Mas, C. | Search bioRxiv

bioRxiv2025

Hydra domain drives SNF2L multimerization and marks ISWI diversification in parasites

ISWI chromatin remodelers are conserved regulators of nucleosome positioning and chromatin accessibility across eukaryotes, yet their evolutionary diversification is poorly understood. In the apicomplexan parasite Toxoplasma gondii, we identify Hydra, a previously unrecognized globular domain embedded within TgSNF2L, one of two ISWI paralogues. Hydra is structurally unique, lacking homology to any known protein fold, and represents a lineage-specific insertion in an otherwise structurally conserved protein family. Biochemical analyses reveal that the isolated Hydra domain self-assembles into stable oligomers, undergoing reversible equilibrium with its monomeric form. Cryo-electron microscopy analysis reveals discrete globular assemblies, though little consistency could be obtained, suggesting a highly dynamic complex. Deletion of Hydra from full-length TgSNF2L disrupts its intrinsic ability to form higher order oligomers in solution, yielding predominantly monomeric and dimeric species. Functionally, the Hydra-driven multimerization of TgSNF2L modulates its availability for chromatin engagement in response to cell-cycle cues. Hydra thus represents the first reported structural innovation within the ISWI family.

molecular biology↗

bioRxiv2025

Analytical Ultracentrifugation as a Tool for Exploring COSAN Assemblies.

The self-assembly of the cobalta bis(dicarbollide) (COSAN) anionic boron clusters into micelles above a critical micelle concentration (cmc) of 10 - 20 mM and its behavior as "sticky nano-ions" facilitating controlled protein aggregation have been previously investigated using scattering techniques, particularly small-angle neutron and X-ray scattering. These techniques effectively provide average structural parameters but, when applied to colloidal systems, often rely on models assuming polydispersity or anisotropic shapes. Thus, complementary techniques are required to confirm or infirm the proposed analyses. Here, we employed sedimentation velocity analytical ultracentrifugation (SV-AUC), which offers the ability to resolve discrete species. We revisited two key questions: (1) the aggregation behavior of COSAN into micelles, a topic still under debate and due to its unconventional amphiphilic nature, and (2) the nature of the protein assemblies induced by COSAN, specifically their size/shape distribution and aggregation number. Our findings confirm the cmc of COSAN of 16 mM and reveal that COSAN micelles exhibit low aggregation numbers (8 in water and 14 in dilute salt), consistent with recent hypotheses. Furthermore, SV-AUC showed that COSAN promotes myoglobin aggregation into discrete oligomeric species with well-defined aggregation numbers, such as dimers, tetramers, and higher-order assemblies, depending on the COSAN-to-protein ratio. These results provide clarity on the discrete nature of COSAN micelle aggregation and protein assembly. This study highlights the complementary role of SV-AUC in understanding supramolecular assemblies, offering useful insights into the behavior of COSAN nano-ions and their interactions with biomacromolecules.

biophysics↗