Biology subjects

Borucu, U.

Publications and source records attributed to Borucu, U..

13 recordsLinked to original sources

bioRxiv2025

Cryo-EM structure of the FtsH periplasmic domain reveals functional dynamics

FtsH, an essential AAA+ metalloprotease, maintains cellular homeostasis by degrading misfolded and membrane-associated proteins. Here, we report cryo-EM structures of the Escherichia coli FtsH periplasmic domain (FtsH-PD) revealing insights into its conformational flexibility. Initial 2D class averages suggested three distinct orientations, right-handed and left-handed maps of FtsH-PD, and a map with a different conformation. The 4.9 [A] structure of FtsH-PD exhibits the conserved +{beta} fold, while the 7.3 [A] map with the different conformation displays a 20{o} clockwise rotation of two alpha helices. These findings support a model where conformational changes are present not only in the FtsH cytosolic domain, but also in the periplasmic domain and potentially facilitate substrate translocation through a combination of mechanisms involving both the FtsH-PD and the HflKC complexed with FtsH, along with lipid-scramblase activity to assist in membrane protein extraction. This study points out novel perspectives on how conformational changes in the periplasmic domain contribute to FtsH substrate degradation mechanisms.

biochemistry↗

bioRxiv2025

Off-Target Structural Insights: ArnA and AcrB in Bacterial Membrane Protein Cryo-EM Analysis

Membrane protein quality control in Escherichia coli involves coordinated actions of the AAA+ protease FtsH, the insertase YidC, and the regulatory complex HflKC. These systems maintain proteostasis by facilitating membrane protein insertion, folding, and degradation. To gain structural insights into a putative complex formed by FtsH and YidC, we performed single-particle cryogenic electron microscopy on detergent-solubilized membrane samples, from which FtsH and YidC were purified using Ni-NTA affinity and size exclusion chromatography. Although SDS-PAGE analysis indicated a high purity of these proteins, cryo-EM datasets unexpectedly yielded high-resolution structures of ArnA and AcrB at 4.0 [A] and 2.9 [A] resolutions, respectivelyArnA is a bifunctional enzyme involved in lipid A modification and polymyxin resistance, while AcrB is a multidrug efflux transporter of the AcrAB-TolC system. ArnA and AcrB are known membrane protein contaminants after Ni-NTA purifications. However, they were also detected in Strep-Tactin affinity purified FtsH-YidC samples. ArnA, typically cytoplasmic, was consistently found in membrane-isolated samples, indicating an association with membrane components. Two-dimensional class averages revealed additional particles resembling GroEL, cytochrome bo oxidase, and the AAA+ cytoplasmic domain of FtsH. Although our preparation was initially considered homogeneous, neither full-size FtsH nor the intact FtsH-YidC complex could be resolved. This case highlights that cryo-EM can reveal structural information beyond the intended target, and that overexpression of membrane proteins involved in quality control may lead to the co-purification of proteins such as ArnA and AcrB.

biochemistry↗

bioRxiv2025

Molecular mechanism for cargo-mediated initiation of kinesin-1 activation

Kinesin-1 is a dynamic heterotetrameric assembly of two heavy and two light chains (KHC and KLC) that mediates microtubule-based intracellular transport of many different cargoes. The complex adopts a compact, autoinhibited state that is activated by cargo-adaptor proteins containing specific short linear peptide motifs (SLiMs). These motifs interact with the tetratricopeptide repeat (TPR) domains of the KLCs. The mechanism coupling SLiM recognition to activation-associated conformational changes in the complex is unknown. Here we combine protein design, computational modelling, biophysical analysis, and electron microscopy to examine the structural and mechanistic consequences of SLiM binding to the KLC-TPR domain within the complete heterotetrameric holoenzyme. We show that coiled coil 1 (CC1) of the KHC docks KLC TPR domains in the autoinhibited complex, forming the shoulder feature observed in electron microscopy. Disrupting this interaction or binding an activating SLiM dislocates the TPR shoulder, freeing the motor domains and promoting transition between its closed, inactive, and open states. Opening the kinesin-1 complex facilitates binding to the microtubule-associated kinesin-1 cofactor, MAP7. Therefore, cargo-mediated dislocation of the TPR shoulder serves as a key initial step in kinesin-1 activation, allosterically linking cargo binding to motor dynamics.

biochemistry↗

bioRxiv2023

CryoET reveals actin filaments within platelet microtubules

Crosstalk between the actin and microtubule cytoskeletons is essential for many cellular processes. Recent studies have shown that microtubules and F-actin can assemble to form a composite structure where F-actin occupies the microtubule lumen. Whether these cytoskeletal hybrids exist in physiological settings and how they are formed is unclear. Here, we show that the short-crossover Class I actin filament previously identified inside microtubules in human HAP1 cells is cofilin-bound F-actin. Lumenal F-actin can be reconstituted in vitro, but cofilin is not essential. Moreover, actin filaments with both cofilin-bound and canonical morphologies reside within human platelet microtubules under physiological conditions. We propose that stress placed upon the microtubule network during motor-driven microtubule looping and sliding may facilitate the incorporation of actin into microtubules.

cell biology↗

bioRxiv2023

Engineering the ADDobody protein scaffold for generation of high-avidity ADDomer super-binders

Adenovirus-derived dodecamer (ADDomer) nanoparticles comprise 60 copies of Adenovirus penton base protein (PBP). ADDomer is thermostable, rendering the storage, transport and deployment of ADDomer-based therapeutics independent of a cold-chain. To expand the scope of ADDomer nanoparticles for new applications, we engineered ADDobodies. ADDobodies represent the crown domain of the PBP, genetically separated from its multimerization domain. We inserted heterologous sequences into hyper-variable loops in the crown domain. The resulting ADDobodies were expressed at high yields in Escherichia coli, are monomeric and maintain thermostability. We solved the X-ray structure of an ADDobody prototype validating our design. We demonstrated that ADDobodies can be used to select a specific binder against a target in in vitro selection experiments using ribosome display, with an enrichment factor of [~]104-fold in one selection round. We show that ADDobodies can be converted back into ADDomers by genetically reconnecting the selected ADDobody with the PBP multimerization domain from a different species, giving rise to a multivalent nanoparticle, called Chimera, confirmed by a 2.2 [A] structure determined by cryogenic electron microscopy (cryo-EM). Chimera comprises 60 binding sites, resulting in ultra-high, picomolar avidity to the target.

synthetic biology↗

bioRxiv2023

Rationally seeded computational protein design

Computational protein design is advancing rapidly. Here we describe efficient routes to two families of -helical-barrel proteins with central channels that bind small molecules. The designs are seeded by the sequences and structures of defined de novo oligomeric barrel-forming peptides. Adjacent helices are connected using computational loop building. For targets with antiparallel helices, short loops are sufficient. However, targets with parallel helices require longer connectors; namely, an outer layer of helix-turn-helix-turn-helix motifs that are packed onto the barrels computationally. Throughout these pipelines, residues that define open states of the barrels are maintained. This minimises sequence sampling and accelerates routes to successful designs. For each of 6 targets, just 2 - 6 synthetic genes are made for expression in E. coli. On average, 80% express to give soluble monomeric proteins that are characterized fully, including high-resolution structures for most targets that match the seed structures and design models with high accuracy.

synthetic biology↗

bioRxiv2023

Antibodies generated in vitro and in vivo elucidate design of a thermostable ADDomer COVID-19 nasal nanoparticle vaccine

COVID-19 continues to damage populations, communities and economies worldwide. Vaccines have reduced COVID-19-related hospitalisations and deaths, primarily in developed countries. Persisting infection rates, and highly transmissible SARS-CoV-2 Variants of Concern (VOCs) causing repeat and breakthrough infections, underscore the ongoing need for new treatments to achieve a global solution. Based on ADDomer, a self-assembling protein nanoparticle scaffold, we created ADDoCoV, a thermostable COVID-19 candidate vaccine displaying multiple copies of a SARS-CoV-2 receptor binding motif (RBM)-derived epitope. In vitro generated neutralising nanobodies combined with molecular dynamics (MD) simulations and electron cryo-microscopy (cryo-EM) established authenticity and accessibility of the epitopes displayed. A Gigabody comprising multimerized nanobodies prevented SARS-CoV-2 virion attachment with picomolar EC50. Antibodies generated by immunising mice cross-reacted with VOCs including Delta and Omicron. Our study elucidates nasal administration of ADDomer-based nanoparticles for active and passive immunisation against SARS-CoV-2 and provides a blueprint for designing nanoparticle reagents to combat respiratory viral infections.

biochemistry↗

bioRxiv2023

Structure of the Commander endosomal trafficking complex linked to X-linked intellectual disability/Ritscher-Schinzel syndrome

The Commander complex is required for endosomal recycling of diverse transmembrane cargos and is mutated in Ritscher-Schinzel syndrome. It comprises two subassemblies; Retriever composed of VPS35L, VPS26C and VPS29, and the CCC complex which contains ten subunits COMMD1-COMMD10 and two coiled-coil domain-containing (CCDC) proteins CCDC22 and CCDC93. Combining X-ray crystallography, electron cryomicroscopy and in silico predictions we have assembled a complete structural model of Commander. Retriever is distantly related to the endosomal Retromer complex but has unique features preventing the shared VPS29 subunit from interacting with Retromer-associated factors. The COMMD proteins form a distinctive hetero-decameric ring stabilised by extensive interactions with CCDC22 and CCDC93. These adopt a coiled-coil structure that connects the CCC and Retriever assemblies and recruits a sixteenth subunit, DENND10, to form the complete Commander complex. The structure allows mapping of disease-causing mutations and reveals the molecular features required for the function of this evolutionarily conserved trafficking machinery.

biochemistry↗

bioRxiv2022

Molecular architecture of the autoinhibited kinesin-1 lambda particle

Despite continuing progress in kinesin enzyme mechanochemistry and emerging understanding of the cargo recognition machinery, it is not known how these functions are coupled and controlled by the alpha-helical coiled coils encoded by a large component of kinesin protein sequences. Here, we combine computational structure prediction with single-particle negative stain electron microscopy to reveal the coiled-coil architecture of heterotetrameric kinesin-1, in its compact state. An unusual flexion in the scaffold enables folding of the complex, bringing the kinesin heavy chain-light chain interface into close apposition with a tetrameric assembly formed from the region of the molecule previously assumed to be the folding hinge. This framework for autoinhibition is required to uncover how engagement of cargo and other regulatory factors drive kinesin-1 activation. Summary statementIntegration of computational structure prediction with electron microscopy reveals the coiled-coil architecture of the autoinhibited compact conformer of the microtubule motor, kinesin-1.

cell biology↗

bioRxiv2022

The Free Fatty Acid-Binding Pocket is a Conserved Hallmark in Pathogenic β-Coronavirus Spike Proteins from SARS-CoV to Omicron

As COVID-19 persists, severe acquired respiratory syndrome coronavirus-2 (SARS-CoV-2) Variants of Concern (VOCs) emerge, accumulating spike (S) glycoprotein mutations. S receptor-binding domain (RBD) comprises a free fatty acid (FFA)-binding pocket. FFA-binding stabilizes a locked S conformation, interfering with virus infectivity. We provide evidence that the pocket is conserved in pathogenic {beta}-coronaviruses ({beta}-CoVs) infecting humans. SARS-CoV, MERS-CoV, SARS-CoV-2 and VOCs bind the essential FFA linoleic acid (LA), while binding is abolished by one mutation in common cold-causing HCoV-HKU1. In the SARS-CoV S structure, LA stabilizes the locked conformation while the open, infectious conformation is LA-free. Electron tomography of SARS-CoV-2 infected cells reveals that LA-treatment inhibits viral replication, resulting in fewer, deformed virions. Our results establish FFA-binding as a hallmark of pathogenic {beta}-CoV infection and replication, highlighting potential antiviral strategies. One-Sentence SummaryFree fatty acid-binding is conserved in pathogenic {beta}-coronavirus S proteins and suppresses viral infection and replication.

biochemistry↗

bioRxiv2021

Structure and mechanism of SARS-CoV-2 Spike N679-V687 deletion variant elucidate cell-type specific evolution of viral fitness

As the global burden of SARS-CoV-2 infections escalates, so does the evolution of viral variants which is of particular concern due to their potential for increased transmissibility and pathology. In addition to this entrenched variant diversity in circulation, RNA viruses can also display genetic diversity within single infected hosts with co-existing viral variants evolving differently in distinct cell types. The BriS{Delta} variant, originally identified as a viral subpopulation by passaging SARS-CoV-2 isolate hCoV-19/England/02/2020, comprises in the spike glycoprotein an eight amino-acid deletion encompassing the furin recognition motif and S1/S2 cleavage site. Here, we elucidate the structure, function and molecular dynamics of this variant spike providing mechanistic insight into how the deletion correlates to viral cell tropism, ACE2 receptor binding and infectivity of this SARS-CoV-2 variant. Moreover, our study reveals long-range allosteric communication between functional regions within the spike that differ in wild-type and deletion variant. Our results support a view of SARS-CoV-2 probing multiple evolutionary trajectories in distinct cell types within the same infected host.

biochemistry↗

bioRxiv2020

Unexpected free fatty acid binding pocket in the cryo-EM structure of SARS-CoV-2 spike protein

COVID-19, caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), represents a global crisis. Key to SARS-CoV-2 therapeutic development is unraveling the mechanisms driving high infectivity, broad tissue tropism and severe pathology. Our cryo-EM structure of SARS-CoV-2 spike (S) glycoprotein reveals that the receptor binding domains (RBDs) tightly and specifically bind the essential free fatty acid (FFA) linoleic acid (LA) in three composite binding pockets. The pocket also appears to be present in the highly pathogenic coronaviruses SARS-CoV and MERS-CoV. Lipid metabolome remodeling is a key feature of coronavirus infection, with LA at its core. LA metabolic pathways are central to inflammation, immune modulation and membrane fluidity. Our structure directly links LA and S, setting the stage for interventions targeting LA binding and metabolic remodeling by SARS-CoV-2. One Sentence SummaryA direct structural link between SARS-CoV-2 spike and linoleic acid, a key molecule in inflammation, immune modulation and membrane fluidity.

biochemistry↗

bioRxiv2019

In situ cryo-electron tomography reveals filamentous actin within the microtubule lumen

Microtubules and filamentous (F-) actin engage in complex interactions to drive many cellular processes from subcellular organisation to cell division and migration. This is thought to be largely controlled by proteins that interface between the two structurally distinct cytoskeletal components. Here, we use cryo-electron tomography to demonstrate that the microtubule lumen can be occupied by extended segments of F-actin in small-molecule induced, microtubule-based cellular projections. We uncover an unexpected versatility in cytoskeletal form that may prompt a significant development of our current models of cellular architecture and offer a new experimental approach for the in-situ study of microtubule structure and contents.

cell biology↗