Biology subjects

Rai, S. K.

Publications and source records attributed to Rai, S. K..

13 recordsLinked to original sources

bioRxiv2026

Intact and single-molecule analysis of heparan sulfate

Establishing tools to couple biological processes to a DNA sequence has transformed our ability to monitor life at the molecular scale due to the scalability, flexibility, and low cost of DNA sequencing. Key examples include DNA-protein (ChIP-seq1), RNA-protein (CLIP-seq2), protein-protein (proximity ligation assay3), and Cas-based recording of cellular events4. In contrast, this paradigm has not yet significantly enhanced studies of glycans, which are mostly limited to non-DNA based chemical and biochemical assays. While classical asparagine-linked and serine/threonine-linked glycans can be directly sequenced using mass spectrometry, glycosaminoglycans - notable players in the extracellular matrix - cannot be easily analyzed in their full-length form. Here we introduce HS-nano-seq, a generalized framework to selectively label, process, and detect features of heparan sulfate on a nanopore sequencing platform. Recognizing that heparan sulfate is biochemically analogous to a nucleic acid, we report purification techniques using rapid nucleic acid strategies and conjugation methods to couple DNA adapters, generating HS-DNA chimeras resolved as discrete species by capillary electrophoresis (CE). The CE assay can distinguish features of chain length and sulfation patterns. At the single-molecule level enabled by nanopore sensing, we classify a library of synthetic heparan sulfate standards and demonstrate that nanopore ionic current fingerprints encode sulfation-dependent structural features of individual HS chains. Analysis of intact, cell-derived HS could discriminate features of individual chains with different sulfation patterns, defining the heterogeneity of binding motifs across cell types and how cells organize and program the tethered extracellular matrix. More broadly, HS-nano-seq establishes a framework for achieving full-length readouts of ECM glycopolymers that are amenable to the same biological interrogation as nucleic acids.

biochemistry↗

bioRxiv2025

Catch and release of sialoglycoRNAs enables sequencing-based profiling across cellular and extracellular material

Glycosylated RNAs (glycoRNAs) represent a recently discovered class of small RNAs, but their systematic characterization has been limited by reliance on metabolic chemical reporters and high RNA input requirements. Here we present rPAL sequencing (rPAL-seq), a sensitive and selective platform for de novo discovery of sialoglycoRNAs. rPAL-seq combines enhanced periodate oxidation of sialic acids with a capture-release workflow and optimized library construction using poly(A) extension coupled with template-switching reverse transcription. The method enabled reproducible profiling from less than 100 ng of input RNA, corresponding to less than 2% of the material required by previous approaches. When applied across 13 human cell lines, rPAL-seq identified lineage-associated glycoRNA patterns alongside a conserved core dominated by uridine-rich snRNAs and snoRNAs, with modification signatures implicating glycosylation on acp3U or related uridine-based modifications. Extending to extracellular vesicles and non-vesicular nanoparticles, rPAL-seq revealed secreted glycoRNA profiles distinct from those of the cellular fraction. rPAL-seq provides a robust, scalable strategy for glycoRNA profiling, opening new avenues for studying this emerging biopolymer.

molecular biology↗

bioRxiv2025

The regulation of virulence gene expression is controlled by phase separation of heterochromatin protein 1 (HP1) in Plasmodium falciparum

Malaria caused by Plasmodium falciparum involves antigenic variation on the infected red blood cell surface by a mutually exclusive expression of virulence (var) genes. The repressed var genes have a distinct genome organization where they localize as a cluster near the nuclear periphery and are bound by PfHP1 (P. falciparum Heterochromatin Protein 1). However, the mode of regulation of var genes by PfHP1 remains unclear. In this study, we show that PfHP1 undergoes phase separation in vitro in an RNA and DNA-dependent manner. Single-molecule DNA tethering experiments further revealed that AT-rich DNA sequences act as nucleation sites for the assembly and compaction of PfHP1-mediated heterochromatinization. We have also identified point mutations in the IDRs (intrinsically disordered regions) of PfHP1 that disrupt its phase separation as well as DNA compaction in vitro. To assess the dynamic properties of PfHP1 condensates in vivo, we performed fluorescence recovery after photobleaching (FRAP) in GFP tagged PfHP1 parasites, which revealed rapid fluorescence recovery, supporting their fluidity and phase separation behaviour. Ectopic expression of PfHP1 phase separation and DNA compaction mutants led to dispersed nuclear localization of PfHP1, in contrast to the punctate appearance of the wild-type protein and altered chromatin binding at var genes. These results were corroborated with DiCre/loxP-based conditional expression of the same PfHP1 mutants, which also led to the de-repression of multiple var genes (as many as 54 out of 60 var genes), mimicking the phenotype of PfHP1 depletion mutants. Hi-C sequencing of PfHP1 mutants revealed loss of interactions in the heterochromatic regions, indicating that PfHP1 phase separation is an essential mechanism for repressive cluster formation. In conclusion, our study demonstrates the role of PfHP1-mediated phase separation in heterochromatin formation and var gene silencing; unveiling a fundamental mechanism that drives antigenic variation in P. falciparum.

molecular biology↗

bioRxiv2024

DNA replication dynamics are associated with genome composition in Plasmodium species

Plasmodium species have variable genome compositions: many have an A/T-content of at least 80% while others are similar in composition to human cells. Here, we made a direct comparison of DNA replication dynamics in two Plasmodium species whose genomes differ by [~]20% A/T-content. This yielded fundamental insights into how DNA composition may affect replication. The highly A/T-biased genome of P. falciparum showed unusual replication dynamics that were not observed in the more balanced P. knowlesi - which had dynamics more like those of human cell lines. We observed that replication forks moved 50% slower in P. falciparum than in P. knowlesi. In P. falciparum, replication forks slowed down over the course of S-phase whereas in P. knowlesi, fork speed increased as in human cells. Furthermore, in both P. knowlesi and human cells, replication forks were strikingly slowed by sequences of particularly high A/T-bias, but in P. falciparum, although replication forks were inherently slow, they were not particularly slow in such biased sequences. Thus, the replisome of P. falciparum may have evolved alongside its extremely biased genome, making it unusually robust to sequence bias. Since several antimalarial drugs act to stall DNA replication, this study may have implications for the effectiveness of, and development of, antimalarial therapies.

microbiology↗

bioRxiv2024

Chaperone-mediated heterotypic phase separation prevents the amyloid formation of the pathological Y145Stop variant of the prion protein

Biomolecular condensates formed via phase separation of proteins and nucleic acids are crucial for the spatiotemporal regulation of a diverse array of essential cellular functions and the maintenance of cellular homeostasis. However, aberrant liquid-to-solid phase transitions of such condensates are associated with several fatal human diseases. Such dynamic membraneless compartments can contain a range of molecular chaperones that can regulate the phase behavior of proteins involved in the formation of these biological condensates. Here, we show that a heat shock protein 40 (Hsp40), Ydj1, exhibits a holdase activity by potentiating the phase separation of a disease-associated stop codon mutant of the prion protein (Y145Stop) either by recruitment into Y145Stop condensates or via Y145Stop-Ydj1 two-component heterotypic phase separation that prevents the conformational conversion of Y145Stop into amyloid fibrils. Utilizing site-directed mutagenesis, multicolor fluorescence imaging, single-droplet steady-state and picosecond time-resolved fluorescence anisotropy, fluorescence recovery after photobleaching, and fluorescence correlation spectroscopy, we delineate the complex network of interactions that govern the heterotypic phase separation of Y145Stop and Ydj1. We also show that the properties of such heterotypic condensates can further be tuned by RNA that promotes the formation of multicomponent multiphasic protein-RNA condensates. Our vibrational Raman spectroscopy results in conjunction with atomic force microscopy imaging reveal that Ydj1 effectively redirects the self-assembly of Y145Stop towards a dynamically-arrested non-amyloidogenic pathway, preventing the formation of typical amyloid fibrils. Our findings underscore the importance of chaperone-mediated heterotypic phase separation in regulating aberrant phase transitions and amyloid formation associated with a wide range of deadly neurodegenerative diseases.

biophysics↗

bioRxiv2024

Chaperone-mediated heterotypic phase separation regulates liquid-to-solid phase transitions into amyloid fibrils

Biomolecular condensates formed via the phase separation of proteins and nucleic acids are thought to regulate a myriad of cellular processes with exquisite spatiotemporal precision. However, such highly dynamic, viscoelastic, mesoscopic, intracellular membraneless bodies can undergo aberrant liquid-to-solid transitions into a range of amyloid-like species. The formation of such pathological assemblies necessitates their clearance by the cellular protein quality control machinery comprising molecular chaperones. Nonetheless, the mechanism underlying the chaperone-mediated regulation of protein homeostasis within biomolecular condensates remains elusive. Here, we present a unique case demonstrating that a heat shock protein 40 (Hsp40), Ydj1, promotes the heterotypic phase separation of intrinsically disordered tau via intermolecular electrostatic and hydrophobic interactions. Through a diverse array of tools involving high-resolution fluorescence imaging, single-droplet steady-state and picosecond time-resolved fluorescence anisotropy, and single-molecule FRET (Forster resonance energy transfer), we elucidate the diverse structural conformations of tau present within phase-separated heterotypic condensates that are otherwise predisposed to aggregation. Our vibrational Raman spectroscopy and electron microscopy data show that the presence of Ydj1 in tau-Ydj1 condensates abolishes the formation of amyloid fibrils, unlike tau-only droplets. By sequentially deleting segments, we identify amyloidogenic hexapeptide motifs located in the hydrophobic microtubule-binding region of tau that foster contacts with the peptide-binding regions of Ydj1, promoting the formation of tau-Ydj1 binary condensates. Additionally, we show that the underlying network of interactions governing these condensates can be further tuned by RNA. Our results underscore an intriguing interplay of molecular drivers that govern chaperone-associated phase separation, with broader implications for the chaperoning of a wide range of intrinsically disordered proteins involved in physiology and disease.

biophysics↗

bioRxiv2023

Single-Molecule FRET Illuminates Structural Subpopulations and Dissects Crucial Molecular Events During Phase Separation of a Prion-Like Low Complexity Domain

Biomolecular condensates formed via phase separation of proteins and nucleic acids are thought to be associated with a wide range of cellular functions and dysfunctions. We dissect critical molecular events associated with phase separation of an intrinsically disordered prion-like low-complexity domain of Fused in Sarcoma by performing single-molecule studies that permit us to access the wealth of molecular information that is skewed in conventional ensemble experiments. Our single-molecule FRET experiments reveal the coexistence of two conformationally distinct subpopulations in the monomeric form. Single-droplet single-molecule FRET studies coupled with fluorescence correlation spectroscopy, picosecond time-resolved fluorescence anisotropy, and vibrational Raman spectroscopy indicate that structural unwinding switches intramolecular interactions into intermolecular contacts allowing the formation of a dynamic network within condensates. A disease-related mutation introduces enhanced structural plasticity engendering greater interchain interactions that can accelerate pathological aggregation. Our findings provide key mechanistic underpinnings of sequence-encoded dynamically-controlled structural unzipping resulting in biological phase separation.

biophysics↗

bioRxiv2023

Massive genome reduction occurred prior to the origin of coral algal symbionts

Dinoflagellates in the Family Symbiodiniaceae (Order Suessiales) are diverse, predominantly symbiotic lineages that associate with taxa such as corals and jellyfish. Their ancestor is believed to have been free-living, and the establishment of symbiosis (i.e., symbiogenesis) is hypothesised to have occurred multiple times during Symbiodiniaceae evolution. Among Symbiodiniaceae taxa, the genus Effrenium is an early diverging, free-living lineage that is phylogenetically positioned between two robustly supported groups of genera within which symbiotic taxa have emerged. The lack of symbiogenesis in Effrenium suggests that the ancestral features of Symbiodiniaceae may have been retained in this lineage. Here we present de novo assembled genomes and associated transcriptome data from three isolates of Effrenium voratum. We compared the Effrenium genomes (1.2-1.9 Gbp in size) and gene features with those of 16 Symbiodiniaceae taxa and other outgroup dinoflagellates. Surprisingly, we find that genome reduction, which is often associated with a symbiotic lifestyle, predates the origin of Symbiodiniaceae. We postulate that adaptation to an extreme habitat (e.g., as in Polarella glacialis) or life in oligotrophic conditions resulted in the Suessiales ancestor having a haploid genome size < 2Gbp, which was retained (or reduced) among all extant algae in this lineage. Nonetheless, our data reveal that the free-living lifestyle distinguishes Effrenium from symbiotic Symbiodiniaceae vis-a-vis their longer introns, more-extensive mRNA editing, fewer ([~]30%) lineage-specific gene families, and lower ([~]10%) level of pseudogenisation. These results demonstrate how genome reduction and the adaptation to symbiotic versus free-living lifestyles intersect, and have driven the diversification and genome evolution of Symbiodiniaceae.

genomics↗

bioRxiv2022

A high-resolution, nanopore-based artificial intelligence assay for DNA replication stress in human cancer cells

DNA replication stress is a hallmark of cancer that is exploited by chemotherapies. Current assays for replication stress have low throughput and poor resolution whilst being unable to map the movement of replication forks genome-wide. We present a new method that uses nanopore sequencing and artificial intelligence to map forks and measure their rates of movement and stalling in melanoma and colon cancer cells treated with chemotherapies. Our method can differentiate between fork slowing and fork stalling in cells treated with hydroxyurea, as well as inhibitors of ATR, WEE1, and PARP1. These different therapies yield different characteristic signatures of replication stress. We assess the role of the intra-S-phase checkpoint on fork slowing and stalling and show that replication stress dynamically changes over S-phase. This method requires sequencing on only a single nanopore flow cell, and the cost-effectiveness and high throughput enables functional screens to determine how human cancers respond to replication-targeted therapies.

cancer biology↗

bioRxiv2022

Heterotypic electrostatic interactions control complex phase separation of tau and prion into multiphasic condensates and co-aggregates

Biomolecular condensates formed via phase separation of proteins and nucleic acids are thought to perform a wide range of critical cellular functions by maintaining spatiotemporal regulation and organizing intracellular biochemistry. However, aberrant phase transitions are implicated in a multitude of human diseases. Here, we demonstrate that two neuronal proteins namely, tau and prion undergo complex coacervation driven by domain-specific electrostatic interactions to yield highly dynamic, mesoscopic liquid-like droplets. The acidic N-terminal segment of tau interacts electrostatically with the polybasic N-terminal intrinsically disordered segment of the prion protein (PrP). We employed a unique combination of time-resolved tools that encompass several orders of magnitude of timescales ranging from nanoseconds to seconds. These studies unveil an intriguing orchestra of molecular events associated with the formation of heterotypic condensates comprising ephemeral, domain-specific, short-range electrostatic nanoclusters. Our results reveal that these heterotypic condensates can be tuned by RNA in a stoichiometry-dependent manner resulting in reversible, multiphasic, immiscible, ternary condensates of different morphologies ranging from core-shell to nested droplets. This ternary system exhibits a typical three-regime phase behavior reminiscent of other membraneless organelles including nucleolar condensates. We also show that upon aging, tau-PrP droplets gradually convert into solid-like co-assemblies by sequestration of persistent intermolecular interactions. Our vibrational Raman spectroscopic data in conjunction with atomic force microscopy and multi-color fluorescence imaging results reveal the presence of amorphous and amyloid-like co-aggregates upon maturation. Our findings provide mechanistic underpinnings of overlapping neuropathology involving tau and PrP and highlight a broader role of complex phase transitions in physiology and disease.

biophysics↗

bioRxiv2021

Spatiotemporal Modulations in Heterotypic Condensates of Prion and α-Synuclein Control Phase Transitions and Amyloid Conversion

Biomolecular condensates formed via liquid-liquid phase separation (LLPS) of proteins and nucleic acids are thought to govern critical cellular functions. These multicomponent assemblies provide dynamic hubs for competitive homotypic and heterotypic interactions. Here, we demonstrate that the complex coacervation between the prion protein (PrP) and -synuclein (-Syn) within a narrow stoichiometry regime results in the formation of highly dynamic liquid droplets. Domain-specific electrostatic interactions between the positively charged intrinsically disordered N-terminal segment of PrP and the negatively charged C-terminal domain of -Syn drive the formation of these highly tunable, reversible, thermo-responsive condensates. Picosecond time-resolved measurements revealed the existence of relatively ordered electrostatic nanoclusters that are stable on the nanosecond timescale and can undergo breaking-and-making on a much slower timescale giving rise to the liquid-like behavior on the second timescale and mesoscopic length-scale. The addition of RNA to these preformed coacervates yields multiphasic, anisotropic, vesicle-like, hollow condensates. LLPS promotes liquid-to-solid maturation of -Syn-PrP condensates resulting in the rapid conversion into heterotypic amyloids. Our results suggest that synergistic interactions between PrP and -Syn in liquid condensates can offer mechanistic underpinnings of their physiological role and overlapping neuropathological features.

biophysics↗

bioRxiv2021

An Intrinsically Disordered Pathological Variant of the Prion Protein Y145Stop Transforms into Self-Templating Amyloids via Liquid-Liquid Phase Separation

Biomolecular condensation via liquid-liquid phase separation of intrinsically disordered proteins/regions (IDPs/IDRs) along with other biomolecules is thought to govern critical cellular functions, whereas, aberrant phase transitions are associated with a range of deadly neurodegenerative diseases. Here we show, a naturally occurring pathological truncation variant of the prion protein (PrP) by a mutation of a tyrosine residue at 145 to a stop codon (Y145Stop) yielding a highly disordered N-terminal IDR that spontaneously phase-separates into liquid-like droplets. Phase separation of this N-terminal segment that is rich in positively charged and aromatic residues is promoted by the electrostatic screening and a multitude of other transient, intermolecular, noncovalent interactions. Single-droplet Raman measurements in conjunction with an array of bioinformatic, spectroscopic, microscopic, and mutagenesis studies revealed that the intrinsic disorder and dynamics are retained in the liquid-like condensates. Lower concentrations of RNA promote the phase transition of Y145Stop at low micromolar protein concentrations under physiological condition. Whereas, higher RNA to protein ratios inhibit condensation indicating the role of RNA in modulating the phase behavior of Y145Stop. Highly dynamic liquid-like droplets eventually transform into dynamically-arrested, ordered, {beta}-rich, amyloid-like aggregates via liquid-to-solid transition upon aging. These amyloid-like aggregates formed via phase separation display the self-templating characteristic and are capable of recruiting and autocatalytically converting monomeric Y145Stop into amyloid fibrils. In contrast to this disease-associated intrinsically disordered Y145 truncated variant, the wild-type full-length PrP exhibited a much lower propensity for phase separation and liquid-to-solid maturation into amyloid-like aggregates hinting at a potentially crucial, chaperone-like, protecting role of the globular C-terminal domain that remains largely conserved in vertebrate evolution. Such an intriguing interplay in the modulation of the protein phase behavior will have much broader implications in cell physiology and disease.

biophysics↗

bioRxiv2020

Comparison of long read methods for sequencing and assembly of a plant genome

Sequencing technologies have advanced to the point where it is possible to generate high accuracy, haplotype resolved, chromosome scale assemblies. Several long read sequencing technologies are available on the market and a growing number of algorithms have been developed over the last years to assemble the reads generated by those technologies. When starting a new genome project, it is therefore challenging to select the most cost-effective sequencing technology as well as the most appropriate software for assembly and polishing. For this reason, it is important to benchmark different approaches applied to the same sample. Here, we report a comparison of three long read sequencing technologies applied to the de novo assembly of a plant genome, Macadamia jansenii. We have generated sequencing data using Pacific Biosciences (Sequel I), Oxford Nanopore Technologies (PromethION) and BGI (single-tube Long Fragment Read) technologies for the same sample. Several assemblers were benchmarked in the assembly of PacBio and Nanopore reads. Results obtained from combining long read technologies or short read and long read technologies are also presented. The assemblies were compared for contiguity, accuracy and completeness as well as sequencing costs and DNA material requirements. Overall, the three long read technologies produced highly contiguous and complete genome assemblies of Macadamia jansenii. At the time of sequencing, the cost associated with each method was significantly different but continuous improvements in technologies have resulted in greater accuracy, increased throughput and reduced costs. We propose updating this comparison regularly with reports on significant iterations of the sequencing technologies.

genomics↗