Structural mechanism for modulation of functional amyloid and biofilm formation by Staphylococcal Bap protein switch.

The Staphylococcal Bap proteins sense environmental signals (such as pH, [Ca2+ ]) to build amyloid scaffold biofilm matrices via unknown mechanisms. We here report the crystal structure of the aggregation-prone region of Staphylococcus aureus Bap which adopts a dumbbell-shaped fold. The middle module (MM) connecting the N-terminal and C-terminal lobes consists of a tandem of novel double-Ca2+ -binding motifs involved in cooperative interaction networks, which undergoes Ca2+ -dependent order-disorder conformational switches. The N-terminal lobe is sufficient to mediate amyloid aggregation through liquid-liquid phase separation and maturation, and subsequent biofilm formation under acidic conditions. Such processes are promoted by disordered MM at low [Ca2+ ] but inhibited by ordered MM stabilized by Ca2+ binding, with inhibition efficiency depending on structural integrity of the interaction networks. These studies illustrate a novel protein switch in pathogenic bacteria and provide insights into the mechanistic understanding of Bap proteins in modulation of functional amyloid and biofilm formation, which could be implemented in the anti-biofilm drug design.

Structure and regulation of human epithelial cell transforming 2 protein.

Epithelial cell transforming 2 (Ect2) protein activates Rho GTPases and controls cytokinesis and many other cellular processes. Dysregulation of Ect2 is associated with various cancers. Here, we report the crystal structure of human Ect2 and complementary mechanistic analyses. The data show the C-terminal PH domain of Ect2 folds back and blocks the canonical RhoA-binding site at the catalytic center of the DH domain, providing a mechanism of Ect2 autoinhibition. Ect2 is activated by binding of GTP-bound RhoA to the PH domain, which suggests an allosteric mechanism of Ect2 activation and a positive-feedback loop reinforcing RhoA signaling. This bimodal RhoA binding of Ect2 is unusual and was confirmed with Förster resonance energy transfer (FRET) and hydrogen-deuterium exchange mass spectrometry (HDX-MS) analyses. Several recurrent cancer-associated mutations map to the catalytic and regulatory interfaces, and dysregulate Ect2 in vitro and in vivo. Together, our findings provide mechanistic insights into Ect2 regulation in normal cells and under disease conditions.

The microtubule end-binding affinity of EB1 is enhanced by a dimeric organization that is susceptible to phosphorylation.

In cells, microtubule dynamics are regulated by plus-end tracking proteins (+TIPs). End-binding protein 1 (EB1, also known as MAPRE1) acts as a master regulator of +TIP networks by targeting the growing ends of microtubules and recruiting other factors. However, the molecular mechanism underlying high-affinity binding of EB1 to microtubule ends remains an open area of research. Using single-molecule imaging, we show that the end-binding kinetics of EB1 change when the polymerization and hydrolysis rates of tubulin dimers are altered, confirming that EB1 binds to GTP-tubulin and/or GDP-Pi-tubulin at microtubule growing ends. The affinity of wild-type EB1 to these sites is higher than that of monomeric EB1 mutants, suggesting that both calponin homology domains present in the EB1 dimer contribute to end binding. Introduction of phosphomimetic mutations into the EB1 linker domain weakens the end-binding affinity and confers a more curved conformation on the EB1 dimer without compromising dimerization, suggesting that the overall architecture of EB1 is important for its end-binding affinity. Taken together, our results provide insights into how the high-affinity end-binding of EB1 is achieved and how this activity may be regulated in cells.

Long non-coding subgenomic flavivirus RNAs have extended 3D structures and are flexible in solution.

Most mosquito-borne flaviviruses, including Zika virus (ZIKV), Dengue virus (DENV), and West Nile virus (WNV), produce long non-coding subgenomic RNAs (sfRNAs) in infected cells that link to pathogenicity and immune evasion. Until now, the structural characterization of these lncRNAs remains limited. Here, we studied the 3D structures of individual and combined subdomains of sfRNAs, and visualized the accessible 3D conformational spaces of complete sfRNAs from DENV2, ZIKV, and WNV by small angle X-ray scattering (SAXS) and computational modeling. The individual xrRNA1s and xrRNA2s adopt similar structures in solution as the crystal structure of ZIKV xrRNA1, and all xrRNA1-2s form compact structures with reduced flexibility. While the DB12 of DENV2 is extended, the DB12s of ZIKV and WNV are compact due to the formation of intertwined double pseudoknots. All 3' stem-loops (3'SLs) share similar rod-like structures. Complete sfRNAs are extended and sample a large conformational space in solution. Our work not only provides structural insight into the function of flavivirus sfRNAs, but also highlights strategies of visualizing other lncRNAs in solution by SAXS and computational methods.

The effect of phosphate ion on the ssDNA binding mode of MoSub1, a Sub1/PC4 homolog from rice blast fungus.

MoSub1 is an ortholog of yeast single stranded DNA binding protein Sub1 or human PC4 from rice blast fungus. All of them share a similar DNA binding region and may have similar biological roles. The well-studied Sub1/PC4 has been reported to play multiple roles in DNA metabolic processes, such as transcription and DNA repair and their DNA binding capacity is significantly affected by phosphorylation. Here, we determined the crystal structure of MoSub1 complexed with ssDNA in a phosphate solution. The crystal structure of the MoSub1-ssDNA complex was solved to a resolution of 2.04 Å. A phosphate ion at the interface of the protein-DNA interaction of the complex bridged the lys84 of the protein and two nucleotides. The DNA was bound in novel mode (L mode) in the MoSub1 complex in the presence of phosphate ions, while DNA bound in the straight mode in the absence of the phosphate ion and in U mode in the same binding motif of the PC4-ssDNA complex. The crystal structure of the complex and a small-angle X-ray scattering analysis revealed that the phosphate ion at the protein-DNA interface affected the DNA binding mode of MoSub1 to oligo-DNA and provided a new structural clue for studying its functions.

Capture and Release of tRNA by the T-Loop Receptor in the Function of the T-Box Riboswitch.

In Gram-positive bacteria, the tRNA-dependent T-box riboswitch system regulates expression of amino acid biosynthetic and aminoacyl-tRNA synthetase genes through a transcription attenuation mechanism. Binding of uncharged tRNA "closes" the switch, allowing transcription read-through. Structural studies of the 100-nucleotide stem I domain reveal tRNA utilizes base pairing and stacking interactions to bind the stem, but little is known structurally about the 180-nucleotide riboswitch core (stem I, stem III, and antiterminator stem) in complex with tRNA or the mechanism of coupling of the intermolecular binding domains crucial to T-box function. Here we utilize solution structural and biophysical methods to characterize the interplay of the different riboswitch-tRNA contact points using Bacillus subtilis and Oceanobacillus iheyensis glycyl T-box and T-box:tRNA constructs. The data reveal that tRNA:riboswitch core binding at equilibrium involves only Specifier-anticodon and antiterminator-acceptor stem pairing. The elbow:platform stacking interaction observed in studies of the T-box stem I domain is released after pairing between the acceptor stem and the bulge in the antiterminator helix. The results are consistent with the model of T-box riboswitch:tRNA function in which tRNA is captured by stem I of the nascent mRNA followed by stabilization of the antiterminator helix and the paused transcription complex.