Fig 1.
Comparison of signaling via two-component systems vs. biomolecular condensates.
TCS (top) represents classical sensing mechanisms in microbes whereby a membrane localized histidine kinase receives an environmental signal and transduces it via response regulators ultimately resulting in regulation of gene expression that occurs over long time scales. Biomolecular condensates (bottom) are an attractive mechanism by which signaling molecules such as kinases can be localized in the presence of environmental fluctuations including stress. The activity and function of localized enzymes within fluid condensates can change rapidly in the presence of stressors. The viscoelastic properties of condensates themselves can be regulated by environmental signals including stress that can transform condensates into solid-like arrested assemblies. Created with BioRender.com.
Fig 2.
Triggers and tuners of condensates.
Formation of condensates is triggered when the local concentration of macromolecules reaches a level called Csat. Csat can be tuned by metabolic, thermal, or mechanical stressors. Once formed, condensates can grow by the addition of more condensate-forming macromolecules or by fusion of fluid condensates. Condensate formation and growth is tightly regulated by a variety of external factors such as fluctuations in pressure, temperature, pH, nutrient levels, osmolarity, and small molecule concentrations. On the other hand, intrinsic factors such as protein–protein interactions, disruptions in gene expression, and protein posttranslational modifications are key regulators of condensate properties and functions. Created with BioRender.com.
Fig 3.
Small molecule-based regulation of condensate properties and function.
(Top) Salient examples of condensates from the aquatic bacterium Caulobacter crescentus that can be regulated by small molecule metabolites. SpmX is an IDP that forms polar condensates that localize the Histidine kinase DivJ. SpmX condensate formation is promoted under ATP depletion while dissolution is favored under high ATP concentrations. This ATP-dependent feedback is in turn exploited to regulate DivJ kinase activity in response to substrate availability. RNase E condensate regulates PNPase activity but is inhibited by phosphate, which is the substrate of this enzyme. RNase E phase separation is regulated by positive feedback from one substrate (polyA) and negative feedback from the other (phosphate). (Bottom) A repertoire of molecules that can tune condensate formation and disassembly. Lipoic acid, lipoamide, cetylpyridinium chloride, ATP, and hexanediol have been shown to dissolve biomolecular condensates. Other molecules with similar structural features such as antibiotics (Mitoxantrone), second messengers (cyclic di-GMP), and quorum-sensing ligands (AHLs) can be putative modulators of bacterial condensates.
Table 1.
Condensates, their function and evidence for phase separation.
Fig 4.
The prevalence of intrinsically disordered proteins in pathogens.
Salient examples of virulence factors in ESKAPE pathogens that have significant intrinsically disordered regions are shown. Distribution of IDPs from ESKAPE pathogens, color coded by the species, are shown as a function of the fraction of acidic (x-axis) and basic (y-axis) residues. See supplementary tables and files for more information on the proteins included in the graph.