Biology subjects

Carter, C. W.

Publications and source records attributed to Carter, C. W..

12 recordsLinked to original sources

bioRxiv2025

Escherichia coli transforms the leucyl-tRNA synthetase gene in vivo into primordial genes

E. coli deletes CP1 and ABD domains from [~]40% of plasmids containing leucyl-tRNA synthetase if and only if both active-site signatures are mutated. Shortened ORFs occurred in all reading frames but form three discrete sets in the same frame. One had only the AVGA signature. Two longer ones both retained the same 24-residue segment containing the AMSAS signature. Large pre-steady-state bursts and steady-state acylation assays confirm that they encode active tRNA synthetases. In these respects, the ORFs resemble models for ancestral Class I aminoacyl-tRNA synthetases (AARS). Both signatures thus appear necessary and sufficient for aminoacylation. Growth at 4{degrees} C produced most of the middle-sized ORF, which is incompatible with the others and appears to result from a distinct mechanism. Three different linkages between the two parts of the active site acylate tRNA minihelix at similar rates. That result greatly expands the sequence space of active ancestral AARS. Widely spaced active-site mutants thus trigger deletions of modules acquired as full-length AARS evolved from simpler catalysts. We propose that the deletions survive because they limit mischarging due to disrupted coupling of active-site residues to domain motion. Such deletions may thus be a general phenomenon, opening broad access to primordial gene discovery. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=84 SRC="FIGDIR/small/647346v2_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@1d87333org.highwire.dtl.DTLVardef@1733f25org.highwire.dtl.DTLVardef@136a74borg.highwire.dtl.DTLVardef@12137ce_HPS_FORMAT_FIGEXP M_FIG Schematic rationale for formation and selection of LeuRS urzyme-like deletions. A. Full length LeuRS uses a complex allosteric network of interactions between Dom A (CP1), Dom B (ABD) and WT active-site catalytic histidine and lysine residues. B. Creation of the double mutant corrupts the allosteric effects of the two domains (faded colors). This creates a cytotoxic protein. C. Deletion of the domains whose functions have been corrupted by disrupting the allosteric network produces variants lacking the inactivated domains. These variants have considerably less cytotoxicity. They likely also resemble evolutionary precursors of the full-length protein. This may represent a general phenomenon for double mutant multi-domain protein genes. C_FIG

evolutionary biology↗

bioRxiv2024

Evolution is coupled with branching across many granularities of life

Across many different scales of life, the rate of evolutionary change is often accelerated at the time when one lineage splits into two. The emergence of novel protein function can be facilitated by gene duplication (neofunctionalisation); rapid morphological change is often accompanied with speciation (punctuated equilibrium); and the establishment of cultural identity is frequently driven by sociopolitical division (schismogenesis). In each case, the change resists rehomogenisation; promoting assortment into distinct lineages that are susceptible to different selective pressures, leading to rapid divergence. The traditional gradualistic view of evolution struggles to detect this phenomenon. We have devised a probabilistic framework that constructs phylogenies, tests hypotheses, and improves divergence time estimation when evolutionary bursts are present. As well as assigning a clock rate of gradual evolution to each branch of a tree, this model also assigns a spike of abrupt change, and independently estimates the contributions arising from each process. We provide evidence of abrupt evolution at the time of branching for proteins (aminoacyl-tRNA synthetases), animal morphologies (cephalopods), and human languages (Indo-European). These three cases provide unique insights: for aminoacyl-tRNA synthetases, the trees are substantially different from those obtained under gradualist models; Cephalopod morphologies are found to evolve almost exclusively through abrupt shifts; and Indo-European dispersal is estimated to have started around 6000 BCE, corroborating the recently proposed hybrid explanation. This work demonstrates a robust means for detecting burstlike processes, and advances our understanding of the link between evolutionary change and branching events. Our open-source code is available under a GPL license.

evolutionary biology↗

bioRxiv2024

AARS Online: a collaborative database on the structure, function, and evolution of the aminoacyl-tRNA synthetases

The aminoacyl-tRNA synthetases (aaRS) are a large group of enzymes that implement the genetic code in all known biological systems. They attach amino acids to their cognate tRNAs, moonlight in various non-translational activities, and are linked to many genetic disorders. The aaRS have a subtle ontology characterized by structural and functional idiosyncrasies that vary from organism to organism, and protein to protein. Across the tree of life, the twenty-two coded amino acids are handled by sixteen evolutionary Families of Class I aaRS and twenty-one Families of Class II aaRS. We introduce AARS Online, an interactive Wikipedia-like tool curated by an international consortium of field experts. This platform systematizes existing knowledge about the aaRS by showcasing a taxonomically diverse selection of aaRS sequences and structures. Through its graphical user interface, AARS Online facilitates a seamless exploration between protein sequence and structure, providing a friendly introduction to the material for non-experts and a useful resource for experts. Curated multiple sequence alignments can be extracted for downstream analyses. Accessible at www.aars.online, AARS Online is a free resource to delve into the world of the aaRS.

biochemistry↗

bioRxiv2023

HetMM: A Michaelis-Menten model for non-homogeneous enzyme mixtures

The Michaelis-Menten model requires its reaction velocities to be measured from a preparation of homogeneous enzymes, with identical or near-identical catalytic activities. However, there are many cases where enzyme preparations do not satisfy this condition, or where one may wish to test the validity of this assumption. We introduce a kinetic model that relaxes this requirement, by assuming there are an unknown number of enzyme species drawn from an unknown probability distribution. This model features one additional parameter over the Michaelis-Menten model, describing the standard deviation of this distribution. We show that the assumption of homogeneity is usually sufficient even in non-homogeneous solutions, and only fails under extreme conditions where Km spans orders of magnitude. We validate this method through simulation studies, demonstrating the method does not overfit to random noise, despite its increase in dimensionality. The two models can be accurately discriminated between even with moderate levels of experimental error. We applied this model to three homogeneous and three heterogeneous biological systems, showing that the standard and heterogeneous models outperform in either case, respectively. Lastly, we show that heterogeneity is not readily distinguished from negatively-cooperative binding under the Hill model. These two fundamentally distinct properties - inequality in catalytic ability and interference between binding sites - give similar Michaelis-Menten curves that are not readily resolved without further experimentation. Our method allows testing for homogeneity and performing parameter inference in a Bayesian framework, and is available online in the user-friendly HetMM package at https://github.com/jordandouglas/HetMM.

biochemistry↗

bioRxiv2023

Domain Acquisition by Class I Aminoacyl-tRNA Synthetase Urzymes Coordinated the Catalytic Functions of HVGH and KMSKS Motifs

Leucyl-tRNA synthetase (LeuRS) is a Class I aminoacyl-tRNA synthetase (aaRS) that catalyzes synthesis of leucyl-tRNAleu for codon-directed protein synthesis on the ribosome. Class I aaRS, which were key to the evolution of genetic coding, contain two discrete signature sequences, HIGH and KMSKS, that participate in transition-state stabilization by the entire eleven-enzyme Class I aaRS superfamily. Combinatorial mutagenesis and thermodynamic cycle analyses of these catalytic signatures in full-length Pyrococcus horikoshii LeuRS and the 129-residue urzyme ancestral model generated from it (LeuAC) provide quantitative insight into the evolutionary gain of function induced by acquisition of the anticodon-binding (ABD) and multiple insertion modules in the catalytic domain. The free energy coupling terms, {Delta}({Delta}G{ddagger}), are small and unfavorable for LeuAC, but large and favorable for LeuRS. Thus, the ABD and other insertion modules induce strong cooperativity between the two signature sequences, which are uncoupled in LeuAC. These results further substantiate the authenticity of LeuAC urzyme catalysis and implicate domain motion in catalysis by the full-length LeuRS. Most importantly, the implication that backbone elements of secondary structures achieve a major portion of the overall transition-state stabilization by LeuAC is also consistent with coevolution of the genetic code and metabolic pathways necessary to produce histidine and lysine sidechains. Bullet PointsO_LIThe LeuRS HVGH and KMSKS signature motifs are energetically coupled by -1.6 kcal/mole. C_LIO_LIThe same motifs are anti-coupled by +0.8 kcal/mole in the 129 residue urzyme, LeuAC. C_LIO_LIAncestral Class I aaRS did not require either histidine or lysine for catalysis. C_LI

biochemistry↗

bioRxiv2023

A Zymography technique to study amino acid activation by aminoacyl-tRNA synthetases (aaRS): A broad spectrum, high- throughput tool to screen activities of aaRS and their Urzyme variants.

Amino acyl tRNA synthetases or aaRSs play a key role in assuring the precision of protein translation. They are highly specific for their cognate amino acid and cognate tRNA substrates during protein synthesis, utilizing ATP to ensure that proper assignments are made between amino acid and anticodon. Specific aaRS for each amino acid are present in all cells. We describe a new zymography technique to qualitatively visualize and semi-quantitatively determine the amino acid activation capacity of each type of aaRS molecule by indirect colorimetric detection of released pyrophosphates during the formation of aminoacyl-AMP. Protein samples containing aaRS are subjected to Native PAGE, followed by incubation in buffer containing cognate amino acid and ATP for sufficient time to generate pyrophosphates (PPi) which are then converted to inorganic phosphates by pyrophosphatase treatment. Finally, the generated and localized phosphates around the aaRS protein inside the gel can be visualized after staining by ammonium molybdate and malachite green solution. This technique has been validated by inspecting the substrate specificities of specific aaRSs. This zymography technique is sufficiently sensitive to detect and authenticate activities of much (i.e., ~10-5-fold) less active aaRS "Urzymes", to study alteration of activities of aaRS by various intrinsic or extrinsic factors and to screen aaRS-specific antimicrobial drugs.

biochemistry↗

bioRxiv2021

Multiplexed action-outcome representation by striatal striosome-matrix compartments detected with a novel cost-benefit foraging task

Learning about positive and negative outcomes of actions is crucial for survival and underpinned by conserved circuits including the striatum. How associations between actions and outcomes are formed is not fully understood, particularly when the outcomes have mixed positive and negative features. We developed a novel foraging ( bandit) task requiring mice to maximize rewards while minimizing punishments. By 2-photon Ca++ imaging, we monitored activity of 5831 identified anterodorsal striatal striosomal and matrix neurons. Surprisingly, we found that action-outcome associations for reward and punishment were combinatorially encoded rather than being integrated as overall outcome value. Single neurons could, for one action, encode outcomes of opposing valence. Striosome compartments consistently exhibited stronger representations of reinforcement outcomes than matrix, especially for high reward or punishment prediction errors. These findings demonstrate a remarkable multiplexing of action-outcome contingencies by single identified striatal neurons and suggest that striosomal neurons are differentially important in action-outcome learning.

neuroscience↗

bioRxiv2020

Class I Aminoacyl-tRNA Synthetase Urzyme and CP1 Modules have Distinct Genetic Origins

The provenance of the aminoacyl-tRNA synthetases (aaRS) poses unusually challenging questions because of their role in the emergence and evolution of genetic coding. We investigate evidence about their ancestry from highly curated structure-based multiple sequence alignments of a small "scaffold" that is structurally invariant in all 10 canonical Class I aaRS. Statistically different values of two uncorrelated phylogenetic metrics--residue by residue conservation derived from Clustal and row-by-row cladistic congruence derived from BEAST2--suggest that the Class I scaffold is a mosaic assembled from distinct, successive genetic sources. These data are especially significant in light of: (i) experimental fragmentations of the Class I scaffold into three partitions that retain catalytic activities in proportion to their length; and (ii) multiple sources of evidence that two of these partitions arose from an ancestral Class I aaRS gene encoding a Class II ancestor in frame on the opposite strand. Two additional metrics output by BEAST2 vary in accordance with the presumed functionality endowed by the various modules. The new evidence supplements previous aaRS phylogenies. It identifies a previously characterized 46-residue Class I "protozyme" as preceding the adaptive radiation of the superfamily containing variations of the Rossmann dinucleotide binding fold related to amino acid discrimination, and thus as root of that molecular tree. Such a rooting is consistent with near simultaneous emergence of genetic coding and the origin of the proteome, resolving a conundrum posed by previous inferences that Class I aaRS evolved long after the genetic code had been implemented in an RNA world. Further, it establishes a timeline for the growth of coding from a binary amino acid alphabet by pinpointing discontinuous enhancements of aaRS fidelity. Author SummaryPhylogenetic analysis uncovers evolutionary connections between different protein superfamily members. We describe complementary, uncorrelated, phylogenetic metrics that support multiple evolutionary histories for different segments within members of the Class I aminoacyl-tRNA synthetase superfamily. Using a carefully curated 3D crystal structure superposition as the primary source of the multiple sequence alignment substantially reduced dependence of these metrics on empirical amino acid substitution matrices. Two metrics are derived from the amino acid distribution observed in each successive position. A third depends on how individual sequences distribute into phylogenetic tree branches for each of the ten amino acids activated by the superfamily. All metrics confirm that a segment previously identified as an inserted element is, indeed, a more recent acquisition, despite its structural conservation. The residue-by-residue conservation metrics reveal significant co-variation of mutational frequencies between a core segment that forms the amino acid binding site and a neighboring segment derived from the more recent insertion element. We attribute that covariation to the differentiation of superfamily members as evolutionary divergence enhanced amino acid specificity. Finally, evidence that the insertion element is a recent acquisition implies a new branching order for much of the proteome.

evolutionary biology↗

bioRxiv2019

Class I and II aminoacyl-tRNA synthetase tRNA groove discrimination created the first synthetase·tRNA cognate pairs and was therefore essential to the origin of genetic coding

The genetic code likely arose when a bidirectional gene began to produce ancestral aminoacyl-tRNA synthetases (aaRS) capable of distinguishing between two distinct sets of amino acids. The synthetase Class division therefore necessarily implies a mechanism by which the two ancestral synthetases could also discriminate between two different kinds of tRNA substrates. We used regression methods to uncover the possible patterns of base sequences capable of such discrimination and find that they appear to be related to thermodynamic differences in the relative stabilities of a hairpin necessary for recognition of tRNA substrates by Class I aaRS. The thermodynamic differences appear to be exploited by secondary structural differences between models for the ancestral aaRS called synthetase Urzymes and reinforced by packing of aromatic amino acid side chains against the nonpolar face of the ribose of A76 if and only if the tRNA CCA sequence forms a hairpin. The patterns of bases 1, 2 and 73 and stabilization of the hairpin by structural complementarity with Class I, but not Class II aaRS Urzymes appears to be necessary and sufficient to have enabled the generation of the first two aaRS*tRNA cognate pairs, and the launch of a rudimentary binary genetic coding related recognizably to contemporary cognate pairs. As a consequence, it seems likely that non-random aminoacylation of tRNAs preceded the advent of the tRNA anticodon stem-loop. Consistent with this suggestion, coding rules in the acceptor-stem bases also reveal a palimpsest of the codon*anticodon interaction, as previously proposed.

genetics↗

bioRxiv2019

Thermodynamic impacts of combinatorial mutagenesis on protein conformational stability: precise, high-throughput measurement by Thermofluor

The D1 switch is a packing motif, broadly distributed in the proteome, that couples tryptophanyl-tRNA synthetase (TrpRS) domain movement to catalysis and specificity, thereby creating an escapement mechanism essential to free-energy transduction. The escapement mechanism arose from analysis of an extensive set of combinatorial mutations to this motif, which allowed us to relate mutant-induced changes quantitatively to both kinetic and computational parameters during catalysis. To further characterize the origins of this escapement mechanism in differential TrpRS conformational stabilities, we use high-throughput Thermofluor measurements for the 16 variants to extend analysis of the mutated residues to their impact on unliganded TrpRS stability. Aggregation of denatured proteins complicates thermodynamic interpretations of denaturation experiments. The free energy landscape of a liganded TrpRS complex, carried out for different purposes, closely matches the volume, helix content, and transition temperatures of Thermoflour and CD melting profiles. Regression analysis using the combinatorial design matrix accounts for >90% of the variance in Tms of both Thermofluor and CD melting profiles. We argue that the agreement of experimental melting temperatures with both computational free energy landscape and with Regression modeling means that experimental melting profiles can be used to analyze the thermodynamic impact of combinatorial mutations. Tertiary packing and aromatic stacking of Phenylalanine 37 exerts a dominant stabilizing effect on both native and molten globular states. The TrpRS Urzyme structure remains essentially intact at the highest temperatures explored by the simulations.

biophysics↗

bioRxiv2018

HIV-1 Protease Uses Bi-Specific S2/S2’ Subsites To Optimize Cleavage of Two Classes of Target Sites

Retroviral proteases (PR) have a unique specificity that allows cleavage of sites with or without a P1 proline. A P1 proline is required at the MA/CA cleavage site due to its role in a post-cleavage conformational change in the capsid protein. However, the HIV-1 PR prefers to have large hydrophobic amino acids flanking the scissile bond, suggesting PR recognizes two different classes of substrate sequences. We analyzed the cleavage rate of over 150 iterations of six different HIV-1 cleavage sites to explore rate determinants of cleavage. We found that cleavage rates are strongly influenced by the two amino acids flanking the amino acids at the scissile bond (P2-P1/P1-P2), with two complementary sets of rules. When P1 is proline, the P2 side chain interacts with a polar region in the S2 subsite of the PR, while the P2 amino acid interacts with a hydrophobic region of the S2 subsite. When P1 is not proline, the orientations of the P2 and P2 side chains with respect to the scissile bond are reversed; P2 residues interact with a hydrophobic face of the S2 subsite while the P2 amino acid usually engages hydrophilic amino acids in the S2 subsite. These results reveal that the HIV-1 PR has evolved bi-functional S2 and S2 subsites to accommodate the steric effects imposed by a P1 proline on the orientation of P2 and P2 substrate side chains. These results also suggest a new strategy for inhibitor design to engage the multiple specificities in these subsites.

biochemistry↗

bioRxiv2017

Interdependence, Reflexivity, Fidelity, Impedance Matching, And The Evolution Of Genetic Coding

Genetic coding is generally thought to have required ribozymes whose functions were taken over by polypeptide aminoacyl-tRNA synthetases (aaRS). Two discoveries about aaRS and their tRNA substrates now furnish a unifying rationale for the opposite conclusion: that the key processes of the Central Dogma of molecular biology emerged simultaneously and naturally from simple origins in a peptide*RNA partnership, eliminating the epistemological need for a prior RNA world. First, the two aaRS classes likely arose from opposite strands of the same ancestral gene, implying a simple genetic alphabet. Inversion symmetries in aaRS structural biology arising from genetic complementarity would have stabilized the initial and subsequent differentiation of coding specificities and hence rapidly promoted diversity in the proteome. Second, amino acid physical chemistry maps onto tRNA identity elements, establishing reflexivity in protein aaRS. Bootstrapping of increasingly detailed coding is thus intrinsic to polypeptide aaRS, but impossible in an RNA world. These notions underline the following concepts that contradict gradual replacement of ribozymal aaRS by polypeptide aaRS: (i) any set of aaRS must be interdependent; (ii) reflexivity intrinsic to polypeptide aaRS production dynamics promotes bootstrapping; (iii) takeover of RNA-catalyzed aminoacylation by enzymes will necessarily degrade specificity; (iv) the Central Dogmas emergence is most probable when replication and translation error rates remain comparable. These characteristics are necessary and sufficient for the essentially de novo emergence of a coupled gene-replicase-translatase system of genetic coding that would have continuously preserved the functional meaning of genetically encoded protein genes whose phylogenetic relationships match those observed today.

evolutionary biology↗