Formation of an actin-like filament concurrent with the enzymatic synthesis of inorganic polyphosphate.

Inorganic polyphosphate (poly P), a chain of hundreds of phosphate residues linked by ATP-like bonds, is found in every cell in nature and is commonly produced from ATP by poly P kinases (e.g., PPK1). Dictyostelium discoideum, the social slime mold, possesses a PPK activity (DdPPK1) with sequence similarity to bacterial PPKs. We find here a previously unrecognized PPK (DdPPK2) in D. discoideum with the sequences and properties of actin-related proteins (Arps) that are similar to muscle actins in size, properties, and globular-filamentous structural transitions. Significantly, the unique actin inhibitors, phalloidin and DNase I, also inhibit synthesis of poly P by DdPPK2. Thus, this particular Arp complex is an enzyme that can polymerize into an actin-like filament concurrent with its synthesis of a poly P chain in a fully reversible reaction.