Search bioRxiv⌕ Search
Home/Archive/bioRxiv

bioRxiv · 10.1101/2025.05.05.652134

SMALL MOLECULES WHICH CAN KEEP THE ECTODOMAINS OF HUMAN FcγRIIIa CLOSED: THERAPY FOR AUTOIMMUNE THROMBOCYTOPENIA PURPURA?

Ashish, F.
View original source ↗Export metadata ↓

Abstract

Comparative analysis of crystal structures of the ectodomains of the unliganded Fc{gamma} receptor A (Fc{gamma}RIIIa), its complex to IgG Fc and two designer proteins named "affimers" revealed that if the two ectodomains of Fc{gamma}RIIIa remain "closed", then they are conformationally incompetent to bind Fc region of antigen or infected cell bound IgG and protect them from degradation in the endosomes. By tightly binding to its ectodomains, these affimers derailed binding of Fc{gamma}RIIIa to Fc portion of IgG leading to latters degradation. This ability to decrease antibody titer suggests use of these affimers as therapeutic interventions in autoimmune conditions like immune thrombocytopenia purpura (ITP) which is caused by autoantibodies against platelets. I queried: are there any small molecules which can dock and lock the ectodomains of Fc{gamma}RIIIa analogous to affimers? Applying MTiOpenScreen on few libraries, I shortlisted 30 drug-like molecules which preferentially docked in the cleft between ectodomains of human Fc{gamma}RIIIa. Their complexes were subsequently analyzed by MD simulations at theoretical pH 7.4 (and 5.7) at 310 K. For all trajectories, we tracked two characteristic molecular descriptors of Fc{gamma}RIIIa: {theta} and {delta}, the angle and distance between the center masses of the two ectodomains, respectively. The values of these descriptors, in conjunction with estimated binding energies from MD simulations allow me to report three new compounds which may act as and/or lead to small molecule interventions in autoimmune conditions like ITP. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=97 SRC="FIGDIR/small/652134v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@1ca47d3org.highwire.dtl.DTLVardef@131a4eeorg.highwire.dtl.DTLVardef@431c70org.highwire.dtl.DTLVardef@87b_HPS_FORMAT_FIGEXP M_FIG C_FIG

Explore related subjects

biophysicsPreprint

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Ashish, F.. 2025-05-10. SMALL MOLECULES WHICH CAN KEEP THE ECTODOMAINS OF HUMAN FcγRIIIa CLOSED: THERAPY FOR AUTOIMMUNE THROMBOCYTOPENIA PURPURA?. https://doi.org/10.1101/2025.05.05.652134

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related preprints

bioRxiv2026

Mechanism of molecular recognition revealed through dynamic drug binding pathways to SARS-CoV-2 main protease

Characterization of drug-binding pathways remains experimentally limited by transient intermediates and computationally challenging due to long timescales intractable for conventional molecular dynamics. To address these challenges, we combined solution NMR titrations with weighted ensemble (WE) enhanced sampling simulations to resolve atomistic pathways of nirmatrelvir binding to the SARS-CoV-2 main protease. NMR titration revealed residue-dependent heterogeneity spanning fast, intermediate, and slow exchange regimes. WE simulations complement the NMR by providing insights into unassigned residues and adding time-resolved and three-dimensional structural context. We map key interactions along two distinct binding pathways, provide dynamic explanations for residues involved in resistance, and capture unique backbone conformations compared to those sampled in unbound or bound states. Our comprehensive binding model is consistent with a combined conformational selection and induced fit mechanism in which early transient contacts are made with residues E47 and L50 and allosteric motions are centered around residue V204 of the distal domain. This synergistic application of WE and titration NMR enables a more comprehensive characterization of drug binding than either method alone, providing an integrated framework that may have broader applicability to defining structure-kinetic relationships and guiding design of next-generation inhibitors.

biophysics↗
bioRxiv2026

A Minimally Perturbative DARPin Probe for Quantitative Fluorescence Imaging of the Human TCR-CD3 Complex

Fluorescence microscopy is a powerful tool for dissecting the molecular mechanisms of T-cell antigen recognition in living cells, but its quantitative insight critically depends on non-perturbative, high-quality probes. Here, we repurpose a small (~15 kDa) CD3epsilon-binding DARPin (designed ankyrin repeat proteins) to a fluorescent label for T-cell receptor (TCR)/CD3 complexes on primary human CD8+ T-cells, with the aim of generating a powerful tool for quantitative analysis, single-molecule tracking, and advanced imaging of TCR dynamics. We show that the DARPin binds CD3{varepsilon} with high affinity and selectivity and using single molecule tracking and brightness analysis, we characterize the TCR-CD3 diffusion behavior and show that the DARPin binds to both CD3epsilon; subunits. Importantly, labeling preserves antigen sensitivity: on supported lipid bilayers presenting cognate pMHC, T-cells remain responsive, assemble synapses, form TCR microclusters, and initiate signaling similar to unlabeled controls. We further demonstrate compatibility with lattice light-sheet microscopy for volumetric imaging of T-cell - APC interactions in living cells. Together, these results establish DARPins as versatile, minimally perturbative probes for high resolution, quantitative studies of T cell synapse organization and signaling.

biophysics↗
bioRxiv2026

Monitoring intramolecular dynamics across two regions of the mouse prion protein during misfolding and oligomerization using fluorescence correlation spectroscopy

It is important to determine whether native state dynamics drive the misfolding and oligomerization of the prion protein, which are important events in prion disease, and how they are modulated by conformational conversion. Native (N) mouse prion protein (moPrP) is known to form small (OS) and large (OL) oligomers rich in {beta}-sheet, and in this study, photoinduced electron transfer-fluorescence correlation spectroscopy (PET-FCS) has been used to characterize intramolecular dynamics within individual monomeric units in both isolated OS and OL, as well as the diffusion properties of the oligomers. It is estimated that OS and OL comprise of about 15 and 55 monomeric units, respectively. Microsecond dynamics at each of the two regions that are the 1-3 and 2-3 interfaces of native protein are distinct in N, OS and OL, although they occur on very similar timescales. Analysis of the evolution of the distribution of diffusion times, determined using the maximum entropy method, indicates heterogeneity in the oligomerization reaction. Analysis of the change in the fluctuations which occur in two different timescales in the native state ensemble shows that they are damped more at the erstwhile 1-3 interface than the erstwhile 2-3 interface. The difference in the extent of damping at the erstwhile 1-3 and 2-3 interfaces can be explained on the basis of the structural changes known to occur across each region. The changes in dynamics occur concurrently in both regions, indicating that the structural changes accompanying conformational conversion also occur simultaneously during the oligomerization of moPrP.

biophysics↗