ATP-driven separation of liquid phase condensates in bacteria

Abstract

Liquid-liquid phase separated (LLPS) states are key to compartmentalise components in the absence of membranes, however it is unclear whether LLPS condensates are actively and specifically organized in the sub-cellular space and by which mechanisms. Here, we address this question by focusing on the ParAB S DNA segregation system, composed of a centromeric-like sequence ( parS ), a DNA-binding protein (ParB) and a motor (ParA). We show that parS -ParB associate to form nanometer-sized, spherical droplets. ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside ParB droplets. Single ParB molecules are able to rapidly diffuse between different droplets, and droplet nucleation is strongly favoured by parS . Notably, the ParA motor is required to prevent the fusion of ParB droplets. These results describe a novel active mechanism that splits, segregates and localises LLPS condensates in the sub-cellular space. 80 85 90 95 100 et al., 2005) , localizes to a large extent (~90% of ParB molecules) to regions containing parS (Sanchez et al., 2015) to form a tight nucleoprotein complex (partition complex). Formation of the partition complex requires weak ParB-ParB dimer interactions mediated by its disordered, low-complexity region. Here, we used super-resolution microscopy and single-particle analysis to investigate the physical mechanisms involved in the formation of partition complexes. We show that partition complexes are small (<50nm), spherical objects.Single, isolated ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside partition complexes. These results suggest a partition of ParB into two phases: a gas-like phase, and a dense, liquid-like condensate phase that shows nanometer-sized, spherical droplets. Next, we show that the nucleation of ParB condensates is strongly favoured by the presence of the centromeric sequence parS .Separation and proper sub-cellular localization of ParB condensates require the ParA motor.Critically, different ParB condensates collapse into a single droplet upon the degradation of ParA. These results describe a novel active mechanism that splits, segregates and localises LLPS condensates in the sub-cellular space. Results ParB assembles into spherical nano-condensatesPrevious studies have revealed that the partition complex is made of ~300 dimers of ParB (Adachi et al., 2006;Bouet et al., 2005) and ~10kb of parS -proximal DNA (Rodionov et al., 1999) . This complex is held together by specific, high-affinity interactions between ParB and parS , and by low-affinity interactions between ParB dimers that are essential for the power-law distribution of ParB around parS sites ( Figure 1A) (Debaugny et al., 2018; Sanchez 455 460 465 470 475 480 485 https://docs.google.com/document/d/1xM7w7WU8bbag5sVCGg1xatPdYe5i1xia3PzXthmzPW8/edit# 24/45 530 535 540 545 550 555 560 https://docs.google.com/document/d/1xM7w7WU8bbag5sVCGg1xatPdYe5i1xia3PzXthmzPW8/edit# 25/45 565 570 575 580 585 590 595 https:/...

Partition complex structure can arise from sliding and bridging of ParB dimers

“…Instead it may be advantageous to form a more compact partition complex to better facilitate the partitioning function of ParABS. Indeed, while F plasmid ParB spreads over a four times larger region than ParB of C. crescentus (11), the resultant partition complex is significantly smaller (a radius (2σ) of 35 nm) (49). Thus, we speculate that plasmid-based ParABS systems may operate in the more compact globular region.…”

Emerging Implications of Phase Separation in Cancer

“…It was suggested that ATP supply is necessary for maintaining liquid‐like behavior in nucleoli and SGs. [ 25,29,128,173 ] Dramatically decreased ATP levels can induce a liquid‐to‐solid transition and change the overall cytoplasmic state in bacteria and yeast. [ 174,175 ] ATP‐related proteins such as helicases and chaperone proteins are commonly implicated in controlling the material properties of RNP granules.…”

Molecular determinants for the layering and coarsening of biological condensates

“…studied ParABS, a system responsible for chromosome and plasmid segregation in bacteria. [ 128 ] While the ParB protein drove phase separation, they found that ParA, specifically ParA's ATPase activity, was necessary to control the size and location of ParABS condensates. Similar mechanisms may also play a role in other condensates, including P granules [ 129 ] and stress granules, [ 130,131 ] where the enzymatic activity has been shown to dissolve condensates.…”