Calcium Involved Directional Organization of Polymer Chains in Polyester Nanogranules in Bacterial Cells

Soil bacteria accumulate polyesters (typically poly([R]-3-hydroxybutyrate (PHB), in which one end of the chain terminates with a carboxyl group) in the form of hydrated, amorphous nanogranules in cells. However, it is not clear what drives the structure of these biomaterials inside bacterial cells. Here, we determined that calcium guides intracellular formation of PHB nanogranules. Our systematic study using the surface zeta potential measurement and the carboxyl-specific SYTO-62 dye binding assay showed that the terminal carboxyl is not exposed to the granule surface but is buried inside native "unit-granules" comprising the mature granule. Extracellular Ca²⁺ was found to mediate the formation of these PHB unit-granules, with uptaken Ca²⁺ stored inside the granules. Comparative [Ca²⁺]-dependent fluorescence spectroscopy revealed that the native granules in Cupriavidus necator H16 act as a Ca²⁺ storage system, presumably for the regulation of its cytosolic Ca²⁺ level, but those from recombinant Escherichia coli do not. This study reveals intimate links between Ca²⁺ and native granule formation, and establishes a novel mechanism that intracellular PHB granules function as Ca²⁺ storage in order to relieve soil bacteria from Ca²⁺ stress.