Biology subjects

Masic, V.

Publications and source records attributed to Masic, V..

4 recordsLinked to original sources

bioRxiv2026

SARS-CoV-2 spike protein-associated sialoglycoconjugates induce nanoscale filipodia to facilitate micro-size platelet clotting

COVID-19 disease is associated with thrombosis, but the pathogenic mechanism remains unclear. Here, we investigate how SARS-CoV-2 spike protein causes platelet activation and aggregation. Our three-dimensional ultrastructural analyses showed that invaginated platelet structures, open canalicular system (OCS), expanded upon activation, trapping viral particles in the process. Binding with platelet OCS concealed SAR-CoV-2 spike-coated particles from virion detection in platelet-depleted blood plasma. Both SARS-CoV-2 spike coated-particles and recombinant spikes specifically induced platelet aggregation with nanoscale filipodia extensions, with the terminal sialic acids of the SARS-CoV-2 spike protein-associated sialoglycoconjugates being the key determinant in platelet activation. Our work illustrates that virus-associated sialic acids, not proteins, are functionally responsible for SARS-CoV-2 induced thrombotic events, providing a mechanistic insight on how glycosylation contributes to disease severity in COVID-19. This study lays the foundation for the development of glycan-modified vaccines with reduced risks of thrombosis.

molecular biology↗

bioRxiv2026

Filament formation is a conserved mechanism of Thoeris SIR2 effector activation

Thoeris defence systems protect bacteria from phages via abortive infection. In type I Thoeris systems, ThsA effectors containing silent information regulator 2 (SIR2) and SMF/DprA-LOG (SLOG) domains are activated by the cyclic ADP-ribose (ADPR) isomer 3'cADPR, triggering abortive infection via nicotinamide adenine dinucleotide (NAD+) depletion. 3'cADPR activates the NADase activity of Bacillus subtilis ThsA tetramers via filament formation of its SIR2 domains, but the molecular details of how 3'cADPR triggers this process remain incompletely understood. Here, we demonstrate that ThsA activation by 3'cADPR-induced SIR2 filament formation is conserved in type I Thoeris systems from Streptococcus equi and Entercoccus facium. We present cryo-electron microscopy structures of the S. equi ThsA filament bound to 3'cADPR and the non-cleavable NAD+ analog carba-NAD+, and of the S. equi ThsA tetramer bound to 3'cADPR. These structures reveal that SIR2 filament formation is required to stabilise an active site conformation that can bind and hydrolyse NAD+. The structures also show that 3'cADPR induces quaternary alterations in the SLOG dimers and consequently the SIR2 tetramer to enable ThsA filament formation. Collectively, our study provides a comprehensive understanding of 3'cADPR-induced activation of type I Thoeris effectors.

microbiology↗

bioRxiv2026

Calcium-phosphate bridge is a novel phosphorylation switch that stabilises protein-complexes during HIV assembly

Calcium (Ca2+) and phosphate (PO43-) are fundamental-element and -chemical group in biology. Specifically, the chemistry of both Ca2+ signalling and phosphorylation switch are independent mechanisms regulating a broad spectrum of biological processes. It is, however, not appreciated that a normal function of phospho-mimic amino acids (aspartate/glutamate) is to interact with Ca2+ at the atomic level. Here, we leveraged HIV-Ca2+ biology in primary cells to describe an unknown layer of regulatory processes via Ca2+-phosphate (PO43-) bridge to support protein complex formation. We identified novel HIV phosphorylation sites overlapping Ca2+ binding domains through phospho-proteomics. Integrating primary cells, molecular virology, structural biology, biophysical and ultrastructural analyses, we presented multiple examples of Ca2+-PO43- bridges that support HIV assembly and function. These include Ca2+-PO43- bridges: (i) stabilising Pr55Gag-Pr160GagPol complex for virus function; (ii) mediating p6Pol dimerization to support virion maturation; and (iii) modulating viral complex formation to package both viral enzymatic- and cellular-proteins. As the convergent enrichment of these signatured calcium-phosphorylation domains occurs across a wide range of viral and cellular proteins, we propose Ca2+-PO43- bridge to be a general principle for Ca2+-coordinated phosphorylation switch to regulate biological processes.

cell biology↗

bioRxiv2022

Chemical structures of cyclic ADP ribose (cADPR) isomers and the molecular basis of their production and signaling

Cyclic ADP ribose (cADPR) isomers are important signaling molecules produced by bacterial and plant Toll/interleukin-1 receptor (TIR) domains via NAD+ hydrolysis, yet their chemical structures are unknown. We show that v-cADPR (2cADPR) and v2-cADPR (3cADPR) isomers are cyclized by O-glycosidic bond formation between the ribose moieties in ADPR. Structures of v-cADPR (2cADPR)-producing TIR domains reveal that conformational changes are required for the formation of the active assembly that resembles those of Toll-like receptor adaptor TIR domains, and mutagenesis data demonstrate that a conserved tryptophan is essential for cyclization. We show that v2-cADPR (3cADPR) is a potent activator of ThsA effector proteins from Thoeris anti-phage defence systems and is responsible for suppression of plant immunity by the effector HopAM1. Collectively, our results define new enzymatic activities of TIR domains, reveal the molecular basis of cADPR isomer production, and establish v2-cADPR (3cADPR) as an antiviral signaling molecule and an effector-mediated signaling molecule for plant immunity suppression. One-Sentence SummaryThe chemical structures of two O-glycosidic bond-containing cyclic ADP ribose isomers, the molecular basis of their production, and their function in antiviral and plant immunity suppression by bacteria are reported.

biochemistry↗