Elucidating the mechanism of polyphosphate granule consolidation in bacteria

Synthesis of polyphosphate (polyP), an ancient anionic biopolymer composed of phosphoryl groups, is universally conserved in the kingdom of life. PolyP has been implicated in promoting fitness under starvation conditions in diverse bacteria, and it has been associated with pleiotropic effects on biofilm formation, motility, cell cycle and oxidative stress resistance. The molecular mechanisms underpinning these effects have traditionally been difficult to explain. In many bacteria, polyP chains come together to form membrane-less superstructures, termed polyP granules. Some bacterial species regulate the organization of these granules. Despite the critical role of polyP in starvation fitness, the composition of these structures and the mechanism(s) underpinning their organization are poorly understood. Our previous work identified polyP-granule associated proteins in opportunistic human pathogen Pseudomonas aeruginosa. However, the mechanism(s) of polyP granule maturation and subcellular organization are not well understood. To understand the fundamental properties of polyP granules, we use in vitro reconstitution to study a simplified multicomponent system of polyP, inorganic multivalent cations, the granule-associated protein AlgP, and DNA. Using light and fluorescence microscopy, our preliminary results suggest that these ternary systems can undergo heterotypic phase transitions. Additionally, the material properties of these condensates are tunable and sensitive to environmental conditions. Interestingly, we also observe that these condensates exhibit unique sub-structure. These studies will inform future efforts to understand the basis of polyP granule composition and consolidation, as well as its functional significance to cellular fitness during starvation stress in diverse bacteria.