Biology subjects

Swasthi, H. M.

Publications and source records attributed to Swasthi, H. M..

4 recordsLinked to original sources

bioRxiv2026

The kinetics and mobility of a ParA ATPase drive carboxysome distribution in Halothiobacillus neapolitanus

Carboxysomes are bacterial microcompartments that drive efficient carbon fixation in autotrophic bacteria. Critical to their function and inheritance is their spatial organization by the ParA-type ATPase, McdA, and its partner protein, McdB. Here, we investigate the -carboxysome McdAB system in Halothiobacillus neapolitanus using biochemical assays, quantitative fluorescence imaging, and mathematical modeling. We find that, unlike most ParA-type ATPases, the ATPase activity of McdA is only stimulated by DNA rather than by its partner protein McdB. Despite this difference, McdB conserves the ability to displace McdA from DNA, suggesting that ATP hydrolysis and DNA unbinding by McdA are not strictly coupled. Together with its ability to diffuse while bound to DNA, McdA forms gradients on the nucleoid that prevent carboxysome aggregation via a Brownian ratchet mechanism. Overall, these findings reveal key differences in a ParA-type ATPase that may be specific for the spatial organization of protein-based organelles in bacteria.

biophysics↗

bioRxiv2025

Tuning the interaction of a ParA-type ATPase with its partner separates bacterial organelle positioning from partitioning

The maintenance of carboxysome distribution (Mcd) system comprises the proteins McdA and McdB, which spatially organize carboxysomes to promote efficient carbon fixation and ensure their equal inheritance during cell division. McdA, a member of the ParA/MinD family of ATPases, forms dynamic gradients on the nucleoid that position McdB-bound carboxysomes. McdB belongs to a widespread but poorly characterized class of ParA/MinD partner proteins, and the molecular basis of its interaction with McdA remains unclear. Here, we demonstrate that the N-terminal 20 residues of H. neapolitanus McdB are both necessary and sufficient for interaction with McdA. Within this region, we identify three lysine residues whose individual substitution modulates McdA binding and leads to distinct carboxysome organization phenotypes. Notably, lysine 7 (K7) is critical for McdA interaction: substitutions at this site result in the formation of a single carboxysome aggregate positioned at mid-nucleoid. This phenotype contrasts with that of an McdB deletion, in which carboxysome aggregates lose their nucleoid association and become sequestered at the cell poles. These findings suggest that weakened McdA-McdB interactions are sufficient to maintain carboxysome aggregates on the nucleoid but inadequate for partitioning individual carboxysomes across it. We propose that, within the ParA/MinD family of ATPases, cargo positioning and partitioning are mechanistically separable: weak interactions with the cognate partner can mediate positioning, whereas effective partitioning requires stronger interactions capable of overcoming cargo self-association forces.

microbiology↗

bioRxiv2022

Cell Surface-localized CsgF Condensate is a Gatekeeper in Bacterial Curli Subunit Secretion

Curli are functional amyloids present on the outer membrane of E. coli. Cell-surface aggregation of CsgA, the major subunit of curli, is a well-orchestrated process. CsgB, the minor subunit of curli, nucleates the aggregation of CsgA while CsgF, a curli accessory protein, ensures proper anchoring of CsgB to the cell surface. The molecular basis of the interactions between CsgF and curli subunits is unclear. Here, we show that CsgF undergoes phase separation in vitro and that the ability of CsgF variants to phase separate tightly correlated with CsgF function in cells during curli biogenesis. Substitution of phenylalanine residues in the CsgF N-terminus both reduced the propensity of CsgF to phase-separate and impaired curli biogenesis. Exogenous addition of purified CsgF complemented csgF - cells. This exogenous addition assay was used to assess the ability of CsgF variants to complement csgF - cells. The presence of CsgF on the cell surface modulated the secretion of CsgA to the cell surface. We also found that the CsgB nucleator protein is a CsgF client. CsgB can form SDS-insoluble aggregates within the dynamic CsgF condensate, and we propose that these multi-component CsgF-B condensates form a nucleation-competent complex that templates CsgA amyloid formation on the cell surface. Together, our study provides insight into the ability of CsgF to phase separate, regulate CsgA secretion, and promote CsgB aggregation in curli assembly.

biochemistry↗

bioRxiv2021

Sub-stoichiometric Hsp104 regulates the genesis and persistence of self-replicable amyloid seeds of a yeast prion protein

The prion-like self-perpetuating conformational conversion is involved in both transmissible neurodegenerative diseases and non-Mendelian inheritance traits. The transmissibility of amyloid-like aggregates is dependent on the stoichiometry of chaperones such as heat shock proteins. To provide the mechanistic underpinning of the generation and persistence of prefibrillar amyloid seeds that are critical for the prion-like propagation, we studied the effect of Hsp104 disaggregase on the assembly mechanism of a yeast prion determinant of Saccharomyces cerevisiae Sup35. At low sub-stoichiometric concentrations, Hsp104 exhibits a dual role and considerably accelerates the formation of seeding-competent prefibrillar amyloids by shortening the lag phase but also prolongs their persistence by introducing unusual kinetic halts and delaying their conversion into matured fibers. Hsp104-mediated amyloid species comprise a more ordered packing and display an enhanced autocatalytic self-templating ability compare to amyloids formed without Hsp104. Our findings underscore the key functional and pathological roles of sub-stoichiometric chaperones in prion-like propagation.

biophysics↗