Biophysical properties of regions flanking the bHLH-Zip motif in the p22 Max protein

Fuzzy protein-DNA interactions and beyond: A common theme in transcription?

Gene expression regulation requires both diversity and specificity. How can these two contradictory conditions be reconciled? Dynamic DNA recognition mechanisms lead to heterogeneous bound conformations, which can be shifted by the cellular cues. Here we summarise recent experimental evidence on how fuzzy interactions contribute to chromatin remodelling, regulation of DNA replication and repair and transcription factor binding. We describe how the binding mode continuum between DNA and regulatory factors lead to variable, multisite contact patterns; polyelectrolyte competitions; on-the-fly shape readouts; autoinhibition controlled by posttranslational modifications or dynamic oligomerisation mechanisms. Increasing experimental evidence supports the rugged energy landscape of the bound protein-DNA assembly, modulation of which leads to distinct functional outcomes. Recent results suggest the evolutionary conservation of these combinatorial mechanisms with moderate sequence constraints in the malleable transcriptional machinery.

Portability of a Small-Molecule Binding Site between Disordered Proteins

Intrinsically disordered proteins (IDPs) are important in both normal and disease states. Small molecules can be targeted to disordered regions, but we currently have only a limited understanding of the nature of small-molecule binding sites in IDPs. Here, we show that a minimal small-molecule binding sequence of eight contiguous residues derived from the Myc protein can be ported into a different disordered protein and recapitulate small-molecule binding activity in the new context. We also find that the residue immediately flanking the binding site can have opposing effects on small-molecule binding in the different disordered protein contexts. The results demonstrate that small-molecule binding sites can act modularly and are portable between disordered protein contexts but that residues outside of the minimal binding site can modulate binding affinity.

Specific and Fuzzy Interactions Cooperate in Modulating Protein Half-Life

Protein degradation is critical for maintaining cellular homeostasis. The 20S proteasome is selective for unfolded, extended polypeptide chains without ubiquitin tags. Sequestration of such segments by protein partners, however, may provide a regulatory mechanism. Here we used the AP-1 complex to study how c-Fos turnover is controlled by interactions with c-Jun. We show that heterodimerization with c-Jun increases c-Fos half-life. Mutations affecting specific contact sites (L165V, L172V) or charge separation (E175D, E189D, K190R) with c-Jun both modulate c-Fos turnover, proportionally to their impact on binding affinity. The fuzzy tail beyond the structured b-HLH/ZIP domain (~165 residues) also contributes to the stabilization of the AP-1 complex, removal of which decreases c-Fos half-life. Thus, protein turnover by 20S proteasome is fine-tuned by both specific and fuzzy interactions, consistently with the previously proposed "nanny" model.

Fuzziness in Protein Interactions-A Historical Perspective

The proposal that coupled folding to binding is not an obligatory mechanism for intrinsically disordered (ID) proteins was put forward 10 years ago. The notion of fuzziness implies that conformational heterogeneity can be maintained upon interactions of ID proteins, which has a functional impact either on regulated assembly or activity of the corresponding complexes. Here I review how the concept has evolved in the past decade, via increasing experimental data providing insights into the mechanisms, pathways and regulatory modes. The effects of structural diversity and transient contacts on protein assemblies have been collected and systematically analyzed (Fuzzy Complexes Database, http://protdyn-database.org). Fuzziness has also been exploited as a framework to decipher molecular organization of higher-order protein structures. Quantification of conformational heterogeneity opens exciting future perspectives for drug discovery from small molecule-ID protein interactions to supramolecular assemblies.

Fuzzy complexes: Specific binding without complete folding

Specific molecular recognition is assumed to require a well-defined set of contacts and devoid of conformational and interaction ambiguities. Growing experimental evidence demonstrates however, that structural multiplicity or dynamic disorder can be retained in protein complexes, termed as fuzziness. Fuzzy regions establish alternative contacts between specific partners usually via transient interactions. Nature often tailors the dynamic properties of these segments via post-translational modifications or alternative splicing to fine-tune affinity. Most experimentally characterized fuzzy complexes are involved in regulation of gene-expression, signal transduction and cell-cycle regulation. Fuzziness is also characteristic to viral protein complexes, cytoskeleton structure, and surprisingly in a few metabolic enzymes. A plausible role of fuzzy complexes in increasing half-life of intrinsically disordered proteins is also discussed.

Peptidylprolyl Isomerase Pin1 Directly Enhances the DNA Binding Functions of Estrogen Receptor α

The transcriptional activity of estrogen receptor α (ERα), the key driver of breast cancer proliferation, is enhanced by multiple cellular interactions, including phosphorylation-dependent interaction with Pin1, a proline isomerase, which mediates cis-trans isomerization of the N-terminal Ser(P)(118)-Pro(119) in the intrinsically disordered AF1 (activation function 1) domain of ERα. Because both ERα and Pin1 have multiple cellular partners, it is unclear how Pin1 assists in the regulation of ERα transactivation mechanisms and whether the functional effects of Pin1 on ERα signaling are direct or indirect. Here, we tested the specific action of Pin1 on an essential step in ERα transactivation, binding to specific DNA sites. DNA binding analysis demonstrates that stable overexpression of Pin1 increases endogenous ERα DNA binding activity when activated by estrogen but not by tamoxifen or EGF. Increased DNA binding affinity is a direct effect of Pin1 on ERα because it is observed in solution-based assays with purified components. Further, our data indicate that isomerization is required for Pin1-modulation of ERα-DNA interactions. In an unbiased in vitro DNA binding microarray with hundreds of thousands of permutations of ERα-binding elements, Pin1 selectively enhances the binding affinity of ERα to consensus DNA elements. These studies reveal that Pin1 isomerization of phosphorylated ERα can directly regulate the function of the adjacent DNA binding domain, and this interaction is further modulated by ligand binding in the ligand-binding domain, providing evidence for Pin1-dependent allosteric regulation of ERα function.

Dynamic protein-DNA recognition: beyond what can be seen

Traditionally, specific DNA recognition is thought to rely on static contacts with the bases or phosphates. Recent results, however, indicate that residues far outside the binding context can crucially influence selectivity or binding affinity via transient, dynamic interactions with the DNA binding interface. These regions usually do not adopt a well-defined structure, even when bound to DNA, and thus form a fuzzy complex. Here, we propose the existence of a dynamic DNA readout mechanism, wherein distant segments modulate conformational preferences, flexibility or spacing of the DNA binding motifs or serve as competitive partners. Despite their low sequence similarity, these intrinsically disordered regions are often conserved at the structural level, and exploited for regulation of the transcription machinery via protein-protein interactions, post-translational modifications or alternative splicing.

Biochemical and structural properties of zebrafish Capsulin produced by Escherichia coli

Capsulin is one of the transcription factors involved in regulating cell differentiation but its biochemical properties and structural characteristics are still unclear. In the present study, we cloned capsulin from zebrafish, which produces large numbers of transparent embryos and has well-characterized developmental stages. By alignment, the deduced amino acid sequence of zebrafish Capsulin, which contains a putative bHLH motif, shares very high homology to that of other species with an 72-82% identity. Zebrafish Capsulin was also targeted to the nucleus of mammalian cells when overexpressed by transient transfection. In order to characterize the structural and biochemical properties of zebrafish Capsulin, a recombinant zebrafish Capsulin protein was expressed and purified in Escherichia coli. By circular dichroism spectroscopy, Capsulin was shown to be 55% α-helical. The size distribution assay by analytical ultracentrifugation indicated that it existed as a monomer-dimer mixture. The results suggested that the recombinant Capsulin has a well-organized and functional structure. Finally, endogenous Capsulin was distributed mainly in the epicardial cells of zebrafish by immunohistochemistry analysis using antibodies raised against zebrafish Capsulin. The present study not only helps us to comparatively analyze capsulin genes across species, but it also provides valuable structural information for further studies of Capsulin biological function in the future.