RNA binding protein thought to be involved in posttranslational regulation. We have solved the structure in solution of the N-terminal domain of FMRP (NDF), a functionally important region involved in multiple interactions. The structure consists of a composite fold comprising two repeats of a Tudor motif followed by a short alpha helix. The interactions between the three structural elements are essential for the stability of the NDF fold. Although structurally similar, the two repeats have different dynamic and functional properties. The second, more flexible repeat is responsible for interacting both with methylated lysine and with 82-FIP, one of the FMRP nuclear partners. NDF contains a 3D nucleolar localization signal, since destabilization of its fold leads to altered nucleolar localization of FMRP. We suggest that the NDF composite fold determines an allosteric mechanism that regulates the FMRP functions.

Key features of the interaction between Pcf11 CID and RNA polymerase II CTD.

The C-terminal domain (CTD) of the large subunit of RNA polymerase II is a platform for mRNA processing factors and links gene transcription to mRNA capping, splicing and polyadenylation. Pcf11, an essential component of the mRNA cleavage factor IA, contains a CTD-interaction domain that binds in a phospho-dependent manner to the heptad repeats within the RNA polymerase II CTD. We show here that the phosphorylated CTD exists as a dynamic disordered ensemble in solution and, by induced fit, it assumes a structured conformation when bound to Pcf11. In addition, we detected cis-trans populations for the CTD prolines, and found that only the all-trans form is selected for binding. These data suggest that the recognition of the CTD is regulated by independent site-specific modifications (phosphorylation and proline cis-trans isomerization) and, probably, by the local concentration of suitable binding sites.

Secondary structure and stability of the selenocysteine insertion sequences (SECIS) for human thioredoxin reductase and glutathione peroxidase.

We have used high resolution NMR and thermodynamics to characterize the secondary structure and stability of the selenocysteine insertion sequences (SECIS) of human glutathione peroxidase (58 nt) and thioredoxin reductase (51 nt). These sequences are members of the two classes of SECIS recently identified with two distinct structures capable of directing selenocysteine incorporation into proteins in eukaryotes. UV melting experiments showed a single cooperative and reversible transition for each RNA, which indicates the presence of stable secondary structures. Despite their large size, the RNAs gave well resolved NMR spectra for the exchangeable protons. Using NOESY, the imino protons as well as the cytosine amino protons of all of the Watson-Crick base pairs were assigned. In addition, a number of non-canonical base pairs including the wobble G.U pairs were identified. The interbase-pair NOEs allowed definition of the hydrogen-bonded structure of the oligonucleotides, providing an experimental model of the secondary structure of these elements. The derived secondary structures are consistent with several features of the predicted models, but with some important differences, especially regarding the conserved sequence motifs.

G-quartet-dependent recognition between the FMRP RGG box and RNA.

Fragile-X syndrome, the most common monogenic form of mental retardation, is caused by down-regulation of the expression of Fragile X Mental Retardation Protein (FMRP). FMRP is a multifunctional, multidomain RNA-binding protein that acts as a translational repressor in neuronal cells. Interaction between FMRP and mRNA targets involves an RGG box, a protein motif commonly thought to mediate unspecific interactions with nucleic acids. Instead, FMRP RGG box has been shown to recognize RNA G-quartet structures specifically and to be necessary in neurons for RNP particle formation and dendritic mRNA localization. In the present study, we have characterized structurally three representative RNA targets of FMRP in their unbound form and in complex with the RGG box. We observe a large heterogeneity in the conformation of the RNA targets and in their RGG binding mode, which could be the basis of recognition specificity. We also found that G-quartet formation occurs not only intramolecularly but can also be mediated by RNA dimerization. These findings suggest a potential role of RNA:RNA interactions in protein:RNA complexes and in RNP particle assembly.

The role of a clinically important mutation in the fold and RNA-binding properties of KH motifs.

We have investigated the role in the fold and RNA-binding properties of the KH modules of a hydrophobic to asparagine mutation of clinical importance in the fragile X syndrome. The mutation involves a well-conserved hydrophobic residue close to the N terminus of the second helix of the KH fold (alpha2(3) position). The effect of the mutation has been long debated: Although the mutant has been shown to disrupt the three-dimensional fold of several KH domains, the residue seems also to be directly involved in RNA binding, the main function of the KH module. Here we have used the KH3 of Nova-1, whose structure is known both in isolation and in an RNA complex, to study in detail the role of the alpha2(3) position. A detailed comparison of Nova KH3 structure with its RNA/KH complex and with other KH structures suggests a dual role for the alpha2(3) residue, which is involved both in stabilizing the hydrophobic core and in RNA contacts. We further show by nuclear magnetic resonance (NMR) studies in solution that L447 of Nova-1 in position alpha2(3) is in exchange in the absence of RNA, and becomes locked in a more rigid conformation only upon formation of an RNA complex. This implies that position alpha2(3) functions as a "gate" in the mechanism of RNA recognition of KH motifs based on the rigidification of the fold upon RNA binding.

Role of dimerization in KH/RNA complexes: the example of Nova KH3.

The K homology module, one of the most common RNA-binding motifs, is present in multiple copies in both prokaryotic and eukaryotic regulatory proteins. Increasing evidence suggests that self-aggregation of KH modules has a functional role. We have used a combination of techniques to characterize the behavior in solution of the third KH domain of Nova-1, a paradigmatic KH protein. The possibility of working on the isolated module allowed us to observe specifically the homodimerization and RNA-binding properties of KH domains. We provide conclusive evidence that self-association of Nova-1 KH3 occurs in solution even in the absence of RNA. Homodimerization involves a specific protein/protein interface. We also studied the dynamical behavior of Nova-1 KH3 in isolation and in complex with RNA. These data provide a model for the mechanism of KH/RNA recognition and suggest functional implications of dimerization in KH complexes. We discuss our findings in the context of the whole KH family and suggest a generalized mode of interaction.