Journal of Molecular Biology

Several proteins from the human proteome have been observed to adopt multiple distinct amyloid filaments, and specific protofilament folds are associated with different diseases. Thereby, it has become necessary to compare pairs of amyloid structures of a given protein. This paper describes the amyloid packing difference (APD), which quantifies the difference between such a pair as the percentage of residues that are involved in unique cross-{beta} packing interactions or that have different side chain orientations relative to the {beta}-strands. Clustering of -synuclein protofilament folds on pairwise APD values recapitulates previously reported clustering based on structural superpositions. Any pair of known protofilament folds of the prion protein, tau, -synuclein, TDP-43 or TAF15 from different diseases have APD values above 20%, whereas all pairs of structures that have been associated with the same disease have APD values below 40%. These observations provide context for the interpretation of APD values of new comparisons.

Recent advances in predicting and modeling conformational ensembles of intrinsically disordered proteins (IDPs) have provided much needed insights into sequence-ensemble relationships. It is thought that conservation of physicochemical properties, but not the exact identity or order of the amino acids, maintains IDP ensemble properties that are crucial for function. However, detailed experimental studies are still required to fully understand the relationships between sequence and function in IDPs. The human CITED proteins, which are fully disordered transcriptional regulators, share conserved C-terminal transactivation domains (CTADs) that interact with the TAZ1 domain of the transcriptional coactivators CBP/p300. The conserved CTADs harbor amino acid substitutions in regions that are known to be important for interactions of CITED2 with TAZ1, but the effects of these substitutions on TAZ1 binding for the other CITED proteins are unknown. Here, we use solution NMR spectroscopy, circular dichroism, and surface plasmon resonance to characterize the conformational ensembles, dynamics, and interactions of the CITED CTADs. The CTADs are disordered in isolation, although the CITED2 CTAD uniquely displays residual helical structure that is sensitive to ionic strength and protein concentration. In contrast, the CITED1 and CITED4 CTADs remain largely disordered and exhibit more uniform dynamics. Quantitative binding measurements reveal differences in thermodynamics and kinetics for the CTADs interactions with TAZ1, with CITED2 binding most tightly and CITED4 exhibiting significantly weaker affinity. Our results highlight the sensitivity of IDP conformational ensembles to minor sequence changes and the impacts that changes in IDP structures and dynamics can have on biological functions.

The RNA-binding protein Fused in Sarcoma (FUS) undergoes phase separation associated with RNA processing. However, the prion-like low complexity (LC) domain of FUS forms solid-like aggregates in neurodegenerative diseases. Whether the formation of {beta}-sheet structure associated with pathology is also physiologically/functionally relevant is debated. Similarly, if mislocalization alone or concomitant aggregation is responsible for FUS gain-of-function toxicity remains to be probed. Here, we introduce {beta}-sheet breaking proline residues into FUS LC with the goal of preventing cross-{beta}-driven aggregation without disrupting essential functions and phase separation. {beta}-sheet-deficient FUS variants maintain native-like global motions, disorder, and phase separation, but no longer show a liquid-to-solid transition (LST). Biochemical partitioning, cellular localization, and auto- and cross-regulatory functions of FUS all remain essentially unchanged. Conversely, FUS-induced neurodegeneration in several Drosophila models is drastically reduced. These findings suggest a strategy for mitigating disease-related toxicity through backbone structure modulation to prevent prion-like domain protein aggregation. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=198 SRC="FIGDIR/small/706410v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@d15f63org.highwire.dtl.DTLVardef@1cd6221org.highwire.dtl.DTLVardef@e58126org.highwire.dtl.DTLVardef@181ec67_HPS_FORMAT_FIGEXP M_FIG C_FIG SUMMARYThe RNA-binding protein Fused in Sarcoma (FUS) undergoes phase separation as part of its physiological function but can aberrantly aggregate into solid-like assemblies in amyotrophic lateral sclerosis and frontotemporal dementia. To dissect the role of {beta}-sheets in both function and pathological transition, we engineered {beta}-sheet-preventing FUS variants via targeted proline residue insertions in the prion-like disordered region. These variants retained native structure, motions, and phase behavior yet showed dramatically reduced aggregation, both as an isolated prion-like domain and in full-length FUS. Crucially, these variants maintained a panel of FUS cellular functions that depend on FUS condensation but prevented FUS toxicity in fly models of neurodegeneration. Our findings implicate {beta}-sheets as key drivers of FUS condensate maturation and neuronal toxicity, highlighting {beta}-sheet modulation as a therapeutic strategy against FUS-related neurodegeneration. HIGHLIGHTSO_LITargeted proline additions disrupt {beta}-sheet formation in FUS without altering native conformations, dynamics, or phase separation behavior C_LIO_LI{beta}-sheet-deficient FUS variants prevent aggregation and liquid-to-solid transitions while retaining key biological functions C_LIO_LIIn vivo models reveal attenuated toxicity of {beta}-sheet-deficient FUS in Drosophila C_LIO_LI{beta}-sheets are identified as central drivers of condensate maturation and neuronal death, offering a therapeutic entry point for modulating prion-like domain pathology C_LI

The oxazolidinone antibiotic linezolid binds to the peptidyl transferase center of the ribosome, where it inhibits a subset of peptide bond formation events. This context-specificity of translation inhibition is dictated by the nature of the amino acid at the penultimate position of the nascent peptide. It remains unknown whether this is a general feature of oxazolidinones and whether it can be modulated by their structural alterations. Here, we show that the oxazolidinone tedizolid also inhibits translation in a context-specific manner, but with dramatically altered selectivity, favoring Ile, His, and Gln as the penultimate residues. Delpazolid, which shares the C5 hydroxymethyl moiety with tedizolid, shows a similar preference. Structural analysis of the ribosome with tedizolid and a stalled nascent peptide showed a compacted, helical conformation of the nascent chain induced by the drug. Our findings reveal that stalling preferences of oxazolidinones can be modulated by structural modifications within this antibiotic class.

Synonymous codons encode the same amino acid but can differ in their usage and translational properties. In previous work we reported statistical differences in backbone dihedral angle distributions associated with synonymous codons in the Escherichia coli proteome. This finding has been questioned due to concerns regarding the statistical methodology used. Here we revisit the dataset using corrected statistical procedures and alternative statistical tests. Across multiple frameworks, the real dataset consistently shows an excess of small p-values relative to randomized controls, indicating detectable codon-associated differences in backbone conformation.

Although the structural basis of selective G-protein coupling to G protein coupled receptors (GPCRs) is well characterized, the mechanisms underlying selective interactions between distinct G subtypes (Gs, Gi, Gq, and G12/13) and G{beta}{gamma} remain poorly understood. While conserved residues in G subtypes are often assumed to have similar functions, they may instead modulate coupling selectivity by altering the frequency and stability of contacts at the G:G{beta}{gamma} interface. Using molecular dynamics (MD) simulations combined with the interpretable machine learning method, Bayesian Network Model (BNM), and protein-protein proximity (BRET) assays, we show that conserved residues in the two closely related Gi/o and Gq/11 subfamilies contribute differentially to G{beta}{gamma} coupling. These conserved residue "hotspots" on Gi and Gq produced divergent functional effects on G{beta}{gamma} coupling, indicating that conservation does not ensure functional equivalence. These findings suggest that local microenvironment and paralog-specific allosteric coupling shape how conserved interface residues contribute to protein-protein coupling. The framework provides a systematic approach for dissecting subtype-specific mechanisms, with implications for drug design and for annotating the functional relevance of disease-associated variants. The computational methods used here are broadly applicable to other homologous protein families.

Pre-mRNA splicing is an essential step in eukaryotic gene expression during which spliceosomes remove introns from nascent RNAs while ligating the adjacent exons. Spliceosomes are cellular nanomachines composed of five small nuclear (snRNA) components and dozens of proteins, most of which are highly conserved. Despite the high conservation of many splicing factors between S. cerevisiae and H. sapiens, several protein components of the S. cerevisiae spliceosome are not essential for growth under normal laboratory conditions. This is particularly surprising for nonessential factors whose conserved domains contact the spliceosomes catalytic core. Uncovering a function for these splicing factors can be challenging since they are not required for viability, may engage in functionally redundant interactions, and may display only weak phenotypes in the absence of secondary mutations in other spliceosome components. One such nonessential factor is the Cwc15 protein. Cwc15s highly conserved N-terminus directly contacts the U2/U6 di-snRNA within the spliceosome catalytic core; yet its precise role in splicing has not been defined in any organism. In this work, we use molecular genetics in S. cerevisiae combined with splicing reporter assays to study Cwc15p function. We propose that Cwc15p not only promotes active site stability during 5 splice site cleavage but also impacts structural transitions into and out of this spliceosome conformation. This function may be critical for splicing in S. cerevisiae under nonoptimal conditions, facilitating use of weak or alternate splice sites, and could have implications for proofreading of spliceosome active site formation. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=146 SRC="FIGDIR/small/713263v1_ufig1.gif" ALT="Figure 1"> View larger version (74K): org.highwire.dtl.DTLVardef@b296c5org.highwire.dtl.DTLVardef@c87b91org.highwire.dtl.DTLVardef@287011org.highwire.dtl.DTLVardef@d59741_HPS_FORMAT_FIGEXP M_FIG C_FIG Article SummaryPre-mRNA splicing is carried out by large macromolecular machines called spliceosomes which are composed of several snRNAs and dozens of proteins. Despite decades of study, the functions of many splicing factors such as S. cerevisiae Cwc15p remain unknown. Cwc15p is highly conserved among eukaryotes and directly contacts the spliceosome catalytic core. Here, we have used genetic and splicing reporter assays to study the function of Cwc15p during splicing in vivo. We propose that Cwc15p both stabilizes the spliceosome active site during 5 splice site cleavage and impacts remodeling of that site.

Protein translation is a highly regulated process influenced by multiple factors at the initiation, elongation, and termination stages. One notable regulatory element of the ribosome is the CAR interaction surface, a three-residue motif in the structure of the ribosome composed of C1274 and A1427 of S. cerevisiae 18S rRNA (corresponding to C1054 and A1196 in E. coli 16S rRNA) and R146 of ribosomal protein Rps3. CAR is highly conserved and positioned adjacent to the amino-acyl (A site) decoding center. It establishes hydrogen bonds with the +1 codon next in line to enter the ribosome A site, acting as an extension of the tRNA anticodon and forming base-stacking interactions with nucleotide 34 of the tRNA. However, despite CARs enzymatically strategic positioning within the ribosome, its functional relationship with the A site remains poorly characterized. Using molecular dynamics (MD) simulations, we examined the interplay between the A site and CAR site, revealing sequence-dependent modulation of H-bonding and {pi}-stacking interactions within and between the two sites. These findings highlight the interplay between the A site and CAR site, suggesting a structural and functional connection between these two regions of the ribosome that may contribute to mRNA sequence-specific tuning of translation elongation. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=91 SRC="FIGDIR/small/714784v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@1d783d3org.highwire.dtl.DTLVardef@f9cd8org.highwire.dtl.DTLVardef@102667corg.highwire.dtl.DTLVardef@967c56_HPS_FORMAT_FIGEXP M_FIG C_FIG

GABAA receptors (GABAARs) are pentameric ligand-gated ion channels (pLGICs) essential for inhibitory synaptic transmission throughout the central nervous system. Despite progress in understanding their three-dimensional structure, the molecular basis for how neurotransmitter binding is transduced to ion channel gating remains poorly understood. Furthermore, relatively little is known about the contributions of distinct subunits to this coupling within typical heteromeric receptors. A highly conserved proline (site 1) in the M2-M3 linker of pLGIC subunits is involved in channel gating - e.g., P273 in the GABAAR {beta}2 subunit. In GABAARs, only the {beta} subunits have an additional proline in the M2-M3 linker (site 2) - e.g., {beta}2(P276) - whereas all other subunits have a non-proline at the homologous site 2 position. Here, we investigate the functional contribution of proline at site 2 in distinct subunits of 1{beta}2{gamma}2 GABAARs. We expressed wild type or mutant 1{beta}2{gamma}2 GABAARs in Xenopus laevis oocytes and used two-electrode voltage clamp electrophysiology to record channel currents in response to GABA and/or other ligands. First, we introduced a proline at site 2 in 1 or {gamma}2 subunits: 1(A280P) and {gamma}2(S291P). Second, we replaced the site 2 proline in the {beta}2 subunit with its homologous non-proline residue from 1 or {gamma}2 subunits: {beta}2(P276A) or {beta}2(P276S). We show that 1(A280P) confers enhanced GABA-sensitivity and spontaneous unliganded channel activity, whereas {gamma}2(S291P) has minor effects on channel activation. In contrast, {beta}2(P276A) or {beta}2(P276S) either had no effect or enhanced GABA-activation, respectively, indicating complex functional dependence on the side chain at site 2 in the {beta}2 subunit. When in combination with other substitutions, the presence or absence of 1(A280P) was consistently correlated with enhanced GABA-sensitivity and spontaneous activity. Thus, introduction of a proline at site 2 in the 1 M2-M3 linker biases the channel towards an activated state and prevents it from remaining closed at rest.

Eukaryotes have distinct nuclear genes for tryptophanyl-tRNA synthetase (TrpRS). Human mitochondrial (Hmt) TrpRS (also WARS2) shares only 14% sequence identity with human cytoplasmic (Hc)TrpRS, but 41% with Bacillus stearothermophilus (Bs)TrpRS. Tryptophan binding to BsTrpRS is largely promoted by hydrophobic interactions and recognition of the indole nitrogen by side chains of Met129 and Asp132. The non-reactive analog indolmycin can recruit unique polar interactions to form an active-site metal coordination that lies off the normal mechanistic path, enhancing affinity to BsTrpRS and other prokaryotic TrpRS enzymes by 1500-fold over its tryptophan substrate. By contrast, human WARS2, complements nonpolar interactions for tryptophan binding with additional electrostatic and hydrogen bonding interactions that are inconsistent with indolmycin binding. We report here a 1.82 [A] crystal structure of an HmtTrpRS* indolmycin*Mn2+*ATP complex, showing that mitochondrial and bacterial enzymes use similar determinants to bind both ATP and indolmycin. ATP forms tight electrostatic interactions between the catalytic metal ion and a non-bridging oxygen atom from each phosphate group. Hydrogen bonds between the oxazolinone group and active-site residues create an off-path ground-state configuration. This arrangement closely mimics that in the corresponding BsTrpRS complex but varies greatly from ATP binding to HcTrpRS, Moreover, isothermal titration calorimetry demonstrates that, as for BsTrpRS, Mg2+*ATP, but not ATP alone, enhances indolmycin binding affinity [~]100-fold with a supplemental {Delta}({Delta}G) of [~] -3 kcal/mol. Structural, thermodynamic, and kinetic similarities confirm our previous conclusion that a reinforced ground-state Mg2+ ion configuration contributes to the high indolmycin affinity in the bacterial system.

Co-translational folding is a critical, yet poorly understood, aspect of protein biogenesis due to its transient, heterogeneous, and experimentally inaccessible nature. Using a myoglobin variant engineered towards increased domain swapping, we show that stable dimers formed during heterologous E. Coli expression revert to the monomeric state following denaturation - renaturation and that domain swapping propensity is significantly affected by synonymous coding. Wider implications for the role of synonymous coding in aggregation and disease are discussed.

Short linear motifs (SLiMs) are small, often transient interaction modules within intrinsically disordered regions (IDRs) of proteins that interact with particular domains and thereby regulate numerous biological processes. The limited sequence information within these short peptides leads to frequent false positive hits in both computational and experimental SLiM identification methods. This makes the description of novel SLiMs challenging and has limited the number of known cases to a few thousand, even though SLiMs play widespread roles in cellular functions. We present SLiMMine, a deep learning-based method to identify SLiMs in the human proteome. By refining the annotations of known, annotated motif classes, we created a high-quality dataset for model training. Using protein embeddings and neural networks, SLiMMine reliably predicts novel SLiM candidates in known classes, eliminates [~]80% of the pattern matching-based motif hits as false-positives, furthermore, it can also be used as a discovery tool to find uncharacterized SLiMs based on optimal sequence environment. In addition, we narrowed the highly general interactor-domain definitions of known SLiM classes to specific human proteins, enabling more precise prediction of a wide range of potential protein-protein interactions (PPIs) in the human interactome. SLiMMine is available in the form of an appealing, user-friendly, multi-purpose web-server at https://slimmine.pbrg.hu/.

Ribosomal proteins, because of their RNA-binding capacity, may engage various cellular RNAs and fulfill nonribosomal roles. Previously, we and others described the intergenic regulation mediated by splicing of RPL22 paralogs in Saccharomyces cerevisiae. Here, we prepared a panel of RPL22A/B intronic mutants with respect to their RNAfold-predicted features and analyzed their properties. We tested the splicing and Rpl22-intron interaction using an intron-containing reporter and a three-hybrid yeast system, respectively. We found that the splicing of RPL22 introns can be inhibited by stabilizing a predicted stem as part of a particular type of conformation (I structure). Stabilizing the formation of an alternate stem (P structure) led to a permissive outcome of splicing. Intriguingly, the regulatory capacity of the main stem loop of the I structure was dependent on the rest of the intronic structure. Rpl22 enhanced splicing inhibition in WT and several of the mutants, which we interpret as stabilization of the I structure by protein binding. Mutagenesis identified both the main and alternative 5ss and additional stem loops as part of the regulatory mechanism. The inhibitory conformation of the intron did not prevent recognition of the 5ss and branch point, but rather stalled splicing at a later stage, before the first catalytic step. We concluded that the structural ensemble of the RPL22 pre-mRNA behaves as an allosteric switch that responds to [Rpl22].

Cellular transport processes along microtubules are often facilitated by multi-motor complexes, which are connected by adapter proteins and cargoes. The nuclear pore protein Nup358, for example, interacts with the dynein adapter Bicaudal D2 (BicD2), which in turn recruits minus-end directed dynein motors and plus-end directed kinesin-1 motors for a nuclear positioning pathway that is essential for brain development. How motor recruitment is regulated by interactions of BicD2 with Nup358 is not well understood. Here, we characterize the structure of a minimal complex of kinesin-1 light chain 2 (KLC2), Nup358 and BicD2 by cryo-electron microscopy and small angle X-ray scattering. KLC2/Nup358 assumes a rod-like structure that increases in thickness, when BicD2 is bound. The addition of BicD2 also shifts the KLC2/Nup358/BicD2 complex towards a 2:2:2 stoichiometry, promoting dimerization at lower protein concentrations than without BicD2. Similarly, the presence of the Nup358/KLC2 interaction results in a shift towards a 2:2:2 stoichiometry. Based on these results, we hypothesize that KLC2 and BicD2 are recruited to Nup358 in a cooperative manner, and cooperativity may be promoted by modulation of the oligomeric state.

Intrinsically disordered arginine-glycine-rich (RG/RGG) regions are highly abundant in the eukaryotic proteome. Proteins containing these motifs participate in fundamental cellular processes, including nuclear import, transcriptional regulation, biomolecular condensate formation, and apoptosis. Mutations or dysfunction of RG/RGG proteins have been implicated in neurodegenerative diseases and cancer. Although some RG/RGG proteins have been shown to drive condensate formation, localize to membrane-less organelles, interact with nuclear import receptors, or undergo arginine methylation, these properties are not shared uniformly across the proteome. The considerable diversity in RG/RGG motif length and amino acid composition raises the question of which sequence features determine their functional behaviour. To address this, we conducted a systematic bioinformatics and experimental analysis, combining synthetic and natural peptides with studies on the RNA-binding protein FUS as a model system. Our results reveal that the sequence composition of RG/RGG motifs is a key determinant of their capacity for RNA-mediated condensate formation, stress granule recruitment, and transportin-1-mediated chaperoning and nuclear import. These findings provide new insight into the sequence grammar of disordered RG/RGG regions and how it encodes the multifunctionality of these proteins in cellular regulation.

The pathological mechanisms and significance for the prevalent hotspot mutations in disordered protein regions are poorly understood. ASXL1 is an obligate co-factor for BAP1 in H2AK119 deubiquitination. ASXL1 mutations are very frequent in myeloid malignancies, and are mostly C-terminal truncating mutations concentrated in a specific disordered region of ASXL1. ASXL1 truncations are gain-of-function mutations that promote myeloid malignancies, but the underlying mechanisms remain poorly understood. Here we show that the frequently truncated mutants of ASXL1 possess an intrinsic property of forming phase-separated biomolecular condensates, and this property is normally suppressed by the frequently deleted regions. A disease-mutant of the endogenous ASXL1 in leukemia cells forms dynamic nuclear co-condensates with other endogenous factors important for gene activation. The ASXL1 disease-mutants can greatly enhance H2A deubiquitination activity of BAP1 in cells and in vitro reconstituted system, enhance myeloid leukemia cell growth, and promote leukemogenesis in a mouse transplant model by turning on myeloid leukemogenic transcriptional programs. However, substitution of residues important for condensation disrupted or impaired all these abilities, suggesting that the condensation property is crucially important for the ASXL1 mutants in promoting cancer. Moreover, we discover that the conserved negative charges in the highly disordered and frequently deleted region on ASXL1 suppress the condensation of the wild type ASXL1. Charge-neutralizing mutations in this region restores condensation of the full-length ASXL1, and are sufficient to turn ASXL1 into a leukemogenic protein. Biochemical, biophysical, and simulation analyses suggest the intramolecular interactions normally mask the N-terminal region in engaging intermolecular interactions required for phase separation, and disease truncations escape from the regulatory interactions and unleash the phase separation property to form nuclear hubs to promote expression of tumorigenic gene programs. Finally, by showing a striking correlation of the mutation frequencies with the condensation properties and leukemogenesis activity for a series of human patient mutations, we suggest that dysregulation of condensation is a central mechanism for ASXL1 mutations in promoting myeloid malignancies. This suggests that dysregulation of condensation may be a key mechanism for some of the prevalent hotspot disease mutations in the disordered proteomes.

Recoverin is a key calcium sensor that controls the desensitization of the visual rhodopsin by GRK1. Previous studies have traditionally been conducted on bovine protein (bRec), while data on human ortholog (hRec) remain scarce. Here, we combine X-ray crystallography, X-ray absorption spectroscopy (XANES), quantum mechanical calculations, molecular dynamics, and functional assays to provide an integrated characterization of hRec. The 2Ca2+-bound hRec structure was solved at 1.60 [A], showing that, unlike bRec, hRec interacts with ROS membranes at physiologically relevant submicromolar Ca2+ levels, due to a species-specific charge distribution that might influence membrane interactions. Both recoverins form a set of Ca2+/Zn2+-bound conformers with improved functional performance. X-ray crystallography (1.85 [A]) and XANES revealed a specific tetrahedral Zn2+ site in 1Ca2+-bound hRec, the first such site reported in the NCS family. In 1Ca2+-bound hRec, zinc promotes the formation of active state, whereas in 2Ca2+-state of bRec, it significantly enhances GRK1 binding, as the latter can complement the Zn2+ coordination. These data refine our understanding of recoverin function in humans and highlight its role as a key link between calcium and zinc signaling in mammalian photoreceptors under normal and pathological conditions.

Humans have 437 catalytically competent protein kinase domains with the typical kinase fold, similar to the structure of Protein Kinase A (PKA). The active form of a kinase must satisfy requirements for binding ATP, magnesium, and substrate. From structural bioinformatics analysis of 248 crystal structures of 54 unique substrate-bound kinases, we derived structural criteria for the active form of typical protein kinases. We include well-known requirements on the DFG motif of the activation loop and the N-terminal domain salt bridge, but also on the positions of the N-terminal and C-terminal segments of the activation loop that must be placed appropriately to bind substrate. With these criteria, only 130 of the 437 human catalytic protein kinases (30%) are in the Protein Data Bank in their active form. Because the active forms of catalytic kinases are needed for understanding substrate specificity and the effects of mutations on catalytic activity in cancer and other diseases, we used AlphaFold2 to produce models of all 437 human protein kinases in the active form. This was accomplished with templates from the PDB that resemble substrate-bound structures, shallow multiple sequence alignments of orthologs and close paralogs of the query protein, and application of the active-kinase criteria to the output models. We selected models for each kinase based on intramolecular ipSAE scores of the activation loop residues of these models, demonstrating that the highest scoring models have the lowest or close to the lowest RMSD to 29 non-redundant substrate-bound structures in the PDB. A larger benchmark of 117 active kinase structures with solved activation loops in the PDB shows that 71% of the highest scoring AlphaFold2 models had backbone RMSD < 1.0 [A] to the benchmark structures and 92% were within 2.0 [A]. Models for all 437 catalytic kinases are available at https://dunbrack.fccc.edu/kincore/activemodels. We believe they may be useful for interpreting mutations leading to constitutive catalytic activity in cancer as well as for templates for modeling substrate and inhibitor binding for molecules which bind to the active state.

Purified proteins are routinely flash frozen for use in functional and structural studies, providing a convenient way to reproduce results across complex experiments. Rho guanine-nucleotide exchange factors (RhoGEFs) are no exception to this practice, yet the effects of freezing on their activity and stability remain largely uncharacterized. This gap potentially affects the characterization of these important enzymes and how results are interpreted with respect to their prospective use as therapeutic targets. Here, we tested the isolated DH/PH tandems of P-Rex1, P-Rex2, and PRG under different cryoprotectant conditions and monitored activity and thermostability over time after flash freezing. Our results show a clear divergence between the activity of fresh and frozen purified RhoGEF protein samples in as little as one week for some conditions. Specifically, the variability in data collected on frozen samples was greatly increased. Despite these differences, thermostability seems to be preserved for much longer timepoints across RhoGEFs. Moreover, despite eventual changes in both activity and thermostability with respect to freezing, there are no obvious changes in global conformation between fresh and frozen samples of the isolated P-Rex2 DH/PH tandem. From our data, there are few generalizable trends between the different RhoGEFs and no single cryoprotective agent tested was a silver bullet to preserve both activity and thermostability across RhoGEFs. Overall, our findings emphasize the unpredictable effects of freezing RhoGEFs. As such, RhoGEF freezing should be carefully characterized for each protein and critically viewed when comparing analyses between different studies.

Escherichia coli YicC enzyme is the founding member of a family of endoribonucleases that is encoded in virtually all bacterial species. Previous structural studies revealed that this ribonuclease binds RNA by a novel mechanism in which the hexameric apoprotein presents an open channel that undergoes a large rotation upon RNA binding and clamps down on the RNA. The current study follows up on these findings by examining the cleavage of various oligonucleotide substrates designed to probe recognition elements required for YicC binding and cleavage. A 26-nucleotide RNA oligomer (oligo), with a KD in the low micromolar range, was the standard to which numerous oligos with altered sequence were compared. In vitro RNase assays and fluorescence anisotropy binding measurements indicated that the preferred substrates for YicC were relatively small RNAs that contain some secondary structure. Larger RNAs or highly structured RNAs were less-than-optimal substrates. Similarly, RyhB RNA, a [~]90-nucleotide, iron-responsive RNA of E. coli, which has been described as a target of YicC binding and/or cleavage, was a poor YicC substrate in our assays. These results suggest that the native substrates for YicC-family members are very small RNAs or RNA fragments derived from larger RNAs.