Probing transient excited states of the bacterial cell division regulator MinE by relaxation dispersion NMR spectroscopy

Metamorphic proteins and how to find them

In the last two decades, our existing notion that most foldable proteins have a unique native state has been challenged by the discovery of metamorphic proteins, which reversibly interconvert between multiple, sometimes highly dissimilar, native states. As the number of known metamorphic proteins increases, several computational and experimental strategies have emerged for gaining insights about their refolding processes and identifying unknown metamorphic proteins amongst the known proteome. In this review, we describe the current advances in biophysically and functionally ascertaining the structural interconversions of metamorphic proteins and how coevolution can be harnessed to identify novel metamorphic proteins from sequence information. We also discuss the challenges and ongoing efforts in using artificial intelligence-based protein structure prediction methods to discover metamorphic proteins and predict their corresponding three-dimensional structures.

The mechanism of MinD stability modulation by MinE in Min protein dynamics

The patterns formed both in vivo and in vitro by the Min protein system have attracted much interest because of the complexity of their dynamic interactions given the apparent simplicity of the component parts. Despite both the experimental and theoretical attention paid to this system, the details of the biochemical interactions of MinD and MinE, the proteins responsible for the patterning, are still unclear. For example, no model consistent with the known biochemistry has yet accounted for the observed dual role of MinE in the membrane stability of MinD. Until now, a statistical comparison of models to the time course of Min protein concentrations on the membrane has not been carried out. Such an approach is a powerful way to test existing and novel models that are difficult to test using a purely experimental approach. Here, we extract time series from previously published fluorescence microscopy time lapse images of in vitro experiments and fit two previously described and one novel mathematical model to the data. We find that the novel model, which we call the Asymmetric Activation with Bridged Stability Model, fits the time-course data best. It is also consistent with known biochemistry and explains the dual MinE role via MinE-dependent membrane stability that transitions under the influence of rising MinE to membrane instability with positive feedback. Our results reveal a more complex network of interactions between MinD and MinE underlying Min-system dynamics than previously considered.

Intrinsic structural dynamics dictate enzymatic activity and inhibition

Enzymes are known to sample various conformations, many of which are critical for their biological function. However, structural characterizations of enzymes predominantly focus on the most populated conformation. As a result, single-point mutations often produce structures that are similar or essentially identical to those of the wild-type enzyme despite large changes in enzymatic activity. Here, we show for mutants of a histone deacetylase enzyme (HDAC8) that reduced enzymatic activities, reduced inhibitor affinities, and reduced residence times are all captured by the rate constants between intrinsically sampled conformations that, in turn, can be obtained independently by solution NMR spectroscopy. Thus, for the HDAC8 enzyme, the dynamic sampling of conformations dictates both enzymatic activity and inhibitor potency. Our analysis also dissects the functional role of the conformations sampled, where specific conformations distinct from those in available structures are responsible for substrate and inhibitor binding, catalysis, and product dissociation. Precise structures alone often do not adequately explain the effect of missense mutations on enzymatic activity and drug potency. Our findings not only assign functional roles to several conformational states of HDAC8 but they also underscore the paramount role of dynamics, which will have general implications for characterizing missense mutations and designing inhibitors.

Fluid protein fold space and its implications

Fold-switching proteins, which remodel their secondary and tertiary structures in response to cellular stimuli, suggest a new view of protein fold space. For decades, experimental evidence has indicated that protein fold space is discrete: dissimilar folds are encoded by dissimilar amino acid sequences. Challenging this assumption, fold-switching proteins interconnect discrete groups of dissimilar protein folds, making protein fold space fluid. Three recent observations support the concept of fluid fold space: (1) some amino acid sequences interconvert between folds with distinct secondary structures, (2) some naturally occurring sequences have switched folds by stepwise mutation, and (3) fold switching is evolutionarily selected and likely confers advantage. These observations indicate that minor amino acid sequence modifications can transform protein structure and function. Consequently, proteomic structural and functional diversity may be expanded by alternative splicing, small nucleotide polymorphisms, post-translational modifications, and modified translation rates.

Distinguishing features of fold-switching proteins

Though many folded proteins assume one stable structure that performs one function, a small-but-increasing number remodel their secondary and tertiary structures and change their functions in response to cellular stimuli. These fold-switching proteins regulate biological processes and are associated with autoimmune dysfunction, severe acute respiratory syndrome coronavirus-2 infection, and more. Despite their biological importance, it is difficult to computationally predict fold switching. With the aim of advancing computational prediction and experimental characterization of fold switchers, this review discusses several features that distinguish fold-switching proteins from their single-fold and intrinsically disordered counterparts. First, the isolated structures of fold switchers are less stable and more heterogeneous than single folders but more stable and less heterogeneous than intrinsically disordered proteins (IDPs). Second, the sequences of single fold, fold switching, and intrinsically disordered proteins can evolve at distinct rates. Third, proteins from these three classes are best predicted using different computational techniques. Finally, late-breaking results suggest that single folders, fold switchers, and IDPs have distinct patterns of residue-residue coevolution. The review closes by discussing high-throughput and medium-throughput experimental approaches that might be used to identify new fold-switching proteins.

Interaction of the bacterial division regulator MinE with lipid bicelles studied by NMR spectroscopy

The bacterial MinE and MinD division regulatory proteins form a standing wave enabling MinC, which binds MinD, to inhibit FtsZ polymerization everywhere except at the midcell, thereby assuring correct positioning of the cytokinetic septum and even distribution of contents to daughter cells. The MinE dimer undergoes major structural rearrangements between a resting six-stranded state present in the cytoplasm, a membrane-bound state, and a four-stranded active state bound to MinD on the membrane, but it is unclear which MinE motifs interact with the membrane in these different states. Using NMR, we probe the structure and global dynamics of MinE bound to disc-shaped lipid bicelles. In the bicelle-bound state, helix α1 no longer sits on top of the six-stranded β-sheet, losing any contact with the protein core, but interacts directly with the bicelle surface; the structure of the protein core remains unperturbed and also interacts with the bicelle surface via helix α2. Binding may involve a previously identified excited state of free MinE in which helix α1 is disordered, thereby allowing it to target the membrane surface. Helix α1 and the protein core undergo nanosecond rigid body motions of differing amplitudes in the plane of the bicelle surface. Global dynamics on the sub-millisecond time scale between a ground state and a sparsely populated excited state are also observed and may represent a very early intermediate on the transition path between the resting six-stranded and active four-stranded conformations. In summary, our results provide insights into MinE structural rearrangements important during bacterial cell division.

Three-dimensional chiral morphodynamics of chemomechanical active shells

Morphogenesis of active shells such as cells is a fundamental chemomechanical process that often exhibits three-dimensional (3D) large deformations and chemical pattern dynamics simultaneously. Here, we establish a chemomechanical active shell theory accounting for mechanical feedback and biochemical regulation to investigate the symmetry-breaking and 3D chiral morphodynamics emerging in the cell cortex. The active bending and stretching of the elastic shells are regulated by biochemical signals like actomyosin and RhoA, which, in turn, exert mechanical feedback on the biochemical events via deformation-dependent diffusion and inhibition. We show that active deformations can trigger chemomechanical bifurcations, yielding pulse spiral waves and global oscillations, which, with increasing mechanical feedback, give way to traveling or standing waves subsequently. Mechanical feedback is also found to contribute to stabilizing the polarity of emerging patterns, thus ensuring robust morphogenesis. Our results reproduce and unravel the experimentally observed solitary and multiple spiral patterns, which initiate asymmetric cleavage in Xenopus and starfish embryogenesis. This study underscores the crucial roles of mechanical feedback in cell development and also suggests a chemomechanical framework allowing for 3D large deformation and chemical signaling to explore complex morphogenesis in living shell-like structures.

A sequence-based method for predicting extant fold switchers that undergo α-helix ↔ β-strand transitions

Extant fold‐switching proteins remodel their secondary structures and change their functions in response to cellular stimuli, regulating biological processes and affecting human health. Despite their biological importance, these proteins remain understudied. Predictive methods are needed to expedite the process of discovering and characterizing more of these shapeshifting proteins. Most previous approaches require a solved structure or all‐atom simulations, greatly constraining their use. Here, we propose a high‐throughput sequence‐based method for predicting extant fold switchers that transition from α‐helix in one conformation to β‐strand in the other. This method leverages two previous observations: (a) α‐helix ↔ β‐strand prediction discrepancies from JPred4 are a robust predictor of fold switching, and (b) the fold‐switching regions (FSRs) of some extant fold switchers have different secondary structure propensities when expressed by themselves (isolated FSRs) than when expressed within the context of their parent protein (contextualized FSRs). Combining these two observations, we ran JPred4 on 99‐fold‐switching proteins and found strong correspondence between predicted and experimentally observed α‐helix ↔ β‐strand discrepancies. To test the overall robustness of this finding, we randomly selected regions of proteins not expected to switch folds (single‐fold proteins) and found significantly fewer predicted α‐helix ↔ β‐strand discrepancies. Combining these discrepancies with the overall percentage of predicted secondary structure, we developed a classifier to identify extant fold switchers (Matthews correlation coefficient of .71). Although this classifier had a high false‐negative rate (7/17), its false‐positive rate was very low (2/136), suggesting that it can be used to predict a subset of extant fold switchers from a multitude of available genomic sequences.

A simple and cost-effective protocol for high-yield expression of deuterated and selectively isoleucine/leucine/valine methyl protonated proteins in Escherichia coli grown in shaker flasks

A simple and cost-effective protocol is presented for expression of perdeuterated, Ile/Leu/Val ¹H/¹³C methyl protonated proteins from 100 ml cultures in M9 ++ /D₂O medium induced at high (OD₆₀₀ ~ 10) cell density in shaker flasks. This protocol, which is an extension of our previous protocols for expression of ²H/¹⁵N/¹³C and ¹H/¹³C labeled proteins, yields comparable quantities of protein from 100 ml cell culture to those obtained using a conventional 1 L culture with M9/D₂O medium, while using three-fold less α-ketoisovaleric (1,2,3,4-¹³C₄; 3,4',4',4'-d₄) and α-ketobutyric (¹³C₄; 3,3-d₂) acid precursors.

An S/T motif controls reversible oligomerization of the Hsp40 chaperone DNAJB6b through subtle reorganization of a β sheet backbone

Chaperone oligomerization is often a key aspect of their function. Irrespective of whether chaperone oligomers act as reservoirs for active monomers or exhibit a chaperoning function themselves, understanding the mechanism of oligomerization will further our understanding of how chaperones maintain the proteome. Here, we focus on the class-II Hsp40, human DNAJB6b, a highly efficient inhibitor of protein self-assembly in vivo and in vitro that forms functional oligomers. Using single-quantum methyl-based relaxation dispersion NMR methods we identify critical residues for DNAJB6b oligomerization in its C-terminal domain (CTD). Detailed solution NMR studies on the structure of the CTD showed that a serine/threonine-rich stretch causes a backbone twist in the N-terminal β strand, stabilizing the monomeric form. Quantitative analysis of an array of NMR relaxation-based experiments (including Carr-Purcell-Meiboom-Gill relaxation dispersion, off-resonance R _1ρ profiles, lifetime line broadening, and exchange-induced shifts) on the CTD of both wild type and a point mutant (T142A) within the S/T region of the first β strand delineates the kinetics of the interconversion between the major twisted-monomeric conformation and a more regular β strand configuration in an excited-state dimer, as well as exchange of both monomer and dimer species with high-molecular-weight oligomers. These data provide insights into the molecular origins of DNAJB6b oligomerization. Further, the results reported here have implications for the design of β sheet proteins with tunable self-assembling properties and pave the way to an atomic-level understanding of amyloid inhibition.

Functional and Regulatory Roles of Fold-Switching Proteins

Fold-switching proteins respond to cellular stimuli by remodeling their secondary structures and changing their functions. Whereas several previous reviews have focused on various structural, physical-chemical, and evolutionary aspects of this newly emerging class of proteins, this minireview focuses on how fold switching modulates protein function and regulates biological processes. It first compares and contrasts fold switchers with other known types of proteins. Second, it presents examples of how various proteins can change their functions through fold switching. Third, it demonstrates that fold switchers can regulate biological processes by discussing two proteins, RfaH and KaiB, whose dramatic secondary structure remodeling events directly affect gene expression and a circadian clock, respectively. Finally, this minireview discusses how the field of protein fold switching might advance.

Conformational equilibrium of MinE regulates the allowable concentration ranges of a protein wave for cell division

The Min system for determining the cell division position at the center in bacteria has a unique character that uses a protein wave (Min wave) that emerges from its components (MinD and MinE). The Min wave emerges under the coupling of chemical reactions and molecular diffusions of MinDE and appears when the concentrations of MinD and MinE are similar. However, the nanoscale mechanism to determine their concentration ranges has remained elusive. In this study, by using artificial cells as a mimic of cells, we showed that the dominant MinE conformations determined the allowable concentration ranges for the emergence of the Min wave. Furthermore, the deletion of the membrane-binding region of MinE indicated that the region was essential for limiting the concentration ranges to be narrower. These findings illustrate a parameter tuning mechanism underlying complex molecular systems at the nanoscale for spatiotemporal regulation in living cells and show a possibility that the regulation of the equilibrium among molecular conformations can work as a switch for cell division.

Local Self-Enhancement of MinD Membrane Binding in Min Protein Pattern Formation

The proteins MinD, MinE and MinC are constitutive for the spatiotemporal organization of cell division in Escherichia coli, in particular, for positioning the division machinery at mid-cell. To achieve this function, the ATPase MinD and the ATPase-activating protein MinE undergo coordinated pole-to-pole oscillations and have thus become a paradigm for protein pattern formation in biology. The exact molecular mechanisms enabling MinDE self-organization, and particularly the role of cooperativity in the membrane binding of MinD, thought to be a key requirement, have remained poorly understood. However, for bottom-up synthetic biology aiming at a de novo design of key cellular features, elucidating these mechanisms is of great relevance. By combining in vitro reconstitution with rationally guided mutagenesis of MinD, we found that when bound to membranes, MinD displays new interfaces for multimerization, which are distinct from the canonical MinD dimerization site. We propose that these additional transient interactions contribute to the local self-enhancement of MinD at the membrane, while their relative lability maintains the structural plasticity required for MinDE wave propagation. This could represent a powerful structural regulation feature not reported so far for self-organizing proteins.