Biology subjects

Swint-Kruse, L.

Publications and source records attributed to Swint-Kruse, L..

12 recordsLinked to original sources

bioRxiv2026

RheoScale 2.0: Revealing the Hidden Roles of Protein Positions via Substitution Patterns

A central challenge in molecular biology is understanding how amino acid substitutions modulate various features of protein function and stability. To illuminate the complexities of this relationship, high-throughput (HTP) assays are increasingly used to assess site-saturating mutagenesis libraries. A common downstream analysis is to average the set of twenty outcomes at each amino acid position for comparison with structural and evolutionary features. Average values clearly identify positions that tolerate most substitutions (neutral positions) and positions where most substitutions abolish activity (toggle positions). However, average values conceal the existence of rheostat positions, where different amino acid substitutions sample a wide range of outcomes. To quantitatively identify rheostat positions, we previously developed a histogram-based analysis that we here expand by: (i) incorporating new position classes observed in experimental studies of rheostat positions; (ii) formalizing a hierarchy of class assignments; (iii) refining error-based identification of neutral positions; and (iv) statistically assessing the robustness of class assignments to changes in experimental and computational parameters. RheoScale 2.0 is implemented in Excel and newly implemented in Python for facile integration with existing HTP pipelines; all parameters are customizable. Example analyses are shown for three HTP datasets of the SARS-CoV-2 papain-like protease. Results illustrate two aspects that influence interpretation of HTP data: First, position assignments (and substitution outcomes) depend highly on the measured feature. Second, many protein positions play multiple roles in the sequence-structure-function relationship. The recognition of varied position roles will advance understanding of pathogen evolution, protein engineering, and variant interpretation for personalized medicine. SummaryRheoScale 2.0 improves how high-throughput mutational data are interpreted by identifying protein positions where amino acid substitutions act like biological dimmer switches. By enabling more nuanced assignment of position behavior, beyond neutral or deleterious outcomes, this analysis framework advances studies of sequence-structure-function relationships and has broad relevance for understanding protein evolution, engineering proteins with desired properties, and interpreting variants linked to human disease. SOFTWARE AVAILABILITYhttps://github.com/liskinsk/RheoScale-calculator

biochemistry↗

bioRxiv2025

Dissecting the effects of single amino acid substitutions in SARS-CoV2 Mpro

Successfully predicting the effects of amino acid substitutions on protein function and stability remains challenging. Recent efforts to improve computational models have included training and validation on high-throughput experimental datasets, such as those generated by deep mutational scanning (DMS) approaches. However, DMS signals typically conflate a substitutions effects on protein function with those on in vivo protein abundance; this limits the resolution of mechanistic insights that can be gleaned from DMS data. Distinguishing functional changes from abundance-related effects is particularly important for substitutions that exhibit intermediate outcomes (e.g., partial loss-of-function), which are difficult to predict. Here, we explored changes in in vivo abundance for substitutions at representative positions in the SARS-CoV-2 Main Protease (Mpro). For this study, we used previously published DMS results to identify "rheostat" positions, which are defined by having substitutions that sample a broad range of intermediate outcomes. We generated 10 substitutions at each of six positions and separately measured effects on function and abundance. Results revealed an [~]45-fold range of change for abundance, demonstrating that it can make significant contributions to DMS outcomes. Moreover, the six tested positions showed diverse substitution sensitivities for function and abundance. Some positions influenced only one parameter. Others exhibited rheostatic effects on both parameters, which to our knowledge, provides the first example of such behavior. Since effects on function and abundance may arise through different biophysical bases, these results underscore the need for datasets that independently measure these parameters in order to build predictors with enhanced mechanistic insights. ABSTRACT for broader audienceChanging one amino acid in a protein can affect its function, its abundance, or both. Understanding these separate effects will help scientists predict how protein changes alter biology, which is important for understanding pathogen evolution, improving personalized medicine, and bioengineering. This work reports a study to experimentally separate these effects for the main protease of SARS-CoV-2 and suggests strategies for building better prediction models for emerging variants of this key viral protein.

biochemistry↗

bioRxiv2023

Fructose-1-kinase has pleiotropic roles in Escherichia coli

In Escherichia coli, the master transcription regulator Catabolite Repressor Activator (Cra) regulates >100 genes in central metabolism. Cra binding to DNA is allosterically regulated by binding to fructose-1-phosphate (F-1-P), but the only documented source of F-1-P is from the concurrent import and phosphorylation of exogenous fructose. Thus, many have proposed that fructose-1,6-bisphosphate (F-1,6-BP) is also a physiological regulatory ligand. However, the role of F-1,6-BP has been widely debated. Here, we report that the E. coli enzyme fructose-1-kinase (FruK) can carry out its "reverse" reaction under physiological substrate concentrations to generate F-1-P from F-1,6-BP. We further show that FruK directly binds Cra with nanomolar affinity and forms higher order, heterocomplexes. Growth assays with a {Delta}fruK strain and fruK complementation show that FruK has a broader role in metabolism than fructose catabolism. The {Delta}fruK strain also alters biofilm formation. Since fruK itself is repressed by Cra, these newly-reported events add layers to the dynamic regulation of E. coli central metabolism that occur in response to changing nutrients. These findings might have wide-spread relevance to other {gamma}-proteobacteria, which conserve both Cra and FruK.

microbiology↗

bioRxiv2023

The intrinsically disordered transcriptional activation domain of CIITA is functionally tuneable by single substitutions: An exception or a new rule?

During protein evolution, some amino acid substitutions modulate protein function ("tuneability"). In most proteins, the tuneable range is wide and can be sampled by a set of protein variants that each contains multiple amino acid substitutions. In other proteins, the full tuneable range can be accessed by a set of variants that each contains a single substitution. Indeed, in some globular proteins, the full tuneable range can be accessed by the set of site-saturating substitutions at an individual "rheostat" position. However, in proteins with intrinsically disordered regions (IDRs), most functional studies - which would also detect tuneability - used multiple substitutions or small deletions. In disordered transcriptional activation domains (ADs), studies with multiple substitutions led to the "acidic exposure" model, which does not anticipate the existence of rheostat positions. In the few studies that did assess effects of single substitutions on AD function, results were mixed: The ADs of two full-length transcription factors did not show tuneability, whereas a fragment of a third AD was tuneable by single substitutions. Here, we tested tuneability in the AD of full-length human class II transactivator (CIITA). Sequence analyses and experiments showed that CIITAs AD is an IDR. Functional assays of singly-substituted AD variants showed that CIITAs function was highly tuneable, with outcomes not predicted by the acidic exposure model. Four tested positions showed rheostat behaviour for transcriptional activation. Thus, tuneability of different IDRs can vary widely. Future studies are needed to illuminate the biophysical features that govern whether an IDR is tuneable by single substitutions.

biochemistry↗

bioRxiv2022

Identification of a covert evolutionary pathway between two protein folds

Although homologous protein sequences are expected to adopt similar structures, some amino acid substitutions can interconvert -helices and {beta}-sheets. Such fold switching may have occurred over evolutionary history, but supporting evidence has been limited by the: (1) abundance and diversity of sequenced genes, (2) quantity of experimentally determined protein structures, and (3) assumptions underlying the statistical methods used to infer homology. Here, we overcame these barriers by applying multiple statistical methods to a family of ~600,000 bacterial response regulator proteins. We found that their homologous DNA-binding subunits assume divergent structures: helix-turn-helix versus -helix+{beta}-sheet (winged helix). Phylogenetic analyses, ancestral sequence reconstruction, and AlphaFold2 models indicated that amino acid substitutions facilitated a switch from helix-turn-helix into winged helix. This structural transformation likely expanded DNA-binding specificity. Our approach uncovers an evolutionary pathway between two protein folds and provides methodology to identify secondary structure switching in other protein families.

bioinformatics↗

bioRxiv2020

"Multiplex" rheostat positions cluster around allosterically critical regions of the lactose repressor protein

Amino acid variation at "rheostat" positions provides opportunity to modulate various aspects of protein function - such as binding affinity or allosteric coupling - across a wide range. Previously a subclass of "multiplex" rheostat positions was identified at which substitutions simultaneously modulated more than one functional parameter. Using the Miller laboratorys dataset of [~]4000 variants of lactose repressor protein (LacI), we compared the structural properties of multiplex rheostat positions with (i) "single" rheostat positions that modulate only one functional parameter, (ii) "toggle" positions that follow textbook substitution rules, and (iii) "neutral" positions that tolerate any substitution without changing function. The combined rheostat classes comprised >40% of LacI positions, more than either toggle or neutral positions. Single rheostat positions were broadly distributed over the structure. Multiplex rheostat positions structurally overlapped with positions involved in allosteric regulation. When their phenotypic outcomes were interpreted within a thermodynamic framework, functional changes at multiplex positions were uncorrelated. This suggests that substitutions lead to complex changes in the underlying molecular biophysics. Bivariable and multivariable analyses of evolutionary signals within multiple sequence alignments could not differentiate single and multiplex rheostat positions. Phylogenetic analyses - such as ConSurf - could distinguish rheostats from toggle and neutral positions. Multivariable analyses could also identify a subset of neutral positions with high probability. Taken together, these results suggest that detailed understanding of the underlying molecular biophysics, likely including protein dynamics, will be required to discriminate single and multiplex rheostat positions from each other and to predict substitution outcomes at these sites.

biochemistry↗

bioRxiv2020

A clinically-relevant polymorphism in the Na+/taurocholate cotransporting polypeptide (NTCP) occurs at a rheostat position

Conventionally, most amino acid substitutions at important protein positions are expected to abolish function. However, in several soluble-globular proteins, we identified a class of non-conserved positions for which various substitutions produced progressive functional changes; we consider these evolutionary “rheostats”. Here, we report a strong rheostat position in the integral membrane protein, Na+/taurocholate cotransporting polypeptide (NTCP), at the site of a pharmacologically-relevant polymorphism (S267F). Functional studies were performed for all 20 substitutions (“S267X”) with three substrates (taurocholate, estrone-3-sulfate and rosuvastatin). The S267X set showed strong rheostatic effects on overall transport, and individual substitutions showed varied effects on transport kinetics (Km and Vmax). However, the outcomes were substrate dependent, indicating altered specificity. To assess protein stability, we measured surface expression and used the Rosetta software suite to model structure and stability changes of S267X. Although buried near the substrate binding site, S267X substitutions were easily accommodated in the NTCP structure model. Across the modest range of changes, calculated stabilities correlated with surface-expression differences, but neither parameter correlated with altered transport. Thus, substitutions at rheostat position 267 had wide-ranging effects on the phenotype of this integral membrane protein. We further propose that polymorphic positions in other proteins might be locations of rheostat positions.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biochemistry↗

bioRxiv2020

Substitutions at Non-Conserved Rheostat Positions Modulate Function by Re-Wiring Long-Range, Dynamic Interactions

Amino acid substitutions at nonconserved protein positions can have non-canonical and “long-distance” outcomes on protein function. Such outcomes might arise from changes in the internal protein communication network, which is often accompanied by changes in structural flexibility. To test this, we calculated flexibilities (“DFI”) and dynamic coupling (“DCI”) for positions in the linker region of the lactose repressor protein (“LacI”). This region contains nonconserved positions for which substitutions alter DNA binding affinity. We first chose to study eleven substitutions at position 52. In computations, substitutions showed long-range effects on flexibilities of DNA binding positions, and the degree of flexibility change correlated with experimentally-measured changes in DNA binding. Substitutions also altered dynamic coupling to DNA binding positions in a manner that captured other experimentally-determined functional changes. Next, we broadened calculations to consider the dynamic coupling between 17 linker positions and the DNA binding domain. Experimentally, these linker positions exhibited a wide range of substitution outcomes: Four conserved positions tolerated almost no substitutions (“toggle”), ten nonconserved positions showed progressive changes from a range of substitutions (“rheostat”), and three nonconserved positions tolerated almost all substitutions (“neutral”). In computations with wild-type LacI, the dynamic couplings between the DNA binding domain and these linker positions showed varied degrees of asymmetry that correlated with the observed toggle/rheostat/neutral substitution outcomes. Thus, we propose that long-range and non-canonical substitutions outcomes at nonconserved positions arise from re-wiring long-range communication among functionally-important positions. Such calculations might enable predictions for substitution outcomes at a range of nonconserved positions.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biophysics↗

bioRxiv2019

Functional tunability from a distance: Rheostat positions influence allosteric coupling between two distant binding sites

For protein mutagenesis, a common expectation is that important positions will behave like on/off \"toggle\" switches (i.e., a few substitutions act like wildtype, most abolish function). However, there exists another class of important positions that manifests a wide range of functional outcomes upon substitution: \"rheostat\" positions. Previously, we evaluated rheostat positions located near the allosteric binding sites for inhibitor alanine (Ala) and activator fructose-1,6-bisphosphate (Fru-1,6-BP) in human liver pyruvate kinase. When substituted with multiple amino acids, many positions demonstrated moderate rheostat effects on allosteric coupling between effector binding and phosphoenolpyruvate (PEP) binding in the active site. Nonetheless, the combined outcomes of all positions sampled the full range of possible allosteric coupling (full tunability). However, that study only evaluated allosteric tunability of \"local\" positions, i.e., positions were located near the binding sites of the allosteric ligand being assessed. Here, we evaluated tunability of allosteric coupling when mutated sites were distant from the allosterically-coupled binding sites. Positions near the Ala binding site had rheostat outcomes on allosteric coupling between Fru-1,6-BP and PEP binding. In contrast, positions in the Fru-1,6-BP site exhibited modest effects on coupling between Ala and PEP binding. Analyzed in aggregate, both PEP/Ala and PEP/Fru-1,6-BP coupling were again fully tunable by amino acid substitutions at this limited set of distant positions. Furthermore, some positions exhibited rheostatic control over multiple parameters and others exhibited rheostatic effects on one parameter and toggle control over a second. These findings highlight challenges in efforts to both predict/interpret mutational outcomes and engineer functions into proteins.

biochemistry↗

bioRxiv2019

Do neutral protein positions really exist? A case study with allostery in human liver pyruvate kinase

Understanding how each residue position contributes to protein function has been a long-standing goal in protein science. Substitution studies have historically focused on conserved protein positions. However, substitutions of nonconserved positions can also modify function. Indeed, we recently identified nonconserved positions that have large substitution effects in human liver pyruvate kinase (hLPYK), including altered allosteric coupling. To facilitate a comparison of which characteristics determine when a nonconserved position does vs. does not contribute to function, the goal of the current work was to identify neutral positions in hLPYK. However, existing hLPYK data showed that three features commonly associated with neutral positions - high sequence entropy, high surface exposure, and alanine scanning - lacked the sensitivity needed to guide experimental studies. We used multiple evolutionary patterns identified in a sequence alignment of the PYK family to identify which positions were least patterned, reasoning that these were most likely to be neutral. Nine positions were tested with a total of 117 amino acid substitutions. Although exploring all potential functions is not feasible for any protein, five parameters associated with substrate/effector affinities and allosteric coupling were measured for hLPYK variants. For each position, the aggregate functional outcomes of all variants were used to quantify a "neutrality" score. Three positions showed perfect neutral scores for all five parameters. Furthermore, the nine positions showed larger neutral scores than 17 positions located near allosteric binding sites. Thus, our strategy successfully enriched the dataset for positions with neutral and modest substitutions.

biochemistry↗

bioRxiv2018

Reconciling in vitro and in vivo activities of engineered, LacI-based repressor proteins: Contributions of DNA looping and operator sequence variation

One way to create new components for synthetic transcription circuits is to re-purpose naturally occurring transcription factor proteins and their cognate DNA operators. For the proteins, re-engineering can be accomplished via domain recombination (to create chimeric regulators) and/or amino acid substitutions. The resulting activities of new protein regulators are often assessed in vitro using a representative operator. However, when functioning in vivo, transcription factors can interact with multiple operators. We compared in vivo and in vitro results for two LacI-based transcription repressor proteins, their mutational variants, and four operator sequences. The two sets of repressor variants differed in their overall in vivo repression, even though their in vitro binding affinities for the primary operator spanned the same range. Here, we show that the offset can be explained by different abilities to simultaneously bind and \"loop\" two DNA operators. Further in vitro studies of the looping-competent repressors were carried out to measure binding to a secondary operator sequence. Surprisingly, binding to this operator was largely insensitive to amino acid changes in the repressor protein. In vitro experiments with additional operators and analyses of published data indicates that amino acid changes in these repressor proteins leads to complicated changes in ligand specificity. These results raise new considerations for engineering components of synthetic transcription circuits and - more broadly - illustrate difficulties encountered when trying to extrapolate information about specificity determinant positions among protein homologs.

biochemistry↗

bioRxiv2017

Protein-protein interactions with fructose-1-kinase alter function of the central Escherichia coli transcription regulator, Cra

In E. coli, the master transcription regulator Cra regulates >100 genes in central metabolism by binding upstream DNA operator sequences. Genes encoding glycolytic enzymes are repressed, whereas those for gluconeogenesis and the citric acid cycle are activated. Cra-DNA binding is allosterically diminished by binding to either fructose-1-phosphate (F-1-P, generated upon fructose import) or fructose-1,6-bisphosphate (F-1,6-BP). F-1,6-BP is generated from F-1-P by the enzyme fructose-1-kinase (FruK) or from other sugars and is a key intermediate in glycolysis. Here, we report that Cra directly interacts with FruK to form a tight protein-protein complex. Further, growth assays with a fruK knockout strain show that FruK has a broader role in metabolism than its known role in fructose catabolism. Biochemical experiments show that F-1,6-BP binding enhances either the Cra/FruK interaction and/or CRA binding to DNA and that FruK can catalyze the reverse reaction of F-1,6-BP to F-1-P. Results were used to propose a model in which the Cra-FruK complex enhances activation of gluconeogenic genes. Finally, since FruK itself is repressed by Cra, these newly-reported events add layers to the dynamic regulation of E. coli central metabolism that occur in response to changing nutrients.

microbiology↗