Biology subjects

Ramm, B.

Publications and source records attributed to Ramm, B..

3 recordsLinked to original sources

bioRxiv2022

A phase-separated biomolecular condensate nucleates polymerization of the tubulin homolog FtsZ to spatiotemporally regulate bacterial cell division

Cell division is spatiotemporally precisely regulated, but the underlying mechanisms are incompletely understood. In the social, predatory bacterium Myxococcus xanthus, the PomX/PomY/PomZ proteins form a single large megadalton-sized complex that directly positions and stimulates cytokinetic ring formation by the tubulin homolog FtsZ. Here, we studied the structure and mechanism of this complex in vitro and in vivo. We demonstrate that PomY forms liquid-like biomolecular condensates by phase separation, while PomX self-assembles into filaments generating a single large cellular structure. The PomX structure enriches PomY, thereby guaranteeing the formation of precisely one PomY condensate per cell through surface-assisted condensation. In vitro, PomY condensates selectively enrich FtsZ and nucleate GTP-dependent FtsZ polymerization, suggesting a novel cell division site positioning mechanism in which the single PomY condensate enriches FtsZ to guide FtsZ-ring formation and division. PomY-nucleated FtsZ polymerization shares features with microtubule nucleation by biomolecular condensates in eukaryotes, supporting this mechanisms ancient origin.

cell biology↗

bioRxiv2021

Mass-sensitive particle tracking (MSPT) to elucidate the membrane-associated MinDE reactioncycle

In spite of their great importance in biology, methods providing access to spontaneous molecular interactions with and on biological membranes have been sparse. So far, it has been consensus that their observation with sufficient sensitivity and time resolution requires the introduction of - predominantly fluorescent-labels to the system. However, the recent advent of mass photometry to quantify mass distributions of unlabelled biomolecules landing on surfaces raised hopes that this approach could be transferred to membranes. Here, we introduce mass-sensitive particle tracking (MSPT), enabling simultaneous label-free tracking and monitoring of molecular masses of single biomolecules diffusing on lipid membranes. We applied this approach to the highly non-linear reaction cycles underlying MinDE protein self-organisation. MSPT allowed us to determine the stoichiometry and turnover of individual membrane-bound MinD/MinDE protein complexes and to quantify their size-dependent diffusion. We found that MinD assembles into complexes larger than the commonly postulated dimer, through lateral interactions of membrane-bound complexes and subunit recruitment from solution. Furthermore, the ATPase-activating protein MinE interconnects MinD into high-molecular-weight heteromeric complexes and affects their subunit turnover and concerted membrane release. This study demonstrates the potential of MSPT to enhance our quantitative understanding of both prokaryotic and eukaryotic membrane-associated biological systems.

biophysics↗

bioRxiv2020

ATP driven diffusiophoresis: active cargo transport without motor proteins

Morphogenesis and homeostasis of biological systems are intricately linked to gradient formation through energy dissipation. Such spatial organization may be achieved via reaction-diffusion or directional cargo transport, as prominently executed by motor proteins. In contrast to these processes that rely on specific protein interactions, active transport based on a non-specific, purely physical mechanism remains poorly explored. Here, by a joint experimental and theoretical approach, we describe a hidden function of the MinDE protein system from E. coli: Besides forming dynamic patterns, this system accomplishes the active transport of large, functionally unrelated cargo on membranes in vitro. Remarkably, this mechanism allows to sort diffusive objects according to their effective size, as evidenced using modular DNA origami-streptavidin nanostructures. We show that the diffusive fluxes of MinDE and cargo couple via density-dependent friction. This non-specific process constitutes a Maxwell-Stefan diffusiophoresis, so far undescribed in a biologically relevant setting. Such nonlinear coupling between diffusive fluxes could represent a generic physical mechanism for the intracellular organization of biomolecules.

biophysics↗