Pearls Polyphosphate and Its Diverse Functions in Host Cells

Polyphosphate (polyP) is a linear polymer of a few to many hundreds of phosphate (Pi) residues linked by high-energy phosphoanhydride bonds (Figure 1A). This ubiquitous polymer is found in bacteria, protists, and mammalian cells, and it was likely present prebiotically [1]. In bacteria, polyP accumulates in volutin or metachromatic granules, which are equivalent to acidocalcisomes [2]. In eukaryotic cells, polyP is present in different compartments, including the cytosol, nucleus, lysosomes, and mitochondria, but is preferentially accumulated in acidic vacuoles such as the yeast vacuole and acidocalcisomes [1,3]. In these organelles, polyP, which is negatively charged, is in close association of inorganic (Ca2+, Mg2+, Zn2+, Fe2+, Na+, K+) and organic (basic amino acids, polyamines) cations. PolyP also combines with calcium and polyhydroxybutyrate forming chan-nels in bacterial membranes, which make them competent fo