Segmental isotope-labeling of the intrinsically disordered protein PQBP1

From Immunogenic Peptides to Intrinsically Disordered Proteins

It is hard to evaluate the role of individual mentors in the genesis of important ideas. In the case of our realization that proteins do not have to be stably folded to be functional, the influence of Richard Lerner and our collaborative work in the 1980s on the conformations of immunogenic peptides provided a base level of thinking about the nature of polypeptides in water solutions that led us to formulate and develop our ideas on the importance of intrinsic disorder in proteins. This review describes how the insights gained into the behavior of peptides led directly to the realization that proteins were not only capable of being functional while disordered, but also that disorder provided a distinct functional advantage in many important cellular processes.

Sortase-mediated segmental labeling: A method for segmental assignment of intrinsically disordered regions in proteins

A significant number of proteins possess sizable intrinsically disordered regions (IDRs). Due to the dynamic nature of IDRs, NMR spectroscopy is often the tool of choice for characterizing these segments. However, the application of NMR to IDRs is often hindered by their instability, spectral overlap and resonance assignment difficulties. Notably, these challenges increase considerably with the size of the IDR. In response to these issues, here we report the use of sortase-mediated ligation (SML) for segmental isotopic labeling of IDR-containing samples. Specifically, we have developed a ligation strategy involving a key segment of the large IDR and adjacent folded headpiece domain comprising the C-terminus of A. thaliana villin 4 (AtVLN4). This procedure significantly reduces the complexity of NMR spectra and enables group identification of signals arising from the labeled IDR fragment, a process we refer to as segmental assignment. The validity of our segmental assignment approach is corroborated by backbone residue-specific assignment of the IDR using a minimal set of standard heteronuclear NMR methods. Using segmental assignment, we further demonstrate that the IDR region adjacent to the headpiece exhibits nonuniform spectral alterations in response to temperature. Subsequent residue-specific characterization revealed two segments within the IDR that responded to temperature in markedly different ways. Overall, this study represents an important step toward the selective labeling and probing of target segments within much larger IDR contexts. Additionally, the approach described offers significant savings in NMR recording time, a valuable advantage for the study of unstable IDRs, their binding interfaces, and functional mechanisms.

Fatal Attraction: The Case of Toxic Soluble Dimers of Truncated PQBP-1 Mutants in X-Linked Intellectual Disability

The frameshift mutants K192S^fs*7 and R153S^fs*41, of the polyglutamine tract-binding protein 1 (PQBP-1), are stable intrinsically disordered proteins (IDPs). They are each associated with the severe cognitive disorder known as the Renpenning syndrome, a form of X-linked intellectual disability (XLID). Relative to the monomeric wild-type protein, these mutants are dimeric, contain more folded contents, and have higher thermal stabilities. Comparisons can be drawn to the toxic oligomerisation in the “conformational diseases”, which collectively describe medical conditions involving a substantial protein structural transition in the pathogenic mechanism. At the molecular level, the end state of these diseases is often cytotoxic protein aggregation. The conformational disease proteins contain varying extents of intrinsic disorder, and the consensus pathogenesis includes an early oligomer formation. We reviewed the experimental characterisation of the toxic oligomers in representative cases. PQBP-1 mutant dimerisation was then compared to the oligomerisation of the conformational disease proteins. The PQBP-1 mutants are unique in behaving as stable soluble dimers, which do not further develop into higher oligomers or aggregates. The toxicity of the PQBP-1 mutant dimers lies in the native functions (in transcription regulation and possibly, RNA splicing) being compromised, rather than proceeding to aggregation. Other examples of stable IDP dimers were discussed and we speculated on the roles of IDP dimerisation in protein evolution.

NMR Structure and Dynamics of TonB Investigated by Scar-Less Segmental Isotopic Labeling Using a Salt-Inducible Split Intein

The growing understanding of partially unfolded proteins increasingly points to their biological relevance in allosteric regulation, complex formation, and protein design. However, the structural characterization of disordered proteins remains challenging. NMR methods can access both the dynamics and structures of such proteins, yet suffering from a high degeneracy of NMR signals. Here, we overcame this bottleneck utilizing a salt-inducible split intein to produce segmentally isotope-labeled samples with the native sequence, including the ligation junction. With this technique, we investigated the NMR structure and conformational dynamics of TonB from Helicobacter pylori in the presence of a proline-rich low complexity region. Spin relaxation experiments suggest that the several nano-second time scale dynamics of the C-terminal domain (CTD) is almost independent of the faster pico-to-nanosecond dynamics of the low complexity central region (LCCR). Our results demonstrate the utility of segmental isotopic labeling for proteins with heterogenous dynamics such as TonB and could advance NMR studies of other partially unfolded proteins.

Methods and Applications of Expressed Protein Ligation

Expressed protein ligation is a method of protein semisynthesis and typically involves the reaction of recombinant protein C-terminal thioesters with N-cysteine containing synthetic peptides in a chemoselective ligation. The recombinant protein C-terminal thioesters are produced by exploiting the action of nature's inteins which are protein modules that catalyze protein splicing. This chapter discusses the basic principles of expressed protein ligation and recent advances and applications in this protein semisynthesis field. Comparative strengths and weaknesses of the method and future challenges are highlighted.

Frameshift PQBP-1 mutants K192Sfs*7 and R153Sfs*41 implicated in X-linked intellectual disability form stable dimers

Polyglutamine tract-binding protein-1 (PQBP-1) is a nuclear intrinsically disordered protein playing important roles in transcriptional regulation and RNA splicing during embryonic and postembryonic development. In human, its mutations lead to severe cognitive impairment known as the Renpenning syndrome, a form of X-linked intellectual disability (XLID). Here, we report a combined biophysical study of two PQBP-1 frameshift mutants, K192S^fs*7 and R153S^fs*41. Both mutants are dimeric in solution, in contrast to the monomeric wild-type protein. These mutants contain more folded contents and have increased thermal stabilities. Using small-angle X-ray scattering data, we generated three-dimensional envelopes which revealed their overall flat shapes. We also described each mutant using an ensemble model based on a native-like initial pool with a dimeric structural core. PQBP-1 is known to repress transcription by way of interacting with the C-terminal domain of RNA polymerase II, which consists of 52 repeats of a consensus heptapeptide sequence YSPTSPS. We studied the binding of PQBP-1 variants to the labelled peptide which is phosphorylated at positions 2 and 5 (YpSPTpSPS) and found that this interaction is significantly weakened in the two mutants.

A WW Domain-Containing Protein Forms Immune Nuclear Bodies against Begomoviruses

The bipartite begomoviruses (Geminiviridae family), which are DNA viruses that replicate in the nucleus of infected cells, encode the nuclear shuttle protein (NSP) to facilitate the translocation of viral DNA from the nucleus to the cytoplasm via nuclear pores. This intracellular trafficking of NSP-DNA complexes is accessorized by the NSP-interacting guanosine triphosphatase (NIG) at the cytosolic side. Here, we report the nuclear redistribution of NIG by AtWWP1, a WW domain-containing protein that forms immune nuclear bodies (NBs) against begomoviruses. We demonstrated that AtWWP1 relocates NIG from the cytoplasm to the nucleus where it is confined to AtWWP1-NBs, suggesting that the NIG-AtWWP1 interaction may interfere with the NIG pro-viral function associated with its cytosolic localization. Consistent with this assumption, loss of AtWWP1 function cuased plants more susceptible to begomovirus infection, whereas overexpression of AtWWP1 enhanced plant resistance to begomovirus. Furthermore, we found that a mutant version of AtWWP1 defective for NB formation was no longer capable of interacting with and relocating NIG to the nucleus and lost its immune function against begomovirus. The antiviral function of AtWWP1-NBs, however, could be antagonized by viral infection that induced either the disruption or a decrease in the number of AtWWP1-NBs. Collectively, these results led us to propose that AtWWP1 organizes nuclear structures into nuclear foci, which provide intrinsic immunity against begomovirus infection.

Long-range regulation of p53 DNA binding by its intrinsically disordered N-terminal transactivation domain

Atomic resolution characterization of the full-length p53 tetramer has been hampered by its size and the presence of extensive intrinsically disordered regions at both the N and C termini. As a consequence, the structural characteristics and dynamics of the disordered regions are poorly understood within the context of the intact p53 tetramer. Here we apply trans-intein splicing to generate segmentally ¹⁵N-labeled full-length p53 constructs in which only the resonances of the N-terminal transactivation domain (NTAD) are visible in NMR spectra, allowing us to observe this region of p53 with unprecedented detail within the tetramer. The N-terminal region is dynamically disordered in the full-length p53 tetramer, fluctuating between states in which it is free and fully exposed to solvent and states in which it makes transient contacts with the DNA-binding domain (DBD). Chemical-shift changes and paramagnetic spin-labeling experiments reveal that the amphipathic AD1 and AD2 motifs of the NTAD interact with the DNA-binding surface of the DBD through primarily electrostatic interactions. Importantly, this interaction inhibits binding of nonspecific DNA to the DBD while having no effect on binding to a specific p53 recognition element. We conclude that the NTAD:DBD interaction functions to enhance selectivity toward target genes by inhibiting binding to nonspecific sites in genomic DNA. This work provides some of the highest-resolution data on the disordered N terminus of the nearly 180-kDa full-length p53 tetramer and demonstrates a regulatory mechanism by which the N terminus of p53 transiently interacts with the DBD to enhance target site discrimination.

Combining DNP NMR with segmental and specific labeling to study a yeast prion protein strain that is not parallel in-register

The yeast prion protein Sup35NM is a self-propagating amyloid. Despite intense study, there is no consensus on the organization of monomers within Sup35NM fibrils. Some studies point to a β-helical arrangement, whereas others suggest a parallel in-register organization. Intermolecular contacts are often determined by experiments that probe long-range heteronuclear contacts for fibrils templated from a 1:1 mixture of ¹³C- and ¹⁵N-labeled monomers. However, for Sup35NM, like many large proteins, chemical shift degeneracy limits the usefulness of this approach. Segmental and specific isotopic labeling reduce degeneracy, but experiments to measure long-range interactions are often too insensitive. To limit degeneracy and increase experimental sensitivity, we combined specific and segmental isotopic labeling schemes with dynamic nuclear polarization (DNP) NMR. Using this combination, we examined an amyloid form of Sup35NM that does not have a parallel in-register structure. The combination of a small number of specific labels with DNP NMR enables determination of architectural information about polymeric protein systems.

Segmental Isotopic Labeling of Proteins for NMR Study Using Intein Technology

Segmental isotopic labeling of samples for NMR studies is attractive for large complex biomacromolecular systems, especially for studies of function-related protein-ligand interactions and protein dynamics (Goto and Kay, Curr Opin Struct Biol 10:585-592, 2000; Rosa et al., Molecules (Basel, Switzerland) 18:440, 2013; Hiroaki, Expert Opin Drug Discovery 8:523-536, 2013). Advantages of segmental isotopic labeling include selective examination of specific segment(s) within a protein by NMR, significantly reducing the spectral complexity for large proteins, and allowing for the application of a variety of solution-based NMR strategies. By utilizing intein techniques (Wood and Camarero, J Biol Chem 289:14512-14519, 2014; Paulus, Annu Rev Biochem 69:447-496, 2000), two related approaches can generally be used in the segmental isotopic labeling of proteins: expressed protein ligation (Muir, Annu Rev Biochem 72:249-289, 2003) and protein trans-splicing (Shah et al., J Am Chem Soc 134:11338-11341, 2012). Here, we describe general implementation and latest improvements of expressed protein ligation method for the production of segmental isotopic labeled NMR samples.

Allosteric modulation of the binding affinity between PQBP1 and the spliceosomal protein U5-15kD

Polyglutamine tract-binding protein 1 (PQBP1) is an intrinsically disordered protein composed of a small folded WW domain and a long disordered region. PQBP1 binds to spliceosomal proteins WBP11 and U5-15kD through its N-terminal WW domain and C-terminal region, respectively. Here, we reveal that the binding between PQBP1 and WBP11 reduces the binding affinity between PQBP1 and U5-15kD. Our results suggest that the interaction between PQBP1 and WBP11 negatively modulates the U5-15kD binding of PQBP1 by an allosteric mechanism.

Intrinsically disordered proteins: emerging interaction specialists

Intrinsically disordered proteins or regions of proteins (IDPs/IDRs) most often function through protein-protein interactions, when they permanently or transiently bind partner molecules with diverse functional consequences. There is a rapid advance in our understanding of the ensuing functional modes, obtained from describing atomic details of individual complexes, proteome-wide studies of interactomes and characterizing loosely assembled hydrogels and tightly packed amyloids. Here we briefly survey the most important recent methodological developments and structural-functional observations, with the aim of increasing the general appreciation of IDPs/IDRs as 'interaction specialists'.