Two important polymers cross paths

Poly P and Its Enzymology Are Important Players in Cell Biology

Poly P, a ubiquitous polymer found in the environment and in all bacterial, fungal, animal, and plant cells, is an obvious energy store with high-energy phospho-anhydride bonds. Poly P is involved in a wide variety of critical cellular functions. It is, for example, essential for bacterial responses to stresses and starvation, as well as for survival (2). As an anion, it chelates Mg²⁺, Mn²⁺, Zn²⁺, and Fe³⁺ and affects their roles in the cell, and it also sequesters toxic metals such as Hg⁺ and Cd²⁺. As a polyanion, the flexible and relatively stable poly P can be localized in the cell and used in response to a wide variety of metabolic needs (3).

The enzymology of poly P has been elucidated primarily in the laboratory of Kornberg at Stanford University (2). The first enzyme activity for poly P synthesis was described in 1956 (4). Now known as PPK1 (poly P kinase 1), the gene for this enzyme was knocked out in Dictyostelium (H. Zhang, personal communication), revealing a second activity, DdPPK2, with remarkably different properties (1). As described below, the enzymatic activity of DdPPK2 is highly unusual in that it depends on and is coincident with assembly of the enzyme into a polymer, a property that it shares with the ubiquitous protein actin.

Actin Assembles into Filaments with Associated Hydrolysis of ATP

Biochemists have been studying actin since the 1940s. Actin is a highly conserved protein with >90% identity between yeast and human and serves a multitude of cellular functions (5). In its filamentous form (F-actin), it provides tracks along which a variety of myosin motors move carrying diverse cellular components to be …