Biology subjects

Kapoor, T. M.

Publications and source records attributed to Kapoor, T. M..

11 recordsLinked to original sources

bioRxiv2024

Structure of the microtubule anchoring factor NEDD1 bound to the γ-tubulin ring complex

The {gamma}-tubulin ring complex ({gamma}-TuRC) is an essential multiprotein assembly, in which {gamma}-tubulin, GCP2-6, actin, MZT1 and MZT2 form an asymmetric cone-shaped structure that provides a template for microtubule nucleation. The {gamma}-TuRC is recruited to microtubule organizing centers (MTOCs), such as centrosomes and pre-existing mitotic spindle microtubules, via the evolutionarily-conserved attachment factor NEDD1. NEDD1 contains an N-terminal WD40 domain that binds to microtubules, and a C-terminal domain that associates with the {gamma}-TuRC. However, the structural basis of the NEDD1-{gamma}-TuRC interaction is not known. Here, we report cryo-electron microscopy (cryo-EM) structures of NEDD1 bound to the human {gamma}-TuRC in the absence or presence of the activating factor CDK5RAP2, which interacts with GCP2 to induce conformational changes in the {gamma}-TuRC and promote its microtubule nucleating function. We found that the C-terminus of NEDD1 forms a tetrameric -helical assembly that contacts the lumen of the {gamma}-TuRC cone, is anchored to GCP4, 5 and 6 via protein modules consisting of MZT1 & GCP3 subcomplexes, and orients its microtubule-binding WD40 domains away from the complex. We biochemically tested our structural models by identifying NEDD1 mutants unable to pull-down{gamma} -tubulin from cultured cells. The structure of the {gamma}-TuRC simultaneously bound to NEDD1 and CDK5RAP2 reveals that both factors can associate with the "open" conformation of the complex. Our results show that NEDD1 does not induce conformational changes in the {gamma}-TuRC, but suggest that anchoring of {gamma}-TuRC-capped microtubules by NEDD1 would be structurally compatible with the significant conformational changes experienced by the {gamma}-TuRC during microtubule nucleation.

biochemistry↗

bioRxiv2024

Distinct modes of coupling between VCP, an essential unfoldase, and deubiquitinases

Errors in proteostasis, which requires regulated degradation and recycling of diverse proteins, are linked to aging, cancer and neurodegenerative disease (1). In particular, recycling proteins from multiprotein complexes, organelles and membranes is initiated by ubiquitylation, extraction and unfolding by the essential mechanoenzyme VCP (2-4), and ubiquitin removal by deubiquitinases (DUBs), a class of [~]100 ubiquitin-specific proteases in humans (5, 6). As VCPs substrate recognition requires ubiquitylation, the removal of ubiquitins from substrates for recycling must follow extraction and unfolding. How the activities of VCP and different DUBs are coordinated for protein recycling or other fates is unclear. Here, we employ a photochemistry-based approach to profile proteome-wide domain-specific VCP interactions in living cells (7). We identify DUBs that bind near the entry, exit, or both sites of VCPs central pore, the channel for ATP-dependent substrate translocation (8-10). From this set of DUBs, we focus on VCPIP1, required for organelle assembly and DNA repair (11-13), that our chemical proteomics workflow indicates binds the central pores entry and exit sites. We determine a [~]3[A] cryo-EM structure of the VCP-VCPIP1 complex and find up to 3 VCPIP1 protomers interact with the VCP hexamer. VCPIP1s UBX-L domain binds VCPs N-domain in a down conformation, linked to VCPs ADP-bound state (2, 14), and the deubiquitinase domain is positioned at the central pores exit site, poised to remove ubiquitin following substrate unfolding. We find that VCP stimulates VCPIP1s DUB activity and use mutagenesis and single-molecule mass photometry assays to test the structural model. Together, our data suggest that DUBs bind VCP at distinct sites and reveal how the two enzyme activities can be coordinated to achieve specific downstream outcomes for ubiquitylated proteins.

biochemistry↗

bioRxiv2023

Allosteric activation of VCP, a AAA unfoldase, by small molecule mimicry

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme activity, activator binding must be permissive to different conformational states needed for enzyme function. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, a AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP Activator 1 (VA1), a compound that dose-dependently stimulates VCP ATPase activity up to [~]3-fold. Our cryo-EM studies resulted in structures ([~]2.9-3.5 [A]-resolution) of VCP in apo and ADP-bound states, and revealed VA1 binding an allosteric pocket near the C-terminus in both states. Engineered mutations in the VA1 binding site confer resistance to VA1, and furthermore, modulate VCP activity to a similar level as VA1-mediated activation. The VA1 binding site can alternatively be occupied by a phenylalanine residue in the VCP C-terminal tail, a motif that is post-translationally modified and interacts with cofactors. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry. SignificanceThe loss of function of valosin-containing protein (VCP/p97), a mechanoenzyme from the AAA superfamily that hydrolyzes ATP and uses the released energy to extract or unfold substrate proteins, is linked to protein aggregation-based disorders. However, druggable allosteric sites to activate VCP, or any AAA mechanoenzyme, have not been identified. Here, we report cryo-EM structures of VCP in two states in complex with VA1, a compound we identified that dose-dependently stimulates VCPs ATP hydrolysis activity. The VA1 binding site can also be occupied by a phenylalanine residue in the VCP C-terminal tail, suggesting that VA1 acts through mimicry of this interaction. Our study reveals a druggable allosteric site and a mechanism of enzyme regulation.

biochemistry↗

bioRxiv2023

Using a function-first "scout fragment"-based approach to develop allosteric covalent inhibitors of conformationally dynamic helicase mechanoenzymes

Helicases, classified into six superfamilies, are mechanoenzymes that utilize energy derived from ATP hydrolysis to remodel DNA and RNA substrates. These enzymes have key roles in diverse cellular processes, such as genome replication and maintenance, ribosome assembly and translation. Helicases with essential functions only in certain cancer cells have been identified and helicases expressed by certain viruses are required for their pathogenicity. As a result, helicases are important targets for chemical probes and therapeutics. However, it has been very challenging to develop selective chemical inhibitors for helicases, enzymes with highly dynamic conformations. We envisioned that electrophilic scout fragments, which have been used for chemical proteomic based profiling, could be leveraged to develop covalent inhibitors of helicases. We adopted a function-first approach, combining enzymatic assays with enantiomeric probe pairs and mass spectrometry, to develop a covalent inhibitor that selectively targets an allosteric site in SARS-CoV-2 nsp13, a superfamily-1 helicase. Further, we demonstrate that scout fragments inhibit the activity of two human superfamily-2 helicases, BLM and WRN, involved in genome maintenance. Together, our findings suggest a covalent inhibitor discovery approach to target helicases and potentially other conformationally dynamic mechanoenzymes.

biochemistry↗

bioRxiv2023

High-content microscopy reveals a morphological signature of bortezomib resistance

Drug resistance is a challenge in anticancer therapy, particularly with targeted therapeutics and cytotoxic compounds. In many cases, cancers can be resistant to the drug prior to exposure, i.e., possess intrinsic drug resistance. However, we lack target-independent methods to anticipate resistance in cancer cell lines or characterize intrinsic drug resistance without a priori knowledge of its cause. We hypothesized that cell morphology could provide an unbiased readout of drug sensitivity prior to treatment. We therefore isolated clonal cell lines that were either sensitive or resistant to bortezomib, a well-characterized proteasome inhibitor and anticancer drug to which many cancer cells possess intrinsic resistance. We then measured high-dimensional single-cell morphology profiles using Cell Painting, a high-content microscopy assay. Our imaging- and computation-based profiling pipeline identified morphological features typically different between resistant and sensitive clones. These features were compiled to generate a morphological signature of bortezomib resistance, which correctly predicted the bortezomib treatment response in seven of ten cell lines not included in the training dataset. This signature of resistance was specific to bortezomib over other drugs targeting the ubiquitin-proteasome system. Our results provide evidence that intrinsic morphological features of drug resistance exist and establish a framework for their identification.

cancer biology↗

bioRxiv2022

Inhibition of the SARS-CoV-2 helicase at single-nucleotide resolution.

The genome of SARS-CoV-2 encodes for a helicase called nsp13 that is essential for viral replication and highly conserved across related viruses, making it an attractive antiviral target. Here we use nanopore tweezers, a high-resolution single-molecule technique, to gain detailed insight into how nsp13 turns ATP-hydrolysis into directed motion along nucleic acid strands. We measured nsp13 both as it translocates along single-stranded DNA or unwinds short DNA duplexes. Our data confirm that nsp13 uses the inchworm mechanism to move along the DNA in single-nucleotide steps, translocating at ~1000 nt/s or unwinding at ~100 bp/s. Nanopore tweezers high spatio-temporal resolution enables observation of the fundamental physical steps taken by nsp13 even as it translocates at speeds in excess of 1000 nucleotides per second enabling detailed kinetic analysis of nsp13 motion. As a proof-of-principle for inhibition studies, we observed nsp13s motion in the presence of the ATPase inhibitor ATP{gamma}S. Our data reveals that ATP{gamma}S interferes with nsp13s action by affecting several different kinetic processes. The dominant mechanism of inhibition differs depending on the application of assisting force. These advances demonstrate that nanopore tweezers are a powerful method for studying viral helicase mechanism and inhibition.

biophysics↗

bioRxiv2021

The MIDAS domain of AAA mechanoenzyme Mdn1 forms catch bonds with two different substrates

Catch bonds are a form of mechanoregulation wherein protein-ligand interactions are strengthened by the application of dissociative tension. Currently, the best-characterized examples of catch bonds are between single protein-ligand pairs. The essential AAA (ATPase associated with diverse cellular activities) mechanoenzyme Mdn1 drives two separate steps in ribosome biogenesis, using its MIDAS domain to extract the ubiquitin-like (UBL) domain-containing proteins Rsa4 and Ytm1 from ribosomal precursors. However, it must subsequently release these assembly factors to reinitiate the enzymatic cycle. The mechanism underlying MIDAS-UBL switching between strongly- and weakly-bound states is unknown. Here, we use single-molecule optical tweezers to investigate the force-dependence of MIDAS-UBL binding. Parallel experiments with Rsa4 and Ytm1 show that forces up to ~4 pN, matching the magnitude of force produced by AAA proteins similar to Mdn1, enhance the MIDAS domain binding lifetime up to tenfold, and higher forces accelerate dissociation. Together, our studies indicate that Mdn1s MIDAS domain forms catch bonds with more than one UBL-substrate, and provide insights into how mechanoregulation may contribute to the Mdn1 enzymatic cycle during ribosome biogenesis.

biophysics↗

bioRxiv2020

Targeting Allostery in the Dynein Motor Domain with Small Molecule Inhibitors

Cytoplasmic dyneins are AAA (ATPase associated with diverse cellular activities) motor proteins responsible for microtubule minus-end-directed intracellular transport. Dyneins unusually large size, four distinct nucleotide-binding sites, and the existence of closely-related isoforms with different functions, pose challenges for the design of potent and selective chemical inhibitors. Here we use structural approaches to develop a model for the inhibition of a well-characterized S. cerevisiae dynein construct by pyrazolo-pyrimidinone-based compounds. These data, along with single molecule experiments and mutagenesis studies, indicate that the compounds likely inhibit dynein by engaging the regulatory ATPase sites in the AAA3 and AAA4 domains, and not by interacting with dyneins main catalytic site in the AAA1 domain. A double Walker B mutant in AAA3 and AAA4 is an inactive enzyme, suggesting that inhibiting these regulatory sites can have a similar effect to inhibiting AAA1. Our findings reveal how chemical inhibitors can be designed to disrupt allosteric communication across dyneins AAA domains.

biochemistry↗

bioRxiv2020

Biochemical reconstitutions reveal principles of human γ-TuRC assembly and function

The formation of cellular microtubule networks is regulated by the{gamma} -tubulin ring complex ({gamma}-TuRC). This [~]2.3 MDa assembly of >31 proteins includes{gamma} -tubulin and GCP2-6, as well as MZT1 and an actin-like protein in a "lumenal bridge". The challenge of reconstituting the{gamma} -TuRC has limited dissections of its assembly and function. Here, we report a complete biochemical reconstitution of the human{gamma} -TuRC ({gamma}-TuRC-GFP), a [~]35S complex that nucleates microtubules in vitro. We extend our approach to generate a stable subcomplex,{gamma} -TuRCmini-GFP, which lacks MZT1 and actin. Using mutagenesis, we show that{gamma} -TuRCmini-GFP nucleates microtubules in a guanine nucleotide-dependent manner and proceeds with similar kinetics as reported for native{gamma} -TuRCs. Electron microscopy reveals that{gamma} -TuRC-GFP resembles the native{gamma} -TuRC architecture, while{gamma} -TuRCmini-GFP adopts a partial cone shape presenting only 8-10{gamma} -tubulin subunits and lacks a well-ordered lumenal bridge. Our structure-function analysis suggests that the lumenal bridge facilitates the self-assembly of regulatory interfaces around a microtubule-nucleating "core" in the{gamma} -TuRC.

biochemistry↗

bioRxiv2020

Force-dependent stimulation of RNA unwinding by SARS-CoV-2 nsp13 helicase

The superfamily-1 helicase non-structural protein 13 (nsp13) is required for SARS-CoV-2 replication, making it an important antiviral therapeutic target. The mechanism and regulation of nsp13 has not been explored at the single-molecule level. Specifically, force-dependent unwinding experiments have yet to be performed for any coronavirus helicase. Here, using optical tweezers, we find that nsp13 unwinding frequency, processivity, and velocity increase substantially when a destabilizing force is applied to the dsRNA, suggesting a passive unwinding mechanism. These results, along with bulk assays, depict nsp13 as an intrinsically weak helicase that can be potently activated by picoNewton forces. Such force-dependent behavior contrasts the known behavior of other viral monomeric helicases, drawing stronger parallels to ring-shaped helicases. Our findings suggest that mechanoregulation, which may be provided by a directly bound RNA-dependent RNA polymerase, enables on-demand helicase activity on the relevant polynucleotide substrate during viral replication.

biophysics↗

bioRxiv2019

Asymmetric molecular architecture of the human γ-tubulin ring complex

The{gamma} -tubulin ring complex ({gamma}-TuRC) is an essential regulator of centrosomal and acentrosomal microtubule formation 1-4. Metazoan{gamma} -TuRCs isolate as [~]2 MDa complexes containing the conserved proteins{gamma} -tubulin, GCP2 and GCP3, as well as the expanded subunits GCP4, GCP5, and GCP6 3,5,6. However, in current structural models,{gamma} -TuRCs assemble solely from subcomplexes of{gamma} -tubulin, GCP2 and GCP3 7. The role of the metazoan-specific subunits in{gamma} -TuRC assembly and architecture are not currently known, due to a lack of high resolution structural data for the native complex. Here, we present a cryo-EM structure of the native human{gamma} -TuRC at 3.8[A] resolution. Our reconstruction reveals an asymmetric, single helical-turn and cone-shaped structure built from at least 34 polypeptides. Pseudo-atomic models indicate that GCP4, GCP5 and GCP6 form distinct Y-shaped assemblies that structurally mimic GCP2/GCP3 subcomplexes and are distal to the{gamma} -TuRC \"seam\". Evolutionary expansion in metazoan-specific subunits diversifies the{gamma} -TuRC by introducing large (>100,000 [A]2) surfaces that could interact with different regulatory factors. We also identify an unanticipated structural bridge that includes an actin-like protein and spans the{gamma} -TuRC lumen. Despite its asymmetric composition and architecture, the human{gamma} -TuRC arranges{gamma} -tubulins into a helical geometry poised to nucleate microtubules. The observed compositional complexity of the{gamma} -TuRC could self-regulate its assembly into a cone-shaped structure to control microtubule formation across diverse contexts, e.g. within biological condensates 8 or alongside existing filaments 9.

biochemistry↗